M8SL2-00-002-812 T855 Initial Tuning and Adjustment B3.11
© Copyright Tait Electronics Limited August 2004. All rights reserved.
While maintaining the low level RF input to the receiver, loosely couple into the
second IF an additional high level signal at 455kHz, you will hear a beat note.
Tune L385 for zero beat.
Note: If a second oscillator is not available, you can connect a frequency counter to
IC710 pin 8 (i.e. after the TCXO buffer) via an oscilloscope probe to measure
the TCXO frequency directly (12.8MHz). At this point the voltage level is
approximately 4V
pp
.
Readjust the front end helicals (#FL410 and #FL420) to give the best sinad.
Change the RF signal level to –75dBm and modulate with ±3kHz deviation
(±2.4kHz) [±1.5kHz] at 1kHz.
Connect an oscilloscope probe to SK320 pin 3 (RSSI 455kHz input) and connect
plugs PL210 and PL220 to give a flat audio response (refer to Section 3.5).
Readjust L310, L320, L330, L340, L350, L360, L370 and L380 to give a maximum
amplitude response on the oscilloscope with minimal amplitude modulation.
Note: If you would like a more accurate method of tuning the IF, refer to the
sweep tuning method described in Section 5.5.5.
Further adjust these coils, along with L390, for minimum audio distortion, ensur-
ing that the 455kHz level (on the oscilloscope) does not fall significantly.
Check that the distortion reading is:
wide bandwidth ≤2%
mid and narrow bandwidth ≤4%.
If required, reconnect plugs PL210 and PL220 to give a de-emphasised audio
response and check that the distortion reading is ≤2% (all bandwidths).
Reduce the RF level until 12dB sinad is reached. The receiver sensitivity should be
better than –117dBm (de-emphasised) or –111dBm (flat), assuming that the audio
levels are not being overdriven (refer to Section 3.11).
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