Daniels VT-3/140-SWA2, Instruction Manual

Page: 57 / 262

Transmitter Main Board Instruction Manual 3-11
10 Adjust the balanced input level control (R31) so that the deviation increases until
compression is observed. The deviation should be ±1.5 kHz or ±3 kHz for
narrowband and wideband channels respectively.
11 Increase the input level (R31) by +20 dBm, it should not increase the deviation more
than maximum. This confirms that the AGC action of preamplifier U1 is working.
12 A 2.4 kHz tone at the desired audio input level should produce the maximum
deviation. Increasing the input level by +20 dBm should not increase the deviation.
This confirms that the limiting action of U4a and U5a is working.
13 Set the audio frequency back to 1 kHz at -8 dBm output. Confirm and record audio
distortion with the appropriate filter on the communications test set.
14 Confirm the audio frequency response by referencing all output deviation
measurements to a 1 kHz input tone at ±20% maximum deviation (±500 Hz for
narrowband or ±1 kHz for wideband). Remove the signal to the balanced input (pins
B18, Z18).
15 Apply a 1 kHz tone at -8 dBm to the microphone audio input. Set the microphone
compression control (R2) to produce ±50% maximum deviation. Reduce the signal
to -10 dBm and adjust the microphone input level control (R2) for ±50% maximum
deviation. Remove the signal.
16. Apply a 100 Hz tone at -18 dBm to the subtone 1 input and adjust the subtone 1 level
control (R42) to produce ±500 Hz deviation. Remove the signal.
3.11 Low Frequency Modulation Configuration
Note: the following information originates from Daniels Electronics document number A0361-06.
3.11.1MT-3 Transmitters
For transmitters, the LTR™ or DCS signal should be applied to the DIRECT MODULATION
input (pin Z28), which is also available on the M-3 and SR-3 motherboards at the control
connectors J1, P1, or J7 on pins B20 (TX A DIR MOD) and A20 (TX B DIR MOD). The signal
from an LTR™ or DCS encoder should be applied through a 47k resistor to reduce the
loading effect of the DIRECT MODULATION input. The signal should be capacitively
coupled to avoid pulling the transmitter frequency off centre. Most LTR™ and DCS encoder
modules already have a capacitive output. Note that the controller generating the low frequency
signal will need to key the transmitter's PTT input. Note that the synthesizer and the audio
processor may require modifications as described later in this document.