Page 98 - Complete Wireless Design

Modulation

Modulation 97

3. Take the amplitude of the center frequency f of the VCO, and the output
out
level of the spurs, and subtract the two. This will be the level of the actual
reference spurs in dBc.

Desensing test. A receiver must be tested for out-of-band signal rejection,
since strong off-frequency interferers can desense the receiver’s low noise
amplifier (LNA) if its front-end filter is not sufficiently selective, causing
BER problems. These powerful out-of-band signals can also cause IMD or
mixing products to form in-band because of the overdriving of the LNA. The
test is performed by combining two RF signal generators, making one gen-
erator the desired signal source by setting it to a center in-band frequency
at 80 dBm while setting the other signal generator to a frequency at either
the lower or upper band edge to function as an undesired out-of-band inter-
ferer at 20 dBm. Now, confirm that the gain for the desired signal does not
decrease below specifications because of amplifier desensing by subtracting
the input signal level in dB from the desired signal level output of the
receiver.

Digital system test and measurement. To confirm proper system operation, the
following are the minimum tests that should be performed on a digitally mod-
ulated receiver. Most of these tests require that a CW test signal be injected at
the receiver’s front end, at the center of one of its channels, and at below P1dB
power levels, with the output taken from the receiver’s last IF output stage.
Some of these tests will require a vector network analyzer.
Receiver tests:

1. Gain, measured in dB.
2. Gain flatness across one channel, measured in dB (sweep entire channel
with frequency generator).
3. Frequency accuracy after a certain warm-up period, measured in Hz.
4. Frequency stability over a certain temperature range after a specified
period of warm-up, measured in Hz.
5. Frequency drift over a set time at 25°C, measured in Hz, from turn-on to
full warm-up.
6. P1dB at output, measured in dBm.
OUT
7. Phase noise of local oscillator at 10 kHz offset from carrier in dBc, mea-
sured in dBc/Hz/10 kHz.
8. Two-tone IMD at output, measured in dBc.
9. In-band internally generated spurs with no input signal, measured in
dBm and Hz.
10. 3-dB bandwidth of a channel, measured in Hz.
11. Minimum discernible signal (MDS) with zero SNR, measured in dBm.

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