Page 320 - Complete Wireless Design

Mixer Design

Mixer Design 319

7.1.4 Passive mixer issues
The selection of a proper DBM for a particular receiver or transmitter appli-
cation will depend on the required P1dB compression point, LO power, device
cost, port isolation, and two-tone intermodulation and mixer-generated prod-
uct suppression.
Undesired frequency product generation, and its suppression, is important
in the entire heterodyning process, so we will delve into this subject a little
further. Output mixer products (Fig. 7.6) are formed by the mixing, in the non-
linear diode elements of a DBM, of the incoming single-tone RF (and its resul-
tant harmonics) with the single-tone LO (and its resultant harmonics). This
creates high-order distortion products that are higher and lower in frequency
than the desired product, which is normally either the sum or difference fre-
quency of the LO and RF in a receiver, or the LO and IF in a transmitter.
Two-tone intermodulation products are created when two tones (f and f )
1 2
are placed at the RF input port of the receiver’s DBM and, mixed with each
other and the LO, give birth to high-order in-band spurious responses at the
IF output port of the mixer. The higher the possible LO oscillator power, the
lower the distortion products. Figure 7.7 demonstrates this point with three
different level mixers (levels 7, 17, and 23), with each using its recommended
LO input power of either 7, 17, or 23 dBm. In Fig. 7.7a, the level 7 mixer’s IF
output shows high third-, fifth-, and seventh-order two-tone IMD products for
a 0 dBm RF input. In Fig. 7.7b, the level 17 mixer decreases the IMD prod-
ucts for the same 0 dBm RF input amplitude. In Fig. 7.7c, the level 23 mixer
shows IMD products much further down than even the level 17 mixer, at
approximately 65 dBc.
A further issue in mixer design is demonstrated in Fig. 7.8. Boosting a level
7 DBM’s LO drive does not in itself drastically improve the IMD product sup-
pression. This can be accomplished, in any significant way, only by increasing

Figure 7.6 Various single-tone mixer spurs generated by the
mixing of the RF (19 MHz) and the LO (20 MHz), and their
harmonics.

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