Page 318 - Complete Wireless Design

Mixer Design

Mixer Design 317

Figure 7.4 A single-balanced
two-diode mixer.

ports. In fact, single-balanced mixers are superior to single-ended mixers in
LO-to-IF and LO-to-RF isolation, as well as in their wider bandwidth opera-
tion. Furthermore, intermodulation distortion suppression is increased over
the single-ended type because any IMDs made up of even harmonics will be
suppressed by the balanced-circuit action, and since twice as many diodes are
typically used with this circuit—along with higher LO power—the same RF
amplitude levels inserted into the single-balanced mixer’s input will create
less IMD to be generated in the first place. Negative attributes (compared to
a single-ended mixer) would be that the LO power must be higher, which
necessitates a more expensive and power-hungry oscillator, and the parts
count is increased, as a perfectly balanced balun and one more (matched) diode
must be used, further increasing costs.
Single-balanced mixers’ name comes from their single-balanced balun,
while double-balanced mixers (Fig. 7.1) are so named because they employ
two baluns. These double-balanced mixers will output only strong IMD prod-
ucts that are constructed of both odd RF and odd LO harmonics. This
decreases the DBM’s total output of mixer products to a quarter of the
amount generated within any simple mixer. However, mixer products are
suppressed to varying levels depending on the quality of the diode match and
the accuracy of the balun balance. So, while a DBM requires twice the LO
power as a single-balanced mixer, as well as double the number of balanced
diodes and baluns, a DBM will have much better IMD suppression, a wider
bandwidth, and a higher intercept point.
Triple-balanced mixers (TBMs), also known as double double-balanced mix-
ers (DDMs), have baluns at all three ports, with two complete diode rings.
They have increased intercept points for decreased mixer product generation
and two-tone intermodulation distortion levels, as well as better port-to-port
isolation and a wider possible IF bandwidth. However, TBMs need higher LO
power and another matched diode ring and balanced balun above that
demanded of a DBM. The price will also be higher.

7.1.3 Passive mixer design
As mentioned, DBM design and construction is best left to the mixer manu-
facturing companies. When utilizing DBMs in your wireless design, purchase
a completed device from one of these companies, as it will be a cheaper solu-
tion than attempting to develop one from scratch. For pointers on selecting
the correct passive mixer for your design, see Sec. 7.1.4, “Passive Mixer
Issues.”

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