Page 324 - Complete Wireless Design

Mixer Design

Mixer Design 323

Operating a mixer in a harmonic mode allows a designer to use a much low-
er LO frequency than would normally be required. Ordinarily, only the sum or
difference frequencies are employed at the IF output of the mixer, but any con-
venient mixing product may be utilized for this purpose: such as f 3f , f
RF LO RF
5f , f 3f , or the f 5f products. However, since these frequencies
LO RF LO RF LO
will be at a lower amplitude than the normal f f or f f products,
RF LO RF LO
supplemental amplification is required at the mixer’s output, as is nonreflec-
tive filtering with a diplexer. The nonreflective filtering is necessary since the
undesired signals and products are reflected back into the IF port of the mix-
er because of the reflective stopbands of a normal output IF filter, causing two-
tone IMD performance to suffer (sometimes by as much as 25 dB).
Some common terminology used to specify a DBM:

Conversion compression—A specification that indicates the maximum value
of the input RF signal level that will obtain a linear increase in IF output
power. For example, level 7 mixers will usually have a conversion
compression of 2 dBm.
Conversion loss—The rated signal level difference between the input and the
output of the mixer at the rated LO input power. For instance, a level 7 ( 7
dBm LO drive) mixer may have a loss in power from input to output of 8 dBm
at midband. Decreasing the LO drive to 0 dBm may increase conversion loss
by 0.5 dB or more.
Cross modulation—If two signals are present at the input port of the
mixer—one modulated and the other CW—this term describes the undesired
transfer of the modulation from one signal to the other.
High-side injection—Occurs when the LO frequency is higher than the RF
frequency in a conversion stage.
Intercept point—Superior two-tone, third-order product suppression
demands a high intercept point. This value is approximately 10 dB higher at
the mixer’s input than the conversion compression rating. Cross-modulation
distortion and desensitization is also reduced with a high intercept point.
Interport isolation—The rating of the feedthrough between the mixer’s LO,
RF, and IF ports. This is the value, in dB, that one port’s signal is attenuated
at another port’s input or output. The most important of these isolation
specifications is the LO attenuation at the IF and RF ports, since LO
feedthrough is a major problem in receiver and transmitter systems design,
and the RF to LO isolation is normally of little concern because of the RF’s
low input levels. Typical LO-to-IF isolation is from 25 to 30 dB.
Low-side injection—Occurs when the LO frequency is lower than the
incoming RF frequency in a conversion stage.
Noise figure (NF)—The noise added by the mixer. Equals the difference
between the noise at the input of the mixer and the output of the mixer, in
dB. When the mixer is driven with the proper LO drive level, the NF will
equal the conversion loss.

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