BARRETT 900 SERIES TRANSCEIVERS
5.8.2.6 DAC Low pass filter
should not be adjusted in the field
5.8.3 RF/ Audio PCB
The adjustment of each of these is critical
and should not be undertaken unless the critical components have been replaced.
5.8.3.1 Input low pass filter
5.8.3.2 45MHz IF Filter
Note:- Whilst the transceiver will operate using
this method the filter shape could be incorrect and adjacent channel selectivity, blocking
performance and transmit purity may be compromised.
If a spectrum analyzer with tracking generator is available, this filter can easily be adjusted by adjusting the
two notch frequencies of 12MHz and 18MHz. The transceiver should be turned off while performing this
adjustment. Connect the tracking generator to TP20, and the spectrum analyzer input to TP21. Adjust L1 for
a notch at 18 MHZ, and L2 for a notch at 12 MHZ. There is no requirement here to check that the filter
response is flat.
This low pass filter has been factory set and
. In an emergency,
satisfactory operation can be obtained by setting both L1 and L2, 3mm into the coil can. There are four
yellow LED's which are lit showing correct operation of the +5, +5D, +5A and +10 supply rails. The two
green LED's illuminate above 8MHz due to local oscillator RF energy.
There are 11 adjustable components on the RF/Audio PCB.
If a spectrum analyser with tracking generator is available, this filter can be adjusted by adjusting the two
notch frequencies of 69MHz and 47MHz. The transceiver should be turned off for this adjustment. Unsolder
SP3 and connect the tracking generator to TP30, and the spectrum analyzer input to TP14. Adjust L16 for a
notch at 69MHz, and L20 for a notch at 47MHz. The frequency response should be flat rom 1MHz to
30MHz
The input low pass filter inductors L16 and L20 should have cores set approximately 5mm inside the cans in
the event of replacement if the correct test equipment is not available.
This filter is the most critical filter in the transceiver. Its setting has a direct bearing on the transceiver
dynamic performance. A spectrum analyser with tracking generator must be used to align this filter and it
should be set with a centre frequency of 45MHz and a span of 50kHz. Solder bridge SP1 and SP2 should
be unsoldered, the sweep signal should be injected at TP28 , the spectrum analyser input should be
connected to TP29. L7 and L8 should be adjusted to achieve the passband shape as indicated in Fig1.1, with
particular attention being given to get the passband as flat as possible. If a spectrum analyser is not available
and either L7 or L8 or the crystal filter has to be replaced, rough alignment can be achieved by peaking the
coils for maximum output whilst injecting a weak signal.
Fig 1.1
0dB
-10dB
-90dB
Center: 45 MHz
Span:50 kHz
-20dB
-30dB
-40dB
-50dB
-60dB
-70dB
-80dB
+10dB
PAGE 41
Summary of Contents for 900 Series
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