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M880-00
3.9
Audio Processor & CTCSS
3.9.1
Two Point Modulation
The T881 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.
Note 1:
In this and following sections deviation settings are given first for wide
bandwidth sets, followed by settings in brackets for narrow bandwidth sets
[ ].
The reference modulation and limiter adjustments are controlled by 256-
Note 2:
step electronic potentiometers (EPOTs), which are adjusted via the "Refer-
ence Modulation" and "Deviation" settings in PGM800Win. This allows the
two point modulation and deviation settings to be adjusted for each chan-
nel.
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 use the "EPOT Fill" option
other channels.
Note 4:
If you are using an RF test set, turn the low pass filter off and set the high
pass filter to 15kHz
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 [±1.5kHz] deviation
at 300Hz.
3.
Change the input frequency to 100Hz and, using PGM800Win, adjust the value of
the "Reference Modulation" EPOT setting for the current channel to obtain ±3kHz
[±1.5kHz] deviation (you can use either the mouse or up and down arrow keys).
1. Use the "EDIT FILL" button on the tool bar or go to "Edit", "Fill", "Epot Settings" on
the menu bar.
Copyright TEL
T881 Initial Tuning & Adjustment
∆
F>±4MHz).
1
in PGM800Win to copy the value to the
before
beginning this procedure.
C3.11
10/07/00
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