Ramsey Electronics COM3010 User Manual page 82

DETAILED INFORMATION
bring up any incoming audio level used during test to a point that the sampling circuit can use
it to full advantage and best dynamic range. The COM3010 can amplify fairly low level signals
up to a usable level so that you don't add distortion errors to your measurement by turning
up the volume of a radio so high that it ruins the SINAD test reading. Remember that
distortion is part of the measurement, and adding it by turning up the volume too loud will
make the receiver seem less sensitive than it really is.
Since the audio sampler is only 8 bits of resolution, this gives us a theoretical limit of 48.5dB
according to the device data sheet. This is directly related to the maximum dB of SINAD that
the meter will read. However with narrow-band radios and general background noise, 30dB
of SINAD is typically about the best you will see with this implementation, but more than
adequate for measuring between 12dB and 20dB.
The SINAD meter of the COM3010 is very accurate for the application of radio testing since
both the generator tone and the SINAD calculations are digitally realized. This means that the
generated tone accuracy is +/-0.1%, and the SINAD sampling frequency and filter are tied
directly to the generator frequency, canceling out error in the generated tone to 0%. This also
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means that the notch filter is very deep, and has steep sides which are ideal. The bandwidth
of the filter is 150Hz, so it is very narrow as to allow much of the noise and distortion through
as possible while still removing the signal. Since it is digitally realized, the filter will never need
tuning and calibration, so you can always be assured the SINAD meter is accurate. The filtered
and unfiltered tones are also sampled with true RMS, another important part of measuring
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accurate SINAD. Many SINAD meters do not use RMS detectors to find the ratio, and results in
substantial error. In all, the SINAD meter can be trusted for years of accurate measurements
on your COM3010.
Your Notes:
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