AN1996 Demodulating at 10.7 MHz IF with the SA605 Rev. 2 — 28 August 2014 Application note Document information Info Content Keywords RSSI extender circuit, RSSI dynamic range, SAW filter, quadrature tank, S curve, tapped-C transform matching network. Abstract This application note discusses RF circuit techniques and principles that will enhance stable receiver operation. Consideration is given to PCB layout, special circuits, such as, the RSSI extender, and passive component selection. Performance data is provided for specific applications at 240 MHz and 45 MHz RF inputs. AN1996 NXP Semiconductors Demodulating at 10.7 MHz IF with the SA605 Revision history Rev Date Description 2 20140828 Application note; second release Modifications: 1 19971023 • The format of this application note has been redesigned to comply with the new identity guidelines of NXP Semiconductors. • Legal texts have been adapted to the new company name where appropriate. Application note; intial release Contact information For more information, please visit: http://www.nxp.com For sales office addresses, please send an email to: [email protected] AN1996 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 28 August 2014 © NXP Semiconductors N.V. 2014. All rights reserved. 2 of 25 AN1996 NXP Semiconductors Demodulating at 10.7 MHz IF with the SA605 1. Introduction The need for high-speed communications is increasing in the market place. To meet these needs, high-performance receivers must demodulate at higher IF frequencies to accommodate for the wider deviations in FM systems. The standard 455 kHz IF frequency, which is easier to work with, and thus more forgiving in production, no longer satisfies the high-speed communication market. The next higher standard IF frequency is 10.7 MHz. This frequency offers more potential bandwidth than 455 kHz, allowing for faster communications. Since the wavelength at 10.7 MHz is much smaller than 455 kHz, the demand for a good RF layout and good RF techniques increases. These demands aid in preventing regeneration from occurring in the IF section of the receiver. This application note will discuss some of the RF techniques used to obtain a stable receiver and reveal the excellent performance achieved in the lab. 1.1 Background If a designer is working with the SA605 for the first time, it is highly recommended that AN1994 (Ref. 2) and AN1995 (Ref. 3) be read. These two application notes discuss the SA605 in great detail and provide a good starting point in designing with the chip. Before starting a design, it is also important to choose the correct part. NXP offers an extensive receiver line to meet the growing demands of the wireless market. Table 1 displays the different types of receivers and their key features. With the aid of this chart, a designer will get a good idea for choosing a chip that best fits their design needs. If low-voltage receiver parts are required in a design, a designer can choose between SA606 or SA636. These low-voltage receivers are designed to operate at 3 V while still providing high performance to meet the specifications for cellular radio. All of these parts can operate with an IF frequency as high as 2 MHz. However, the SA636 can operate with a standard IF frequency of 10.7 MHz and also provide fast RSSI speed. Additionally the SA636 has a Power-down mode to conserve battery power. AN1996 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 28 August 2014 © NXP Semiconductors N.V. 2014. All rights reserved. 3 of 25 xxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxx x x x xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxx xx xx xxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxx xxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxx x x xxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxx xxx NXP Semiconductors AN1996 Application note Table 1. FM/IF family overview Specification SA602A SA604A SA605 SA606 SA636 VCC 4.5 V to 8 V 4.5 V to 8 V 4.5 V to 8 V 2.7 V to 7 V 2.7 V to 5.5 V ICC 2.4 mA at 6 V 3.3 mA at 6 V 5.7 mA at 6 V 3.5 mA at 3 V 6.5 mA at 3 V Number of pins 8 16 20 20 20 Packages SA602AD/01: SO8 SA604AD/01: SO16 SA605D/01: SO20 SA606DK/01: SSOP20 SA636BS: HVQFN20 SA605DK/01: SSOP20 SA606DK/02: SSOP20 SA636DK/01: SSOP20 SA606DK/03: SSOP20 12 dB SINAD (RF = 45 MHz; IF = 455 kHz); 1 kHz tone; 8 kHz deviation 120 dBm / 0.22 V 120 dBm / 0.22 V 120 dBm / 0.22 V 117 dBm / 0.31 V 112 dBm / 0.54 V (RF = 240 MHz; IF = 10.7 MHz) Process ft 8 GHz 8 GHz 8 GHz 8 GHz 8 GHz For lower-cost version and less performance SA612A SA614A SA615 SA616 - • • Features • Audio and data pins IF bandwidth of 25 MHz No external matching required for standard 455 kHz IF filter • • • • • Audio and data pins IF bandwidth of 25 MHz No external matching required for standard 455 kHz IF filter • • • Audio and data pins IF bandwidth of 25 MHz No external matching required for standard 455 kHz IF filter • • • • • • Low-voltage Internal RSSI and audio op amps No external matching required for standard 455 kHz IF filter IF bandwidth of 2 MHz • • Power-down mode Low-voltage Fast RSSI time IF bandwidth of 25 MHz Internal RSSI and audio op amps No external matching required for standard 10.7 MHz IF filter RSSI output section 90 dB 90 dB 90 dB 90 dB 90 dB Accuracy 1.5 dB 1.5 dB 1.5 dB 1.5 dB 1.5 dB - 1.4 s - - - - 21.3 s - - - Rise time[1] - 1.5 s - - 1.2 s Fall time[1] - 19.4 s - - 2 s 455 kHz IF Rise time[1] Fall time[1] 10.7 MHz IF AN1996 4 of 25 © NXP Semiconductors N.V. 2014. All rights reserved. Dynamic range Demodulating at 10.7 MHz IF with the SA605 Rev. 2 — 28 August 2014 All information provided in this document is subject to legal disclaimers. 