Page 322 - Complete Wireless Design

Mixer Design

Mixer Design 321

mally the easiest and cheapest solution for two-tone, third-order product
improvement: by decreasing the input two-tone RF signal by 1 dB, we will
decrease the output two-tone, third-order products by 3 dB. However, the
inverse is also true: Increasing the RF two-tone input by 1 dB will increase
the output two-tone, third-order products by 3 dB. And as these third-order
products are the most dangerous spurious signals because they can fall in
band at the mixer’s IF port, they must be attenuated to the lowest level the
system requires.
To assure ourselves of decent intermodulation distortion performance and
conversion loss variations, a Level 7 mixer should never be run with an RF
input higher than 3 dBm (with the LO drive at the rated power level); a
level 10 mixer never above 0 dBm; a level 13 mixer never above 3 dBm; and
a level 17 mixer never higher than 7 dBm. In fact, decreasing these RF
input levels to 20 dB below the LO drive is commonly done to reduce IMD
generation to even lower amplitudes, while, unfortunately, also increasing
the relative LO feedthrough.
It is possible to calculate the highest (in amplitude) two-tone, third-order
spur level that is down from our desired signal by:
TOIM 2 (TOIP RF )
SUP IN
where TOIM third-order intermodulation suppression down from the
SUP
signal of interest, in dBc, at the mixer’s output port
TOIP third-order input intercept point of the mixer, dBm
RF power, in dBm, of the RF signal at the input to the mixer
IN
Appropriate frequencies for the LO and IF should be selected during the fre-
quency-planning stage that will minimize the number and strength of the mix-
er products present within the IF bandpass of the DBM. This is most
conveniently performed by employing the appropriate software, such as
Blattenberger’s RF Workbench, or The Engineer’s Club’s MixerSpur. Both of
these low-cost programs will graphically indicate if there are any dangerous
mixer spurs within the IF passband.
As all mixers have not only a nominal, but also a minimum and a maximum
LO drive level as recommended by the manufacturer for the particular DBM,
we may sometimes desire a minimum drive level for two reasons: sufficient LO
power may not be available and/or LO feedthrough must be minimized.
However, two-tone IMD suppression, conversion losses, and return loss will all
suffer as a result. Slightly increasing the LO drive level above the nominal val-
ue will end in a higher NF and a higher LO feedthrough; but will improve the
mixer’s two-tone IMD performance and mixer product suppression and
decrease the conversion losses across the band. As indicated above, running
the mixer at the recommended LO drive level is the best compromise for supe-
rior mixer performance.
In designing an upconverting superheterodyne receiver, the incoming RF
signal should be placed at the passive mixer’s IF port, while the now higher

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