Page 325 - Complete Wireless Design

Mixer Design

324 Chapter Seven

7.2 Active Mixers
7.2.1 Introduction
Active mixers vary from passive diode mixers in that they can supply a con-
version gain instead of a loss; they require far less LO power, are much less
sensitive to port terminations, have better ultimate LO-to-IF isolation, and
produce less mixer spurs. However, wide adoption of active mixers, such as the
Gilbert cell type, has been dramatically hindered by a very poor IP3, high NF
(around 15 dB), and the need for a DC supply voltage. The first two problems
have limited the active mixer’s role to the later stages of a receiver where NF
matters little, and where the dynamic range of the signal is more under con-
trol by the AGC.
Active mixers are available up to RF frequencies of 5 GHz, with IF frequen-
cies of 2 GHz, and are double balanced.

7.2.2 Types of active mixers
As with passive mixers, there are different types of active mixers.
The single-ended FET mixer of Fig. 7.10 consists of a JFET, some biasing
components, and two tuned tanks. The RF input signal is dropped across the
first tuned input tank and sent to the JFET’s gate. An LO signal is inserted
into the source lead, with the resultant converted signal removed from the
JFET’s drain and placed across the tuned output tank. This second tank is
tuned to the desired IF output frequency, with most of the mixing, RF signal,
and LO frequencies being severely attenuated by this circuit. The secondary
circuit of the output transformer takes this signal, reduces the high output
impedance, and places it into the IF amplifiers.
A dual-gate MOSFET mixer of the type shown in Fig. 7.11 employs a MOS-
FET, some biasing components, and a single tuned tank. The RF signal is sent
through the coupling capacitor into the second gate, while the LO is inserted
into the first gate, with the sum and difference frequency, along with the mix-
er products, being sent on to the tuned circuit. Since this output tank is tuned
to exactly the desired IF, all other frequencies are attenuated, while the IF
itself is dropped across the transformer’s primary. The IF is then removed
from the transformer’s secondary and sent on to the IF strip for further ampli-
fication and filtering.
Another low-cost active mixer is the single-ended transistor mixer of Fig.
7.12. 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 only the desired IF will
be of any significant amplitude because of the primary and secondary tuned
tank circuits.
A popular Gilbert cell mixer is shown in Fig. 7.13. The RF signal is inserted
into the base of Q of the modified emitter-coupled amplifier (composed of Q
1 1

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