1 kHz tone; 70 kHz deviation xxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxx x x x xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxx xx xx xxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxx xxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxx x x xxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxx xxx FM/IF family overview …continued Specification SA602A SA604A SA605 SA606 SA636 17 dB - 13 dB 17 dB 13 dB 3rd-order intercept point (input) 13 dB f1 = 45 MHz; f2 = 45.06 MHz - 10 dBm 9 dBm 11 dBm (f1 = 240.05 MHz; f2 = 240.35 MHz) NXP Semiconductors AN1996 Application note Table 1. Mixer Max. conversion power gain (RF = 45 MHz; IF = 455 kHz) 5 dB - 5 dB 6.2 dB 11 dB at 240 MHz 1.5 k - 4.7 k 8 k 4.7 k 3.5 pF 3 pF 3.5 pF at 240 MHz Output resistance 1.5 k - 1.5 k 1.5 k 330 Total IF gain - 100 dB 100 dB 100 dB 96 dB (includes 6 dB pad) Total IF bandwidth - 25 MHz 25 MHz 2 MHz 25 MHz Input impedance - 1.6 k 1.6 k 1.5 k 330 Output impedance - 1.0 k 1.0 k 330 330 Gain - 40 dB 40 dB 44 dB 44 dB Bandwidth - 41 MHz 41 MHz 5.5 MHz 40 MHz - 1.6 k 1.6 k 1.5 k 330 3 pF IF Section IF amplifier IF limiter Input impedance - 330 330 330 330 Gain - 60 dB 60 dB 58 dB 58 dB Bandwidth - 28 MHz 28 MHz 4.5 MHz 28 MHz Output No IF filters in the circuit. AN1996 5 of 25 © NXP Semiconductors N.V. 2014. All rights reserved. [1] impedance[1] Demodulating at 10.7 MHz IF with the SA605 Rev. 2 — 28 August 2014 All information provided in this document is subject to legal disclaimers. Noise Figure at 45 MHz RF input resistance and capacitance at 45 MHz AN1996 NXP Semiconductors Demodulating at 10.7 MHz IF with the SA605 1.2 Objective The objective of this application note is to show that the SA605 can perform well at an IF frequency of 10.7 MHz. Since most NXP Semiconductors receiver demoboards are characterized at RF = 45 MHz/IF = 455 kHz, we decided to continue to characterize at this frequency. This way we could compare how much degradation (for different IFs) there was with a RF = 45 MHz/IF = 455 kHz versus RF = 45 MHz/ IF = 10.7 MHz. As we will discuss later, there was minimal degradation in performance. We also tested at RF = 240 MHz/IF = 10.7 MHz. The 240 MHz RF is sometimes referred to as the first IF for double conversion receivers. Testing the board at RF = 83.16 MHz (which is also a common first IF for analog cellular radio) and IF = 10.7 MHz was not done because the conversion gain and noise figure does not change that much compared to 45 MHz input. Therefore, we can expect the same type of performance at 83.16 MHz. The RF = 240 MHz/IF = 10.7 MHz demoboard is expected to perform less than the RF = 45 MHz/IF = 10.7 MHz demoboard because the mixer conversion gain decreases while the noise figure increases. These two parameters will decrease the performance of the receiver as the RF frequency increases. For systems requiring low voltage operation, IF = 10.7 MHz and fast RSSI speed, the SA636 will be the correct choice. 2. Board setup and performance graphs Figure 1 and Figure 2 show the SA605 schematics for the 240 MHz and 45 MHz boards, respectively. Table 2 lists the basic functions of each external component for both Figure 1 and Figure 2. AN1996 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 28 August 2014 © NXP Semiconductors N.V. 2014. All rights reserved. 6 of 25 AN1996 NXP Semiconductors Demodulating at 10.7 MHz IF with the SA605 D1 R18 J1 RF IN 56k C2 10pF C3 330pF C7 1-5pF C1 1-5pF L5 8.2μH L1 56nH MXR OUT RF IN C6 15pF L2 82nH C8 330pF C10 10μF VCC C26 47pF C9 .1μF BYPASS DECOUP OSC OUT OSC IN MUTE IN VCC RSSI OUT AUDIO OUT DATA OUT QUAD IN IF IN DECOUP R9 100k C16 39pF C32 220pF IF OUT GND LIM IN DECOUP DECOUP LIM OUT C31 220pF TO RSSI (PIN 7) C21 0.1μF C17 C25 0.1μF 0.1μF C22 0.1μF L3 8.2μH C18 0.1μF R10 100k C30 220pF 8.2μH C19 47pF C20 47pF C14 1nF C13 150pF R20 5.1k Q1 BC857B IFT1 5.6μH AUDIO R19 100k C29 47pF L4 C15 1pF RSSI C12 .01μF C27 C28 47pF 47pF C23 0.1μF U1 SA605D C11 .1μF BAS16 3 2 C24 1nF J2 LO VCC FLT1 1 FLT1 1 3 2 R17 220 R16 R11 100k 120 DATA an1996_1 Fig 1. SA605 schematic: RF = 240 MHz, LO = 229.3 MHz, IF = 10.7 MHz AN1996 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 28 August 2014 © NXP Semiconductors N.V. 2014. All rights reserved. 7 of 25 AN1996 NXP Semiconductors Demodulating at 10.7 MHz IF with the SA605 D1 R18 J1 RF IN 56k C2 120pF C3 330pF C1 5-30pF L5 8.2 μH L1 620nH C7 5-30pF RF IN C6 150pF L2 390nH C8 330pF C10 10μF V CC C26 47pF C9 .1 μF MXR OUT BYPASS DECOUP OSC OUT OSC IN MUTE IN V CC RSSI OUT AUDIO OUT DATA OUT QUAD IN IF IN DECOUP C16 39pF IF OUT GND LIM IN DECOUP DECOUP LIM OUT 8.2 μH C12 .01 μF C17 0.1 μF C22 0.1 μF L3 8.2 μH C19 47pF FLT1 1 C20 47pF C14 1nF 3 2 R17 220 R16 R11 100k C13 150pF TO RSSI (PIN 7) C25 C18 0.1 μF R10 100k C31 220pF C21 0.1 μF IFT1 5.6 μH AUDIO C32 220pF C30 220pF L4 C23 0.1 μF C15 1pF RSSI C29 47pF R20 10k Q1 BC857B 0.1 μF R9 100k R19 100k 2 U1 SA605D C11 .1 μF BAS16 3 C27 C28 47pF 47pF C24 1nF J2 LO V CC FLT1 1 120 DATA Fig 2. an1996_2 SA605 schematic: RF = 45 MHz, LO = 55.7 MHz, IF = 10.7 MHz Table 2. AN1996 Application note SO layout schematic component list Component Description U1 SA605D/01 FLT1 10.7 MHz ceramic filter Murata SFE10.7MA5-A (280 kHz bandwidth)[1] FLT2 10.7 MHz ceramic filter Murata SFE10.7MA5-A (280 kHz bandwidth)[1] C1 part of the tapped-C