LMX2240 Intermediate Frequency Receiver General Description Features The LMX2240 is a monolithic, integrated intermediate frequency receiver suitable for use in Digital European Cordless Telecommunications (DECT) systems as well as other mobile telephony and wireless communications applications. It is fabricated using National’s ABiCTM IV BiCMOS process (fT e 15 GHz). The LMX2240 consists of a high gain limiting amplifier, a frequency discriminator, and a received signal strength indicator (RSSI). The high gain limiting amplifier and discriminator operate in the 40 MHz to 150 MHz frequency range, and the limiter has approximately 70 dB of gain. The use of the limiter and the discriminator provides a low cost, high performance demodulator for communications systems. The RSSI output can be used for channel quality monitoring. The LMX2240 is intended to support single conversion receivers. This device saves power, size, and cost by eliminating the second local oscillator (LO), second converter (mixer), and additional filters. The LMX2240 is recommended for systems with channel bandwidths of 300 kHz to 2.5 MHz. The LMX2240 is available in a 16-pin JEDEC surface mount plastic package. Y Y Y Y Y Y Y Y Typical operation at 110 MHz RF sensitivity to b75 dBm; RSSI sensitivity to b 82 dBm High gain (70 dB) limiting amplifier Average current consumption: 480 mA for DECT handset (burst mode) a 3V operation Power down mode for increased current savings Part of a complete receiver solution with the LMX2216 LNA/Mixer, the LMX2315/20 Phase-locked Loop, and the LMX2411 Baseband Processor Compliant to ARi1TM specification Applications Y Y Y Y Y Digital European Cordless Telecommunications (DECT) Portable wireless communications (PCS/PCN, cordless) Wireless local area networks (WLANs) Digital cellular telephone systems Other wireless communications systems Functional Block Diagram TL/W/11755 – 1 ABiCTM and ARi1TM are trademarks of National Semiconductor Corporation. C1995 National Semiconductor Corporation TL/W/11755 RRD-B30M115/Printed in U. S. A. LMX2240 Intermediate Frequency Receiver April 1995 Connection Diagram Small Outline Package TL/W/11755 – 2 Top View Order Number LMX2240M See NS Package Number M16A Pin Description Pin No. Pin Name I/O 1 PD I Power Down; a HIGH signal switches the part to power down mode. 2 RSSI Out O Voltage output of the received signal strength indicator (RSSI). 3 NC 4 GND 5 GND 6 MID O 7 Demod Out O 8 VCC (Mixer) Description No connection Ground Ground Mid-range output of the discriminator; can be used for comparator threshold. Demodulated output of the discriminator. Source voltage for the mixer (discriminator). 9 VCC (Lim.) 10 Quad In I Source voltage for the limiter. 11 Lim. Out O 12 GND Ground 13 GND Ground 14 Comp. Compensation pin for the limiter. See Applications Information for capacitor value. 15 Comp. Compensation pin for the limiter. See Applications Information for capacitor value. 16 IF In I Quadrature input. A DC path from source through an inductor must be present at this pin, but, there must be no series resistance (a parallel resistor to the inductor is acceptable). Limiter output to the quadrature tank. IF input to the limiter. 2 Absolute Maximum Ratings Recommended Operating Conditions If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/Distributors for availability and specifications. Power Supply Voltage (VCC) Storage Temperature Range (TS) Lead Temperature (TL) (Soldering, 10 seconds) Min Supply Voltage (VCC) 3V Operating Temperature (TA) 6.5V b 65§ C to a 150§ C Max Units 2.85 3.15 V b 10 a 70 §C a 260§ C Electrical Characteristics The following specifications apply for supply voltage VCC e a 3V g 5%, fIN e 120 MHz, and TA e 25§ C unless otherwise specified Symbol Parameter IDD Supply Current IPD Power Down Current fmax Maximum IF Input Frequency fmin Minimum IF Input Frequency Conditions Min 120 Value Typ Max Units 8 10 mA 115 200 mA 150 MHz 10 MHz IF LIMITER NF IF Limiter Noise Figure 11.5 12.5 dB AV Limiter Gain ZL e 1000X 70 dB sens Limiter/Disc. Sensitivity BER e 0.001 b 75 dBm IFin IF Limiter Input Impedance IFout IF Limiter Output Impedance Vmax Maximum Input Voltage Level Vout Output Swing Lim Input Limiting Point 150 350 225 X 250 X 500 mVPP 500 VPP b 70 dBm 1.2 VPP DISCRIMINATOR Vout Discriminator Output Peak-to-Peak Voltage (Note 1) See Test Circuit 1.0 VOS Disc. Output DC Voltage (Pin 7) 1.4 1.7 V MID Mid-Range Output (Pin 6) 1.4 1.7 V DISCin Disc. Input Impedance 1000 X DISCout Disc. Output Impedance 150 X 70 dB RSSI RSSI RSSIout RSSI Dynamic Range RSSI Output Voltage RSSI Slope Pin e b80 dBm 0.35 0.5 0.8 Pin e 0 dBm 1.15 1.5 1.8 11 16 mV/dB 3 dB Pin e b70 dBm to b20 dBm RSSI Linearity V V Note 1: The discriminator output peak-to-peak voltage is measured by operating the discriminator mixer with two separate inputs (i.e., as a mixer). A beat frequency of 1 kHz is generated, and this tone’s output swing is guaranteed to be at least 1.0 VPP. When the mixer is configured as a discriminator with the limiter and a tank circuit, the guaranteed 1.0 VPP output translates to (1.0V *(36/180) e ) 200 mVPP demodulated output, assuming at least 36§ phase shift across the band of interest from the tank circuit. 