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RF SIGNAL INPUT
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A single-ended active JFET mixer circuit
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FIGURE 714 A dual-gate MOSFET mixer circuit
transformer s primary The IF is then removed from the transformer s secondary and sent on to the IF strip for further amplification and filtering Another low-cost active mixer is the single-ended transistor type of Fig 715 Both the signal and the LO are inserted into the base and mixed together by the nonlinear Class AB biased transistor Obviously, unless a diplexer is placed at the input, the RF and LO have no real isolation between their ports The original RF signal and the LO frequency, as well as all mixing products, are present at the transistor s collector but, due to the
RF INPUT R2 RE C1
One method of using a transistor as a nonlinear mixer stage
FIGURE 716 A simpli ed depiction of a Gilbert cell integrated mixer
primary and secondary tuned tank circuits, only the desired IF frequency will be of any significant amplitude Gilbert cell mixers, the most advanced of the active types, are commonly available up to RF frequencies of 58 GHz, with IF frequencies of 2 GHz, and are double balanced A popular Gilbert cell mixer is shown in Fig 716 The RF signal is inserted into the base of Q1 of the modified emitter-coupled amplifier (comprised of Q1 and Q2), while the square wave LO is input into the base of Q3 The LO causes Q3 and Q6 to be on, while next Q4 and Q5 transistors are turned on, causing mixing of the LO and the RF, creating the IF signal When Gilbert cell IC mixers are operated at low frequencies, a square wave local oscillator must be used to decrease the steadily rising NF of the mixer, caused by the longer off times of the quad transistor s (Q3, Q4, Q5, and Q6) A square wave minimizes these off times Many active mixers can be operated past their maximum rated frequency, but there may then be a conversion loss instead of a conversion gain Depending on design, these mixers are also available for both balanced and unbalanced operation, and can be found in low-cost surface-mount packages
723 Active Mixer Design
Presented below are some low-cost discrete active mixer designs for consumer applications However, the use of modern Gilbert cell integrated mixers is sometimes the quickest and easiest route to take in today s fast-paced wireless market
Dual-Gate Single-Ended Narrowband MOSFET Mixer for up to 250 MHz (Fig 717)
This is a simple, inexpensive mixer that is a good choice for lower frequency RF applications in consumer wireless equipment If the RF frequency is not exceptionally high, this MOSFET mixer will supply conversion gain It does not require the high levels of LO drive that the passive mixers do, but has much lower intermodulation performance The output port should be diplexed or padded to decrease IMD products Stage gain will vary with LO amplitude and terminating impedances
MATCH 50 TO G1 OF MOSFET AT RF IF MATCH RO TO 50
RF INPUT 50 RFC
L C C
IF OUTPUT 50
RG2 C LO RS LO INPUT (VLO IN) 50 CC
A narrowband active dual-gate MOSFET single-ended mixer design
1 Select an RF dual-gate N-channel E-MOSFET that has plenty of gain at and significantly above the highest RF frequencies of interest 2 CLO = CC/10 (if LO not buffered) 3 RG2 = 100 k 4 RS = 560 5 CC = < 1 6 VLO 6 Vpp 7 Vdd = 12 V 8 RD = 2 k to 56 k (RD pulls the MOSFET s drain down to the value of RD, rather than to that of the MOSFET s low-frequency high-output impedance This use of RD helps IMD levels, but since ZOUT drops as the frequency increases, RD is not required at the higher frequencies) 9 Match the input and output of the MOSFET to 50 using S-parameters This MOSFET mixer should exhibit the following specifications: VG2 = 1 V (supplied by self bias to MOSFET) P1dB 1 dBm (output) TOIP 17 dBm (output) RFIN < 12 dBm (for decreased IMD levels) GAIN 12 dB MAG ( +10 dB) LO-to-RF isolation 30 dB NF 8 to 10 dB