network to match the front-end mixer C2 part of the tapped-C network to match the front-end mixer C3 used as an AC short to Pin 2 and to provide a DC block for L1, which prevents the upsetting of the DC biasing on Pin 1 C6 part of the tapped-C network to match the LO input C7 part of the tapped-C network to match the LO input C8 DC blocking capacitor C9 supply bypassing C10 supply bypassing (this value can be reduced if the SA605 is used with a battery) C11 used as a filter, cap value can be adjusted when higher RSSI speed is preferred over lower RSSI ripple C12 used as a filter C13 used as a filter C14 used to AC ground the quad tank C15 used to provide the 90 phase shift to the phase detector C16 quad tank component to resonant at 10.7 MHz with IFT1 and C15 All information provided in this document is subject to legal disclaimers. Rev. 2 — 28 August 2014 © NXP Semiconductors N.V. 2014. All rights reserved. 8 of 25 AN1996 NXP Semiconductors Demodulating at 10.7 MHz IF with the SA605 Table 2. SO layout schematic component list …continued Component Description C17 IF limiter decoupling capacitor C18 DC block for L3 which prevents the upsetting of the DC biasing on Pin 14 C19 part of the tapped-C network for FLT2 C20 part of the tapped-C network for FLT2 C21 IF amplifier decoupling capacitor C22 DC blocking capacitor C23 IF amplifier decoupling capacitor and DC block for L3 which prevents the upsetting of the DC biasing on Pin 14 C24 provides DC block for L5 which prevents the upsetting of the DC biasing on Pin 20 C25 IF limiter decoupling capacitor C26 part of the tapped-C network for FLT1 C27 part of the tapped-C network for FLT1 C28 part of the tapped-C network for FLT1 C29 part of the tapped-C network for FLT1 R9 used to convert the current into the RSSI voltage R10 converts the audio current to a voltage R11 converts the data current to a voltage R16 used to kill some of the IF signal for stability purposes R17 used in conjunction with R16 for a matching network for FLT2 L1 part of the tapped-C network to match the front-end mixer L2 part of the tapped-C network to match the front-end mixer L3 part of the tapped-C network to match the input of FLT2 L4 part of the tapped-C network to match the input of FLT1 L5 part of the tapped-C network to match the input of FLT1 RSSI extender circuit R18 provides bias regulation, the gain will stay constant over varying VCC R19 for biasing, buffer RF DC voltage R20 provides the DC bias, RSSI gain (when R20 increases, RSSI gain decreases) C30 DC blocking capacitor which connects the ceramic filter’s output to the PNP transistor’s input C31 decoupling capacitor, and should be removed for measuring RSSI systems speed C32 peak detector charge capacitor D1 diode to stabilize the bias current Q1 NXP BC857B PNP transistor IFT1 part of the quad tank circuit [1] AN1996 Application note If a designer wants to use different IF bandwidth filters than the ones used in this application note, the quad tank’s S-curve may need to be adjusted to accommodate the new bandwidth. All information provided in this document is subject to legal disclaimers. Rev. 2 — 28 August 2014 © NXP Semiconductors N.V. 2014. All rights reserved. 9 of 25 AN1996 NXP Semiconductors Demodulating at 10.7 MHz IF with the SA605 There are minor differences between Figure 1 and Figure 2. The RF and LO tapped-C component values are changed to accommodate for the different RF and LO test frequencies (RF = 240 MHz and 45 MHz and LO = 229.3 MHz and 55.7 MHz). The other difference is the value of R20. This resistor value was changed to optimize the RSSI curve’s linearity (see RSSI extender section in this application note for further details). The recommended SA605 layout is shown in Figure 3. This layout can be integrated with other systems. 5.2cm SA605 4.8cm T O P S IL K S C R E E N T O P V IE W B O T T O M V IE W an1996_3 Fig 3. SA605 SO demoboard layouts (not actual size) Figure 4 and Figure 5 show the performance graphs for the SA605 at 240 MHz and 45 MHz RF inputs. AN1996 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 28 August 2014 © NXP Semiconductors N.V. 2014. All rights reserved. 10 of 25 AN1996 NXP Semiconductors Demodulating at 10.7 MHz IF with the SA605 10.00 0.00 AUDIO --10.00 --20.00 --30.00 THD + N(dB) --40.00 --50.00 AM REJ (dB) --60.00 NOISE --70.00 --80.00 --125 --115 --105 --95 --85 --75 --65 --55 --45 --35 --25 --15 --5 RF LEVEL (dBm) 5.0 4.5 4.0 3.5 3.0 RSSI (V) 2.5 2.0 1.5 1.0 0.5 0.0 --130 --120 --110 --100 --90 --80 --70 --60 RF IN (dBm) --50 --40 --30 --20 --10 0 10 an1996_4 Fig 4. SA605 SO performance graphs at 240 MHz 10.00 0.00 AUDIO --10.00 --20.00 --30.00 THD + N(dB) --40.00 --50.00 AM REJ (dB) --60.00 NOISE --70.00 --80.00 --125 --115 --105 --95 --85 --75 --65 --55 --45 --35 --25 --30 --20 --15 --5 RF LEVEL (dBm) 5.0 4.5 4.0 3.5 RSSI (V) 3.0 2.5 2.0 1.5 1.0 0.5 0.0 --130 --120 --110 --100 --90 --80 --70 --60 --50 RF IN (dBm) --40 --10 0 10 an1996_5 Fig 5. AN1996 Application note SA605 SO performance graphs at 45 MHz All information provided in this document is subject to legal disclaimers. Rev. 2 — 28 August 2014 © NXP Semiconductors N.V. 2014. All rights reserved. 