5-24
Figure 5. Common path distortion appears as beats at regular
intervals
Recommended Setup for CPD Analysis
If the test point is not band-limited to the reverse spectrum (i.e. on
the low band side of a diplex filter) a low pass filter may be needed
to eliminate the possibility of instrument related intermodulation
products that mimic CPD. The susceptibility to intermodulation is
dependent on the total number of carriers present on the input to the
instrument. The measurement technique is similar to using a band
pass filter for intermodulation tests (composite second/third order).
The objective is to avoid bombarding the RF input of the instrument
with a large number of relatively high level signals (typically 78
channels in a 550 MHz system), while looking for low level signals.
The problem is that when attenuation is removed to see the low level
beats more clearly, the front end of the instrument may be over-
loaded with the high level signals on the same test point, and gener-
ate beats of its own.
A low pass filter may be needed (see figure 2) when connecting the
instrument to a bi-directional amplifier test point – a test point with
Summary of Contents for 3SR
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