Page 316 - Complete Wireless Design

Mixer Design

Mixer Design 315

Figure 7.1 A double-balanced mixer stage.

are available in a module, with the diodes and transformers already balanced
within a single, low-cost dual in-line package.
Lower-end passive mixers are available that employ either a single diode
(Fig. 7.2a) or double diodes (Fig. 7.2b). These mixers are not double-balanced
and, in contrast to an active mixer, must be supplied with a high-amplitude
LO signal (but not as high as a DBM’s). Nonetheless, they are cheap and
require few components.

7.1.2 Types of passive mixers
There are several types of passive mixer designs available, depending on cost
and performance levels required. Some of these passive diode mixers have
already been introduced above, but will be further investigated in this section.
Figure 7.3 shows a one-diode, single-ended mixer. This type of device is
found only in low-cost circuits, with the isolation between ports being sup-
plied by bandpass and low-pass filters that are separated in frequency. The
mixer circuit can also take advantage of the low level of LO power needed to
drive the single-diode mixing element compared to the higher drive levels
required of a DBM. The single-ended mixer shown, however, has a rather nar-
row bandwidth, poor port-to-port isolation, a low intercept point, and inferior
intermodulation distortion suppression. If we would like to increase the spec-
ifications and overall quality of this device, we will need to increase the num-
ber of diodes. This will allow a higher LO drive input level, which
automatically forces an increase in the mixer’s 1-dB compression point
because the P1dB is always about 10 dB below the LO for all diode mixers.
Thus, the higher the LO drive that can be inserted into a mixer, the higher
the P1dB possible. As we are then demanding a more powerful LO, this will
unfortunately cost more and radiate a higher level of EMI.

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