11 of 25 AN1996 NXP Semiconductors Demodulating at 10.7 MHz IF with the SA605 3. RF input The SA605 board is set up to receive an RF input of 240 MHz (see Figure 1). This is achieved by implementing a tapped-C network. The deviation should be set to 70 kHz to achieve 110 dBm to 112 dBm for 12 dB SINAD. However, the deviation can be increased to 100 kHz, depending on the bandwidth of the IF filter and the Q of the quad tank. Because we wanted to test the board at 45 MHz, we changed the values of the tapped-C network for the RF and LO ports (see Figure 2). We found that a 116 dBm to 118 dBm for 12 dB SINAD could be achieved. With these results, we were pretty close to achieving performance similar to our standard 455 kHz IF board. A designer can also make similar RF and LO component changes if he/she needs to evaluate the board at a different RF frequency. It should be noted that if a designer purchases a stuffed SA605 demoboard from NXP Semiconductors, its setup will be for an RF input frequency of 240 MHz. AN1994 will aid the designer in calculating the tapped-C values for other desired frequencies, while AN1995 will be of value for making S11 bench measurements. Just remember that the input impedance will differ for different RF frequencies. 4. LO input The LO frequency should be 229.3 MHz for the RF = 240 MHz demoboard and have a drive level of 10 dBm to 0 dBm (this also applies for the RF = 45 MHz and LO = 55.7 MHz). The drive level is important to achieve maximum conversion gain. The LO input also has a matched tapped-C network for efficiency purposes which makes for good RF practices. If a designer wanted to change the matching network to inject a different LO frequency, he/she could follow the steps in AN1994 and assume that the input impedance is around 10 k for low frequency inputs. The main goal is to get maximum voltage transfer from the signal generator to the inductor. An external oscillator circuit was used to provide greater flexibility in choosing different RF and LO frequencies; however, an on-board oscillator can be used with the SA605. New high frequency fundamental crystals, now entering the market, can also be used for high LO frequency requirements. Most receiver systems, however, will use a synthesizer to drive the LO port. 5. 10.7 MHz ceramic filters The input and output impedance of the 10.7 MHz ceramic IF filters are 330 . The SA605 input and output impedances are roughly 1.5 k. Therefore, a matching circuit had to be implemented to obtain maximum voltage transfer. Tapped-C networks were used to match the filters input and output impedance. But in this case, we decided to go with non-tuning elements to reduce set-up time. Figure 6 shows the values chosen for the network. Although our total deviation is 140 kHz, we used 280 kHz IF bandwidth filters to maximize for fast RSSI speed. The SINAD performance difference between using 180 kHz bandwidth filter versus 280 kHz band shaping filter was insignificant. AN1996 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 28 August 2014 © NXP Semiconductors N.V. 2014. All rights reserved. 12 of 25 AN1996 NXP Semiconductors Demodulating at 10.7 MHz IF with the SA605 FROM MIXER (PIN 20) C26 47pF L5 8.2 μH 10.7MHz CERAMIC FILTER C 27 47pF C24 1nF C29 47pF FLT1 Z = 330 Ω C 28 47pF L4 8.2 μH TO IF AMP INPUT (PIN18) C23 0.1 μF Z = 330 Ω T A P P E D -C N E T W O R K T A P P E D -C N E T W O R K IF LIMITER IN (PIN14) FLT2 R17 220 Ω FROM IF AMP OUT (PIN 16) C22 0.1 μF R16 120 Ω 10.7MHz CERAMIC FILTER C19 47pF C20 47pF L3 8.2 μH C23 .1 μF an1996_6 Fig 6. AN1996 Application note Matching configuration for FLT1 and FLT2 All information provided in this document is subject to legal disclaimers. Rev. 2 — 28 August 2014 © NXP Semiconductors N.V. 2014. All rights reserved. 13 of 25 AN1996 NXP Semiconductors Demodulating at 10.7 MHz IF with the SA605 6. Stabilizing the IF section from regeneration Because the gain in the IF section is 100 dB and the wavelength for 10.7 MHz is small, the hardest design phase of this project was to stabilize the IF section. The steps below show the methods used to obtain a stable layout. 1. The total IF section (IF amp and limiter) gain is 100 dB, which makes it difficult to stabilize the chip at 10.7 MHz. Therefore, a 120 resistor (R16 of Figure 1) was used to kill some of the IF gain to obtain a stable system. Remark: Expect AM rejection performance to degrade as you decrease the IF gain externally. 2. Since the tapped-C inductors for FLT1 and FLT2 are not shielded, it is important not to place them too close to one another. Magnetic coupling will occur and may increase the probability of regeneration. 3. It was also found that if the IF limiter bypass capacitors do not have the same physical ground, the stability worsens. Referring to Figure 1, the IF limiter bypass capacitors (C17, C25) are connected to assure a common ground. 4. The positioning of ground feedthroughs are vital. A designer should put feedthroughs near the IF bypass capacitors ground points. In addition, feedthroughs are needed underneath the chip. Other strategic locations are important for feedthroughs where insufficient grounding occurs. 5. Shielding should be used after the best possible stability is achieved. The SA605 demoboard is stable, so shielding was not used. However, if put into a bigger system, shielding should be used to keep out unwanted RF frequencies. As a special note, if a good shield is used, it can increase the R16 resistor value such that there is less IF gain to kill to achieve stability. This means the RSSI dynamic range is improved. So if a designer does not want to implement the RSSI extender circuit, but is still concerned with SINAD and RSSI range, he/she can experiment with R16 and shielding because there is a correlation between them (see RSSI extender section in this application note for more information). In addition, AM rejection performance will improve due to the greater availability of the total IF gain. The key to stabilizing the IF section is to kill the gain. This was done with a resistor (R16 in Figure 6) to ground. All the other methods mentioned above are secondary compared to this step. Lowering the value of this resistor reduces the gain and the increasing resistor value kills less gain. For our particular layout, 120 was chosen to obtain a stable board, but we were careful not to kill too much gain. One of the downfalls of killing too much gain is that the SINAD reading will become worse and the RSSI dynamic range is reduced. 