design ideas Fast Time Division Duplex (TDD) Transmission Using an Upconverting Mixer with a High Side Switch Vladimir Dvorkin Many wireless infrastructure time division duplex (TDD) transmit applications require fast on/off switching of the transmitter, typically within one to five microseconds. There are several different ways to implement fast Tx on/ off switching, including the use of RF switches in the signal path, or on/off switching of the supply voltage for different stages of the transmitter chain. The advantages of the latter method are low cost, very good performance and power saving during the Tx off-time. In particular, a good place to apply supply switching is at the transmit upconverting mixer because this removes both the transmit signal and all other mixing products from the mixer RF output. The LT5579 high performance upconverting mixer fits various TDD and Burst Mode transmitter applications with output frequencies up to 3.8GHz. Fast on/off supply voltage (VCC) switching for the LT5579 is as simple as adding an external high side power supply switch (note that this technique is equally effective for the lower frequency upconverting mixer, LT5578). HIGH SIDE V CC SWITCH FOR A BURST MODE TRANSMITTER USING THE LT5579 MIXER The high side VCC switch circuit in Figure 1 uses a P-channel MOSFET (IRLML6401) with an RDS(ON) of less than 0.1Ω. An N-channel enhancement mode FET (2N7002), connected from the drain of IRLML6401 to ground, further improves fall time. 1.0µF ON STATE VCC = 3.29V VCC = 3.3V 11Ω 33pF HIGH SIDE VCC SWITCH The input driver for the high side VCC switch is a high speed CMOS inverter (MC74HC1G04) capable of driving capacitive loads. The IRLML6401 input capacitance is typically 830pF and the 2N7002 input capacitance is under 50pF. For faster rise times, two high speed CMOS drivers can be used in parallel. Likewise, for faster fall times, a different N-channel MOSFET with lower on-resistance can be used. With the LT5579 supply current of 220mA, the power supply voltage drop across the MOSFET is only 11mV. The response time of the high side VCC switch is shown in Figure 2. Total turn-on time is only 650ns and total turn-off time is 500ns. These measurements were performed using two RF bypass capacitors at the mixer VCC pin (33pF and 270pF). Higher value RF bypass (continued on page 27) 270pF 33pF 39nH S IRLML6401 Tx IF INPUT 240MHz G D D VCC ON/OFF CONTROL The 2N7002’s RDS(ON) is less than 4Ω, which is sufficient for this application. 4:1 33pF Tr-r 33pF IF – 2N7002 11Ω INPUT CONTROL SIGNAL FOR HIGH SIDE VCC SWITCH RF OUTPUT 2140MHz LT5579 LO+ LO– LT5579 RF OUTPUT GND S LO INPUT Figure 1. Upconverting mixer with high side VCC switch RF HIGH SIDE VCC SWITCH OUTPUT 39nH MC74HC1G04 G VCC IF + 1µs/DIV Figure 2. VCC turn-on and turn-off waveforms July 2010 : LT Journal of Analog Innovation | 25 design ideas The LTC2393-16 with its fully differential inputs can improve SNR by as much as 6dB over conventional differential input ADCs. This ADC is well suited for applications that require low distortion and a large dynamic range. VCM AIN+ C3 R1 10µF 1k R3 249Ω 0V TO 4.096V AIN– IN+ C1 0.002µF NPO C4 10µF 0V TO 4.096V R5 49.9Ω R2 1k R4 249Ω VCM R6 49.9Ω LTC2393-16 IN– VCM VCM C2 10µF Figure 3. AC-coupled differential input PCB LAYOUT The circuits shown are quite simple in concept. However, when dealing with a high speed 16-bit ADC, PC board layout must also be considered. Always use a ground plane. Keep traces as short as possible. If a long trace is required for a bias node such as VCM, use additional bypass capacitors for each component attached to the node and make the trace as wide as possible. Keep bypass capacitors as close to the supply pins as possible. Each bypass capacitor should have its own low impedance Figure 4. Sample layout for LTC2393-16 return to ground. The analog input traces should be screened by ground. The layout involving the analog inputs should be as symmetrical as possible so that parasitic elements cancel each other out. Figure 4 shows a sample layout for the LTC2393-16. Figure 4 is a composite of the top metal, ground plane and silkscreen layers. See the DC1500A Quick Start Guide at www.linear.com for a complete LTC2393-16 layout example. CONCLUSION The LTC2393-16 with its fully differential inputs can improve SNR by as much as 6dB over conventional differential input ADCs. This ADC is well suited for applications that require low distortion and a large dynamic range. Realizing the potential low noise, low distortion performance of the LTC2393-16 requires combining simple driver circuits with proper component selection and good layout practices. n (LT5579 continued from page 25) capacitors can be used, which would result in correspondingly slower rise/fall times. The LT5579 upconverting mixer circuit shown in Figure 1 was optimized and tested at an RF output frequency of 2140MHz. The RF output envelope in Figure 2 shows a dip about 300ns after the VCC switch turns on, followed by another, smaller dip at about the 500ns point. Both dips represent the mixer’s internal feedback circuit reaction to the ramping supply voltage. LO leakage to the RF output of the LT5579 was measured at –40dBm when VCC is on and –46dBm for VCC off. The LO port of the LT5579 is internally matched and has a return loss of 10dB to 18dB over a frequency range of 1100MHz to 3200MHz. When the LT5579 mixer is in the off state, the return loss of the LO port is about 3dB to 5dB across the same frequency range of 1100MHz to 3200MHz. It is advisable to use an LO injection VCO with a buffered output for better reverse isolation, and to avoid any VCO pulling while the LO port impedance changes when switching between the on and off states. CONCLUSION LT5579 and LT5578 mixers without an ENABLE pin can be used in TDD applications with external VCC switching. Using only three parts (IRLML6401, 2N7002 and an MC74HC1G04), a high performance high side VCC switch allows turn-on and turn-off in under 1µs. n July 2010 : LT Journal of Analog Innovation | 27