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136 Chapter 5
C:\Spk\Ref\RefGuideRevE\CdmascrnE2.fm
CDMA Measurements - Screens and Control Fields
CDMA Measurements
Figure 5-2 Time Offset
Carrier Feedthrough
Carrier feedthrough is displayed when the Rho measurement is
selected. Carrier feedthrough is a common cause of bad rho
measurements. Carrier feedthrough is a result of RF carrier signal
feeding through the I/Q modulator and getting on the output circuitry
without getting modulated. A good carrier feedthrough level is lower
than −25 dB (such as −29 dB).
As shown in the diagram, a perfect constellation is offset by the
magnitude of the carrier feedthrough. In the frequency domain, carrier
feedthrough can show up as an uncorrelated energy spike that can be
seen on the spectrum analyzer by closely scanning the top of the trace.
The effects of carrier feedthrough can also show up as higher noise
levels on the CODE DOM screen. The inactive Walsh codes will be
pushing the −27 dB specification for noise. Carrier feedthrough can be
caused by the lack of isolation across the mixer and cavity of the
transmitter's I/Q modulator. Shielding can help reduce carrier
feedthrough.