7. RSSI dynamic range There are two main factors which determine the RSSI dynamic range: • How stable is the board? • How much gain is killed externally? AN1996 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 28 August 2014 © NXP Semiconductors N.V. 2014. All rights reserved. 14 of 25 AN1996 NXP Semiconductors Demodulating at 10.7 MHz IF with the SA605 If the board is unstable, a high RSSI voltage reading will occur at the bottom end of the curve. If too much gain is taken away, the upper half of the curve is flattened. Thus the dynamic range can be affected. Figure 7 shows how the linear range can be decreased under the conditions mentioned above. STRONG RSSI (V) IF GAIN DECREASING STABILITY DECREASING RSSI DYNAMIC RANGE RSSI DYNAMIC RANGE WEAK WEAK RF INPUT (dBm) STRONG an1996_7 Fig 7. RSSI dynamic range It is important to choose the appropriate resistor to kill enough gain to get stability but not too much gain to affect the upper RSSI curve dynamic range. Because we had to kill some IF gain to achieve good board stability and good SINAD readings, our RSSI overall dynamic range was reduced on the upper end of the curve. Because SINAD and the RSSI dynamic range are two important parameters for most of our customers, we decided to add an ‘RSSI extender’ modification to the board to get the best of both worlds. Together with the RSSI external modification and the ‘stability resistor’, we can now achieve excellent SINAD readings and maintain a wide RSSI dynamic range. 8. RSSI extender circuit The RSSI extender circuit increases the upper dynamic range roughly about 20 dB to 30 dB for the 240 MHz demoboard. The SA605 demoboard has 90 dB to 100 dB of linear dynamic range when the RSSI modification is used. Referring to Figure 8, one can see that one transistor is used with a few external components. The IF input signal to the PNP transistor is tapped after the ceramic filter to ensure a clean IF signal. The circuit then senses the strength of the signal and converts it to current, which is then summed together with the RSSI output of the chip. AN1996 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 28 August 2014 © NXP Semiconductors N.V. 2014. All rights reserved. 15 of 25 AN1996 NXP Semiconductors Demodulating at 10.7 MHz IF with the SA605 R18 56kΩ VCC D1 BAS16 R20 5.1kΩ for 240MHz 10kΩ for 45MHz R19 100kΩ INPUT (FROM FLT1 OUTPUT) C30 220pF Q1 BC857B C32 220pF C31 220pF TO RSSI (PIN 7) an1996_8 R18 — provides bias regulation, the gain will stay constant over varying VCC R19 — for biasing, buffer RF DC voltage R20 — provides the DC bias, RSSI gain (when R20 increases, RSSI gain decreases) C30 — DC blocking capacitor that connects the ceramic filter’s output to the PNP transistor’s input C31 — decoupling capacitor C32 — peak detector charge capacitor D1 — diode to prevent improper current flow Q1 — NXP BC857B PNP transistor Fig 8. External RSSI extender circuit The PNP transistor stage has to be biased as a class B amplifier. The circuit provides two functions. It is a DC amplifier and an RF detector. The gain of the RSSI extender can be controlled by R20 and R9 (Gain = R9 / R20). Adjusting R20 is preferable because it controls the upper half of the RSSI curve, whereas adjusting R9 shifts the whole RSSI curve. If a different RF frequency is supplied to the mixer input, it is important to set the external RSSI gain accordingly. When the RF input was changed from 240 MHz to 45 MHz, the conversion gain of the mixer increased. Therefore, the earlier gain settings for the RSSI extender was too much. A lower gain setting had to be implemented such that a smoother transition would occur. 9. Quad tank The quad tank is tuned for 10.7 MHz ( f = 1 2 LC ). Figure 1 shows the values used (C14,C15, C16, IFT1) and Figure 9 shows the S-curve. The linear portion of the S-curve is roughly 200 kHz. Therefore, it is a good circuit for a total deviation of 140 kHz. It is possible to deviate at 200 kHz, but this does not leave much room for part tolerances. AN1996 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 28 August 2014 © NXP Semiconductors N.V. 2014. All rights reserved. 16 of 25 AN1996 NXP Semiconductors Demodulating at 10.7 MHz IF with the SA605 4 AUDIO (V) 3 2 1 0 10.0 10.5 10.7 11.0 11.5 IF IN (MHz) an1996_9 Fig 9. 10.7 MHz quad tank S-curve If more deviation is needed, a designer can lower the S-curve with a parallel resistor connected to the quadrature tank. A designer should play with different value resistors and plot the S-curve to pick the best value for the design. To key in on the resistor value with minimum effort, a designer can put a potentiometer in parallel with the quad tank and tune it for best distortion. Then the designer can use fixed value resistors that are close to the potentiometer’s value. Fixed quad tank component values can be used to eliminate tuning, but a designer must allow for part tolerances and temperature considerations. For better performance over temperature, a resonator/discriminator can be used. Thus, no tuning is required for the quad tank section, which will save on production costs. 