3 Typical Application Block Diagram TL/W/11755 – 3 Functional Description THE RECEIVED SIGNAL STRENGTH INDICATOR (RSSI) The RSSI circuit has a range of 70 dB. Its output voltage is proportional to the logarithm of the input signal level. The RSSI circuit has a sensitivity of b82 dBm. The output voltage of the circuit ranges from 0.5V to 1.5V typically. OVERVIEW The LMX2240 IF demodulator is a low power IF processor that includes a frequency discriminator, an IF hard limiting amplifier, and a received signal strength indicator (RSSI). The LMX2240 is capable of differentially demodulating an FM or AM signal with as high an IF as 150 MHz, avoiding a costly second down-conversion. The RSSI output can be used for time gated channel measurements required in TDMA and other systems. Other features include high receiver sensitivity and a power down mode to allow for standby operation. THE FREQUENCY DISCRIMINATOR The frequency discriminator is a Gilbert cell mixer that requires an external tank circuit to create a 90§ phase shift at the desired frequency. The output of this circuit is centered at 1.5V by an internal level shifting circuit, and a mid-range voltage (at 1.5V) is also provided. The sensitivity of the discriminator to phase inaccuracies is 5.5 mV/degree (see Applications Information). This means that for a phase imbalance of 10§ , the received eye diagram will be shifted by about 55 mV off of the 1.5V mid-range voltage. For the typical case, this amounts to about 10% of the output eye diagram (for 400 mVPP output). THE LIMITING AMPLIFIER The limiting amplifier has a typical gain of 70 dB and a sensitivity of about b75 dBm. This allows it to be used in the DECT system with 20 dB net RF gain in front of it to achieve a sensitivity of b95 dBm. The limiter is a five stage amplifier with internal compensation at each stage to ensure stability. Two external compensation capacitors are also required to further enhance stability. The input to the limiter is a relatively low impedance to allow easy matching to typical IF surface acoustic wave (SAW) filters. The output of the limiter is connected off chip to an external quadrature tank circuit as well as connected internally to the discriminator (mixer). The output impedance of the limiter is 250X (typical). 4 Typical Performance Characteristics Limiter Gain vs Frequency with Temperature as a Parameter TL/W/11755 – 4 Limiter Gain vs Frequency with Supply Voltage as a Parameter TL/W/11755 – 5 5 Typical Performance Characteristics (Continued) Current Consumption vs Supply Voltage with Temperature as a Parameter Discriminator Output Peak-to-Peak Voltage vs Supply with Temperature as a Parameter TL/W/11755–6 TL/W/11755 – 7 Mid-Range (Reference) Voltage vs Supply with Temperature as a Parameter Power Down Current vs Temperature TL/W/11755–8 TL/W/11755 – 9 Limiter Output Power vs Frequency with Voltage as a Paramerer Limiter Output Power vs Frequency with Temperature as a Paramerer TL/W/11755–10 TL/W/11755 – 11 6 Typical Performance Characteristics (Continued) RSSI Output vs Input Power with VCC as a Parameter TL/W/11755 – 12 RSSI Output vs Input Power with Temperature as a Parameter TL/W/11755 – 13 7 Automatic Test Circuit TL/W/11755 – 14 C1 e 1000 pF g 10% NPO Ceramic C2 e 1000 pF g 10% NPO Ceramic C3 e 1000 pF g 10% NPO Ceramic R1 e 25X g 5% (/4W Thin Film Carbon R2 e 1 kX g 5% (/4W Thin Film Carbon R3 e 1 kX g 5% (/4W Thin Film Carbon L1 e 10 mH g 5% Air Coil C4 e 1000 pF g 10% NPO Ceramic R4 e 20X g 5% (/4W Thin Film Carbon C5 e 1000 pF g 10% NPO Ceramic R5 e 3.9 kX g 5% (/4W Thin Film Carbon 8 Typical Application Example TL/W/11755 – 15 C1 e C2 e C3 e C5 e C6 e 100 pF g 10% NPO Ceramic C7 e C9 e 0.01 mF g 10% NPO Ceramic C4 e 1 pF g 10% NPO Ceramic C8 e 82 pF g 10% X7R Ceramic R1 e 4 kX g 5% (/4W Thin Film Carbon R2 e 880X g 5% (/4W Thin Film Carbon Tank e Toko Ý638AH-0294 All supporting components 0603 surface mount except tank. 9 Applications Information With a circuit that gives an output peak-to-peak voltage of 1.0 VPP (min) with ideal quadrature, the slope is seen to be 5.5 mV/degree. With a practical quadrature tank circuit at 110.6 MHz, the phase shift over a 1 MHz bandwidth is about 45§ –50§ , which translates to an output peak-to-peak voltage of about 250 mVPP. THE INTERMEDIATE FREQUENCY LIMITER The IF limiter has a large amount of gain at high enough frequency to cause concern about oscillation. To ensure that the limiter does not oscillate, a few precautions should be taken. The compensation capacitors that are used should