Page 328 - Complete Wireless Design

Mixer Design

Mixer Design 327

7.2.3 Designing active mixers
Presented below are some low-cost discrete active mixer designs, but the IC
mixer is normally the quickest and easiest route to take in today’s fast-paced
wireless market.

Dual-gate single-ended narrowband MOSFET mixer (Fig. 7.14). This design will
function up to 200 MHz with the proper active and passive components. The
output can be diplexed or padded to slightly decrease IMD products. Stage
gain will vary with LO amplitude and terminating impedances. The design
proceeds as follows:

1. C C /10 (if LO is not buffered).
LO C
2. R 100 kilohm.
G2
3. R 560 ohm.
S
4. C 1 ohm (X ).
C C
5. V ≈ 6 V peak to peak.
LO
6. V 12 V.
DD
7. R 2 to 5.6 kilohms. (R pulls the MOSFET’s drain down to the value of
D D
R , rather than to that of the MOSFET’s low-frequency, high-output imped-
D
ance. This use of R helps IMD levels, but is not required at the higher fre-
D
quencies, since Z drops as the frequency increases.)
OUT
8. Match the input and output of the MOSFET to 50 ohms using S parameters.
9. The mixer should exhibit the following specifications:
V 1 V (supplied by self-bias to MOSFET)
G2
P1dB ≈ 1 dBm (output)
TOIP ≈ 17 dBm (output)
RF 12 dBm (for decreased IMD levels)
IN
GAIN ≈ 12 dB MAG (≈ 10 dB)
LO-to-RF isolation ≈ 30 dB
NF ≈ 8 to 10 dB

Dual-gate MOSFET single-ended mixer (Fig. 7.15). This mixer is good for unde-
manding applications up to approximately 400 MHz. The mixer’s output can be
diplexed or padded to slightly decrease IMDs. Voltage gain of the mixer varies
with LO amplitude and terminating impedances, but should be around 10.

1. Select an RF dual-gate N-channel E-MOSFET operational at the highest
frequencies of interest.

2. C and C 1/(6.28 f).
B C
1
3. C
T 2 2
4[f ( L )]
IF 1
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