10. RSSI system speed The RSSI rise and fall times are important in applications that use pulsed RF in their design. The way we define the speed is how fast the RSSI voltage can travel up and down the RSSI curve. Figure 10 shows a representation of this. Five different pulsed RF levels were tested to get a good representation of the RSSI speed. One can predict that the stronger the pulsed signal, the higher the RSSI voltage and the longer it will take for the fall time to occur. Generally speaking, the rise time is determined by how long it takes to charge up an internal capacitor. The fall time depends on how long it takes to discharge this capacitor. AN1996 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 28 August 2014 © NXP Semiconductors N.V. 2014. All rights reserved. 17 of 25 AN1996 NXP Semiconductors RSSI (V) Demodulating at 10.7 MHz IF with the SA605 tR tF - 80 - 60 - 40 - 20 0 RF LEVEL (dBm) an1996_10 Fig 10. RSSI curve with pulsed RF levels It is also important to understand that there are two types of RSSI speeds. The first type is the RSSI chip speed and the second is the RSSI system speed. The RSSI chip speed will be faster than the system speed. The bandwidth of the external filters and other external parts can slow down the RSSI system speed dramatically. Figure 11 shows the bench setup for the RSSI system speed measurements. The pulsed RF was set for 10 kHz and the RSSI output was monitored with a digital oscilloscope. Figure 12 shows how the rise and fall times were measured on the oscilloscope. PHILIPS SIGNAL GENERATOR 10kHz SQ. WAVE HP8640B SIG GEN “RF SOURCE” HP8640B SIG GEN “LO SOURCE” HP PRINTER RF LEVEL: --20dBm FREQ: 240MHz DEV: ±70kHz MOD: Pulsed HP 54503 DIGITAL O’SCOPE PULSED RF RF LEVEL: --10dBm FREQ: 229.3MHz DEV: Off RF IN HP6216A RSSI OUT POWER SUPPLY LO IN VCC DUT: 605 or 625 an1996_11 Fig 11. RSSI speed setup AN1996 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 28 August 2014 © NXP Semiconductors N.V. 2014. All rights reserved. 18 of 25 AN1996 NXP Semiconductors Demodulating at 10.7 MHz IF with the SA605 AMPLITUDE (V) RF PULSED INPUT TO THE 625 625 RSSI OUTPUT RESPONSE tR tF TIME ( μs) an1996_12 Fig 12. Oscilloscope display of RSSI system rise and fall times The modifications done on the SA605 board are shown in Figure 13. The RSSI caps C11 and C31 were eliminated, and the RSSI resistor values were changed. We wanted to see how much time was saved by using a smaller RSSI resistor value. C31 FLT1 FLT2 SA605 RSSI 1 RF INPUT @ 240MHz: 0dBm --20dBm --40dBm --60dBm --80dBm 7 R9 = 10kΩ, C11 51kΩ, or 100k Ω an1996_13 Fig 13. SA605 RSSI test circuit configuration The RSSI system speed for the 240 MHz SA605 demo board is shown in Figure 14. Again, the only modification was that the RSSI caps (C11 and C31) were taken out and the RSSI resistor value (R9) was varied. For different RF levels, the speed seems to vary slightly, but this is expected. The higher the RSSI voltage, the longer it will take to come back down the RSSI curve for the fall time. AN1996 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 28 August 2014 © NXP Semiconductors N.V. 2014. All rights reserved. 19 of 25 AN1996 NXP Semiconductors Demodulating at 10.7 MHz IF with the SA605 40 C11 and C31 REMOVED FROM CIRCUIT RSSI = 10k Ω RISE RSSI = 10k Ω FALL RSSI = 51k Ω RISE 30 TIME (microseconds) RSSI = 51k Ω FALL RSSI = 100k Ω RISE RSSI = 100k Ω FALL 20 10 0 0 - 20 - 40 RF INPUT (dBm) - 60 - 80 an1996_14 Fig 14. RSSI systems rise and fall time with different RSSI resistor values Looking more closely at Figure 14, one can note that the 0 dBm input level has a faster fall time than the 20 dBm level. This occurs because of the limited dynamic range of the test equipment. The equipment does not have sufficient on/off range, so at 0 dBm the ‘off’ mode is actually still on. Therefore, you do not get a true reading. At 0 dBm the RSSI voltage is lower than 20 dBm. The reason why this happens is because the RSSI linearity range stops at 10 dBm. When the RF input drive is too high (for example, 0 dBm), the mixer conversion gain decreases, which causes the RSSI voltage to drop. AN1996 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 28 August 2014 © NXP Semiconductors N.V. 2014. All rights reserved. 20 of 25 AN1996 NXP Semiconductors Demodulating at 10.7 MHz IF with the SA605 11. Questions and answers Question: What should the audio level at Pin 8 be? Answer: The audio level is at 580 mV (peak-to-peak) looking directly at the audio output pin and does not include a C-message filter. However, the audio output level will depend on two factors: the ‘Q’ of the quadrature tank and the deviation used. The higher the quad tank’s ‘Q’, the larger the audio level. Additionally, the more deviation applied, the larger the audio output. But the audio output will be limited to a certain point. Question: Am I required to use the 10 F supply capacitor? Answer: No, a smaller value can be used. The 10 F capacitor is a suggested value for evaluation purposes. Most of the time a power supply is used to evaluate our demo boards. If the supply is noisy, it will degrade the receiver performance. We have found that a lower value capacitor can be used when the receiver is powered by a