Page 315 - Complete Wireless Design

Mixer Design

314 Chapter Seven

There are three basic classifications for both active and passive mixers.
Briefly:
Unbalanced mixers have an IF output consisting of f , f , f f , f f ,
S LO S LO S LO
and other spurious outputs. They will also exhibit little isolation between
each of the mixer’s three ports, resulting in undesired signal interactions
and feedthrough to another port.
Single-balanced mixers will at least strongly attenuate either the original
input signal or the LO—but not both—while sending less of the above
mixing products on to its output than the unbalanced type.
Double-balanced mixers (DBMs) supply superior IF-RF-LO interport
isolation, while outputting only the sum and difference frequencies of the
input signal and the local oscillator, and significantly attenuating three-
quarters of the possible mixer spurs. This makes the job of filtering and
selecting a frequency plan a much easier task.

7.1 Passive Mixers
7.1.1 Introduction
Passive mixers permit a much higher RF input signal level than active mixers
before severe distortion products within the output IF becomes unacceptable.
These distortion products are in the form of intermodulation distortion (IMD),
along with compression distortion. The IMD may fall in-band, or cause other
signals to fall in-band, possibly swamping out or creating interference to the
baseband signal. This causes additional noise, which will degrade system per-
formance and BER.
Passive mixers also possess a lower noise figure than active mixers, which
is very important for any stage within the front end of a low-noise receiver.
However, passive mixers will have an insertion loss of around 7 dB, instead of
the insertion gain that active mixers produce.
The passive mixer conversion losses are caused by the mixing diode’s inter-
nal resistance, port impedance mismatches, mixer product generation, and the
inevitable 3 dB that is wasted in the undesired sum or difference frequency.
(This sum or difference frequency is removed by filtering, thus cutting a mix-
er’s final output power in half).
Figure 7.1 is a common double-balanced mixer, which utilizes a diode ring
to achieve frequency conversion of the input signal. The mixer’s diodes are
being constantly switched on and off within the ring by the LO, while the RF
signal is alternately sent through the diodes, this mixes the two signals in a
nonlinear manner, producing the IF output frequency. DBMs can function up
to 8 GHz (and beyond) by using hot-carrier (Schottky) diodes, which possess
low noise and high conversion efficiency.
DBMs made of discrete lumped components and placed on the wireless
devices’ PC board are seldom utilized today. Instead, double-balanced mixers

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