Tait T860 Series II, Service Manual

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C3.10 T867 Initial Tuning & Adjustment
M860-00-Preliminary
30/06/98 Copyright TEL
3.7 Audio Processor
3.7.1 Two Point Modulation
The T867 utilises two point modulation to obtain a wide audio bandwidth independent
of the synthesiser loop filter response. This is achieved by simultaneously frequency
modulating the VCO and phase modulating the synthesiser reference frequency. The
relative signal levels fed to the two modulators are quite critical and cause interaction
when setting up.
Both modulating signals require readjustment when the exciter is shifted in frequency
greater than the switching range (i.e.
∆
F>±4MHz).
Note 1: In this and following sections deviation settings are given first for wide
bandwidth sets, followed by settings in brackets for mid bandwidth sets ( )
and narrow bandwidth sets [ ].
Note 2: Reference modulation and limiter adjustment are controlled by
PGM800Win. Electronic potentiometers (256 step) are used to allow chan-
nel-by-channel adjustment of deviation and two point modulation.
Note 3: To optimise the modulation response across the switching range, repeat
steps 1-4 below for each channel that will be used (usually needed only for
data applications). In applications where the modulation response is less
critical (e.g. voice use only), carry out steps 1-4 below on the middle channel
and cut and paste the value to all other channels.
3.7.2 Modulator Adjustment
1. Inject an audio signal of 500Hz 1.5V rms (+5dBm) into the CTCSS input (D-range
1 (PL100) pin 8).
Key the exciter by earthing the Tx-Key line.
2. Adjust the output from the audio generator to obtain ±3kHz (±2.4kHz) [±1.5kHz]
deviation at 500Hz.
3. Change the input frequency to 50Hz and adjust “reference modulation” via
PGM800Win to obtain ±3kHz (±2.4kHz) [±1.5kHz] deviation.
4. Change the input frequency back to 500Hz.
Repeat steps 2 and 3 above until the deviations achieved at the two input frequen-
cies are within 0.2dB of each other. You will need to do this at least four times.
5. Sweep the audio between 50 and 300Hz for peaks.
Note: A peak between 50 and 300Hz will indicate a fault condition, i.e:
- incorrect set-up
or - modulation circuitry fault.
The specification window is ±1dB relative to 150Hz from 65 to 260Hz.