Page 331 - Complete Wireless Design

Mixer Design

330 Chapter Seven

minimize LO drive power, and output a relatively clean IF signal from the
mixer’s output port:
1. Z and Z are microstrip transmission lines, and will equal 50 ohms.
1 9
2. C will block DC but pass the desired RF frequency with less than 1
1
ohm X .
c
3. Z and Z provide the proper input impedance match at the RF frequency
2 3
for the GaAs FET device.
4. Z acts as an RFC to the desired RF frequency, while Z functions as a
4 5
capacitor. They form bias decoupling for the negative V supply.
CC
5. R functions as a low-frequency termination to maintain mixer stability.
1
Values between 10 to 50 ohms should suffice.
6. C is used to bypass the IF away from the RF port.
2
7. V should be adjusted from 5 to 1 V for best mixer operation.
G
8. A JFET must be selected that can operate at a frequency far above the
expected RF input frequency.
9. Z and Z will match the S of the FET at the LO frequency.
6 7 22
10. Z functions as an RFC to attenuate the LO from entering the bias supply
8
( V ) or the IF output port, but allows the DC and IF to pass unhindered.
D
C passes the LO to ground, and acts as the RF ground for Z .
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11. C bypasses the IF to ground to decouple from V .
5 D
12. V should initially be set to 5 V and then decreased for optimum per-
D
formance.
13. L and C are chosen to match the IF frequency to the FET’s output, while
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low-pass filtering the IF output for increased isolation.
14. C is a DC block, but passes the IF with little attenuation. Could be series
7
resonant to attenuate other frequencies besides the IF.
15. C is a DC block, and should be chosen to operate at its series resonant fre-
3
quency at the LO to assist in blocking the undesired IF, while increasing
port isolation.

Integrated circuit double-balanced mixer (Fig. 7.17). Active mixers, such as the
HP IAM-82028, are useful in non-noise-sensitive applications that require a
low LO input power (0 dBm) and load-insensitive performance. The IAM-
82028 Gilbert cell–based mixer operates with a flat RF-to-IF conversion gain
of 15 dB over a wide RF input range of 0.05 to 5 GHz. This mixer also enjoys
an IF output capability of DC to 2 GHz, an output P1dB of 12 dBm (which
depends on V : 7 V yields 2 P1dBm; 12 V yields 12 P1dBm), and
CC CC CC
will function with a V of between 7 and 13 V. To design, simply add the cou-
CC
pling/decoupling components as shown, supply the proper grounds and V ,
CC
and the mixer is complete.

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