battery. But it is probably safer to stay at a reasonable capacitor size. Question: Can I use different IF filters for my required bandwidth specifications? Answer: Yes, you can order different IF filters with different bandwidths. Some of the standard manufacturers have 180 kHz, 230 kHz, and 280 kHz bandwidths for 10.7 MHz ceramic filters. Just be sure that the quad tank ‘S-curve’ is linear for your required bandwidth. The SA605 demoboard has a 200 kHz linearity for the quad tank. So 70 kHz deviation is perfect. We have also found that even though the IF filter’s bandwidth might be more than our requirements, it does not really degrade overall receiver performance. But to follow good engineering practices, a designer should order filters that are closest to their requirements. Going with wider bandwidth filters will give you better RSSI system speed. Question: I want to use part of your demo board for my digital receiver project. Can you recommend a good 10.7 MHz filter with accurate 10.7 MHz center frequency which can provide minimum phase delay? Answer: At the present time, I only know of one manufacturer that is working on a filter to meet digital receiver requirements. Murata has a surface mount 10.7 MHz filter. The number is FX-6502 (SFECA 10.7). It was specifically designed for Japanese digital cordless phones. You can adapt these filters to our SA605 demoboard. We also used these filters in our layout and got similar SINAD and RSSI system speed performance compared to the standard 10.7 MHz filters (280 kHz bandwidth). I believe the difference between the filters will be apparent for digital demodulation schemes. AN1996 Application note All information provided in this document is subject to legal disclaimers. Rev. 2 — 28 August 2014 © NXP Semiconductors N.V. 2014. All rights reserved. 21 of 25 AN1996 NXP Semiconductors Demodulating at 10.7 MHz IF with the SA605 Question: Why does the AM rejection performance look better on the SA605, 455 kHz IF board than the SA605 10.7 MHz IF demoboard? Answer: For the 455 kHz IF demoboard there is more IF gain available compared to the 10.7 MHz IF board. Recall that for the 10.7 MHz IF board, some of the IF gain was killed externally for stability reasons. Since the IF gain helps improve AM rejection performance, by killing IF gain, AM rejection is decreased. Question: The SA605 10.7 MHz IF demoboard is made for the SO package. Can I use your SSOP package and expect the same level of performance? Answer: We have not done a SSOP layout yet. But if the same techniques are used, I am sure the SSOP package will work. The SA636DK demoboard is available in SSOP package. Question: I tried to duplicate your RSSI system reading measurements using your demoboard and I get slower times. What am I doing wrong? Answer: The RSSI system speed measurements are very tricky. Make sure your cable lengths are not too long. I have found that when making microsecond measurements, lab setup is of utmost importance. Also, make sure the RSSI caps (C11 and C31) are removed from the circuit. Also be sure that the bandwidth of your IF filters is not slowing down the RSSI system speed (Cf: section on RSSI system speed). Question: I am going to use your design in my NTT cordless digital phone. Can you recommend a 240.05 MHz filter? Answer: Murata SF2055A 240.050 MHz SAW filter is a filter you can use for your application. 12. Abbreviations Table 3. AN1996 Application note Abbreviations Acronym Description AM Amplitude Modulation FM Frequency Modulation IF Intermediate Frequency LC inductor-capacitor network LO Local Oscillator NTT Nippon Telegraph and Telephone PNP bipolar transistor with P-type emitter and collector and an N-type base RF Radio Frequency RSSI Received Signal Strength Indicator SINAD Signal-to-Noise And Distortion ratio All information provided in this document is subject to legal disclaimers. Rev. 2 — 28 August 2014 © NXP Semiconductors N.V. 2014. All rights reserved. 22 of 25 AN1996 NXP Semiconductors Demodulating at 10.7 MHz IF with the SA605 13. References AN1996 Application note [1] SA605, “High performance low power mixer FM IF system” — Product data sheet; NXP Semiconductors; www.nxp.com/documents/data_sheet/SA605.pdf [2] AN1994, “Reviewing key areas when designing with the SA605” — Application note; NXP Semiconductors; www.nxp.com/documents/application_note/AN1994.pdf [3] AN1995, “Evaluating the SA605 SO and SSOP demoboard” — Application note; NXP Semiconductors; www.nxp.com/documents/application_note/AN1995.pdf All information provided in this document is subject to legal disclaimers. Rev. 2 — 28 August 2014 © NXP Semiconductors N.V. 2014. All rights reserved. 23 of 25 AN1996 NXP Semiconductors Demodulating at 10.7 MHz IF with the SA605 14. Legal information 14.1 Definitions Draft — The document is a draft version only. The content is still under internal review and subject to formal approval, which may result in modifications or additions. NXP Semiconductors does not give any representations or warranties as to the accuracy or completeness of information included herein and shall have no liability for the consequences of use of such information. 