be chosen to roll off any unwanted frequencies below the band of interest. The capacitor should be a high Q, RF type ceramic chip capacitor. For DECT, the capacitor value should be 100 pF, and the capacitors should be soldered as close to the LMX2240 as possible. This will create a pass band from 40 MHz to 150 MHz. The AC coupling capacitor at the input to the limiter (from the SAW filter) should be the same value as the compensation capacitors. Assume the FM modulated signal is denoted as s(t) e cos (0ct a m(t)) , where m(t) e m # (4) t b(t) dt , b% and b(t) is the modulating baseband signal. The constant m is defined as m e 2DfTb. The signal s(t) must be delayed by some u so that THE DISCRIMINATOR There are two types of discriminator that can be used to demodulate FM signals. The first is a delay line discriminator, which uses a delay in one path of the received signal to introduce a phase difference between it and the received signal. The operation of the delay line discriminator is derived in the inset box. The other type of discriminator relies on a quadrature tank to directly introduce a phase shift in the received signal. This is the type of implementation that is commonly used in mobile communications because of its relative ease of construction and low cost. The discriminator operates best when the inputs to it are hard-limited (i.e., square edges). If the input signal is small enough such that the IF amplifier cannot limit it, the output voltage swing of the limiter will suffer. Typically, the minimum voltage swing the discriminator can see and still fully switch is about 100 mVPP. The two inputs to the discriminator can be of different peak-to-peak voltage swings as long as both are over the lower limit. This allows the quadrature tank circuit to have some insertion loss. In fact, up to 8 dB insertion loss can be tolerated while still ensuring that the discriminator output won’t suffer. The quadrature circuit can also affect the discriminator output voltage swing. The discriminator output voltage swing specified assumes perfect quadrature at the frequency of interest (mixer operation). With available analog components, perfect quadrature is not possible. This is due in part to the high frequency of the IF and the proportionally very narrow bandwidth of the desired signal. For example, a DECT signal is about 1 MHz wide, which is k 1% of the IF at which the demodulation occurs. This makes the quadrature circuit difficult to achieve. With moderately high Q components, however, a reasonable phase shift can be achieved with a single pole tank. This is illustrated by the following equation: the output of the discriminator is given by DISCout e cos(0c t ) # cos(0c t a w), (1) l(t) e s(t a u) e cos (0c(t a u) a m(t a u)) . If the delay u is such that 0ct e 2nq a q , n e 0, 1, 2, 3, . . . , 2 then s(t a u) e sin(0ct a m(t a u)), and multiplying (4) and (7) yields s(t) l(t) e cos (0ct a m(t)) sin (0ct a m(t a u)) 1 e sin (20ct a m(t) a m(t a u)) 2 1 a sin (m(t a u) b m(t)) . 2 (5) (6) (7) (8) The double frequency component can be filtered off with a lowpass filter. If u is kept small, 1 1 [m(t a u) b m(t)] sin (m(t a u) b m(t)) & 2 2 tau m e b(t) dt b b% 2 t m b(t) dt (9) b% 2 a t u m e b(t) dt 2 t m &u b(t) . 2 The object for a delay line, then, is to maximize the delay while retaining the approximations necessary to satisfy (9), u k 0.1 Tb. # # # which results in DISCout e cos(0c t a 0c t a w) a cos(0c t b 0c t b w). (2) When the double frequency component is filtered out with a low pass filter, the cosine of the phase remains DISCout e cos(b w) e cos(w). (3) It can be seen that at 90§ phase shift, the output will be zero. At 0§ , the output will be 0.5, and at 180§ , it will be b0.5. The output swing is then set by the multiplication of the cosine term with the discriminator output amplifier’s gain. 10 11 LMX2240 Intermediate Frequency Receiver Physical Dimensions inches (millimeters) 16-Lead Molded Package (SO) Order Number LMX2240M For Tape and Reel Order Number LMX2240MX NS Package Number M16A LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform, when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. National Semiconductor Corporation 1111 West Bardin Road Arlington, TX 76017 Tel: 1(800) 272-9959 Fax: 1(800) 737-7018 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. National Semiconductor Europe Fax: (a49) 0-180-530 85 86 Email: cnjwge @ tevm2.nsc.com Deutsch Tel: (a49) 0-180-530 85 85 English Tel: (a49) 0-180-532 78 32 Fran3ais Tel: (a49) 0-180-532 93 58 Italiano Tel: (a49) 0-180-534 16 80 National Semiconductor Hong Kong Ltd. 13th Floor, Straight Block, Ocean Centre, 5 Canton Rd. Tsimshatsui, Kowloon Hong Kong Tel: (852) 2737-1600 Fax: (852) 2736-9960 National Semiconductor Japan Ltd. Tel: 81-043-299-2309 Fax: 81-043-299-2408 National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.