M850-00
T856/857 Initial Tuning & Adjustment
C3.11
Copyright TEL 31/09/98
3.9 Audio Processor
3.9.1 Two Point Modulation
The T856 and T857 utilise 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 fre-
quency. 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.
3.9.2 Modulator Adjustment
1. Inject an audio signal of 300Hz 1.5V rms (+5dBm) into the CTCSS input (D-range
1 (PL100) pin 8).
Key the transmitter by earthing the Tx-Key line.
2. Adjust the output from the audio generator to obtain ±3kHz (±2.4kHz) [±1.5kHz]
deviation at 300Hz.
3. Change the input frequency to 100Hz and adjust IC220 via PGM800Win “refer-
ence modulation” to obtain ±3kHz (±2.4kHz) [±1.5kHz] deviation.
4. Change the input frequency back to 300Hz.
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 67 to 260Hz.
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