14.2 Disclaimers Limited warranty and liability — Information in this document is believed to be accurate and reliable. However, NXP Semiconductors does not give any representations or warranties, expressed or implied, as to the accuracy or completeness of such information and shall have no liability for the consequences of use of such information. NXP Semiconductors takes no responsibility for the content in this document if provided by an information source outside of NXP Semiconductors. In no event shall NXP Semiconductors be liable for any indirect, incidental, punitive, special or consequential damages (including - without limitation - lost profits, lost savings, business interruption, costs related to the removal or replacement of any products or rework charges) whether or not such damages are based on tort (including negligence), warranty, breach of contract or any other legal theory. Notwithstanding any damages that customer might incur for any reason whatsoever, NXP Semiconductors’ aggregate and cumulative liability towards customer for the products described herein shall be limited in accordance with the Terms and conditions of commercial sale of NXP Semiconductors. Right to make changes — NXP Semiconductors reserves the right to make changes to information published in this document, including without limitation specifications and product descriptions, at any time and without notice. This document supersedes and replaces all information supplied prior to the publication hereof. Suitability for use — NXP Semiconductors products are not designed, authorized or warranted to be suitable for use in life support, life-critical or safety-critical systems or equipment, nor in applications where failure or malfunction of an NXP Semiconductors product can reasonably be expected AN1996 Application note to result in personal injury, death or severe property or environmental damage. NXP Semiconductors and its suppliers accept no liability for inclusion and/or use of NXP Semiconductors products in such equipment or applications and therefore such inclusion and/or use is at the customer’s own risk. Applications — Applications that are described herein for any of these products are for illustrative purposes only. NXP Semiconductors makes no representation or warranty that such applications will be suitable for the specified use without further testing or modification. Customers are responsible for the design and operation of their applications and products using NXP Semiconductors products, and NXP Semiconductors accepts no liability for any assistance with applications or customer product design. It is customer’s sole responsibility to determine whether the NXP Semiconductors product is suitable and fit for the customer’s applications and products planned, as well as for the planned application and use of customer’s third party customer(s). Customers should provide appropriate design and operating safeguards to minimize the risks associated with their applications and products. NXP Semiconductors does not accept any liability related to any default, damage, costs or problem which is based on any weakness or default in the customer’s applications or products, or the application or use by customer’s third party customer(s). Customer is responsible for doing all necessary testing for the customer’s applications and products using NXP Semiconductors products in order to avoid a default of the applications and the products or of the application or use by customer’s third party customer(s). NXP does not accept any liability in this respect. Export control — This document as well as the item(s) described herein may be subject to export control regulations. Export might require a prior authorization from competent authorities. Translations — A non-English (translated) version of a document is for reference only. The English version shall prevail in case of any discrepancy between the translated and English versions. 14.3 Trademarks Notice: All referenced brands, product names, service names and trademarks are the property of their respective owners. All information provided in this document is subject to legal disclaimers. Rev. 2 — 28 August 2014 © NXP Semiconductors N.V. 2014. All rights reserved. 24 of 25 AN1996 NXP Semiconductors Demodulating at 10.7 MHz IF with the SA605 15. Contents 1 1.1 1.2 2 3 4 5 6 7 8 9 10 11 12 13 14 14.1 14.2 14.3 15 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Objective. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Board setup and performance graphs. . . . . . . 6 RF input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 LO input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 10.7 MHz ceramic filters . . . . . . . . . . . . . . . . . 12 Stabilizing the IF section from regeneration . 14 RSSI dynamic range . . . . . . . . . . . . . . . . . . . . 14 RSSI extender circuit. . . . . . . . . . . . . . . . . . . . 15 Quad tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 RSSI system speed . . . . . . . . . . . . . . . . . . . . . 17 Questions and answers . . . . . . . . . . . . . . . . . 21 Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . 22 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Legal information. . . . . . . . . . . . . . . . . . . . . . . 24 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Disclaimers . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Trademarks. . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Please be aware that important notices concerning this document and the product(s) described herein, have been included in section ‘Legal information’. © NXP Semiconductors N.V. 2014. All rights reserved. For more information, please visit: http://www.nxp.com For sales office addresses, please send an email to: [email protected] Date of release: 28 August 2014 Document identifier: AN1996