Copyright TEL
31/01/96
3.6
Alignment Of Receiver Front End And IF
Note 1:
In this and following sections deviation settings are given first for wide
band sets, followed by settings in brackets for narrow band sets [ ].
Note 2:
Frequency Range
136-156MHz:
With the power supply switched off, solder bridge links
1-6 and link B in the front end.
148-174MHz:
Solder bridge link A and ensure links 1-6 and link B are
not connected.
Align the synthesiser as instructed in Section 3.5. For multichannel operation the
receiver should be aligned on a frequency in the middle of the required band.
Inject a strong on-channel RF signal with 3kHz deviation [1.5kHz] at 1kHz into
the antenna socket and adjust L304, L305, L309 & L310 to give best sinad.
Continually decrease the RF level to maintain 12dB sinad.
Roughly tune IF coils L315/L316/L317, VC301 and quad coil L319 for best sinad.
While maintaining a low level unmodulated RF input to the receiver, loosely cou-
ple into the first IF an additional high level signal at 21.4MHz - a beat note will be
heard.
Trim the synthesiser TCXO (IC2) for zero beat.
Readjust the front end doublets (L304, L305, L309 & L310) to give best sinad.
Change the RF signal level to -75dBm and modulate with 3kHz deviation
[1.5kHz] at 1kHz.
Connect an oscilloscope probe to the RSSI test point (pad 115 or 125) and connect
plugs PL101 and PL103 to give a flat audio response (refer to Section 3.3).
Readjust IF coils L315/L316/L317, VC301 and quad coil L319 to give a maximum
amplitude response on the oscilloscope with minimal amplitude modulation.
Further adjust these coils, along with L319, for minimum audio distortion, ensur-
ing that the 455kHz level (on the oscilloscope) does not fall significantly.
Check that the distortion reading is less than 2%.
Reconnect plugs PL101 and PL103 to give a de-emphasised audio response (if
required) and reduce the RF level until 12dB sinad is reached. The receiver sensi-
tivity should be better than -117dBm, assuming that the audio levels are not being
overdriven (refer to Section 3.8).
Содержание T830 Series
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