User's Manual

RPT 633
TM1184 Issue 1 Page 5 - 7
c) Measure the output frequency to the nearest 10Hz; switch the data polarity to
the binary O state by connecting TP14 to TP9 and again measure the output
frequency. Adjust RV2 and repeat this process until the difference between the
two frequencies is as given in the TOTAL DEVIATION column in Table 1 for the
appropriate transmitter channel spacing, with a tolerance of ±100Hz.
d) Using a plastic trimming tool adjust VC5 until the mean of the two frequencies
measured at (c) is within 100Hz of the specified center frequency of the
transmitter.
e) Repeat sub-paragraphs (c) and (d) as necessary.
f) Switch the mode switch to ANALOGUE by setting LK3 on the VOX keying
module to AC and adjust RV3 to give the specified centre frequency of the
transmitter to a tolerance of ±100Hz.
Table 1: Data Deviation
CHANNEL DEVIATION TOTAL
SPACING DEVIATION
kHz kHz kHz
25 ±4.5 9
20 ±3.6 7.2
12.5 ±2.25 4.5
10 ±1.8 3.6
Setting Analogue Modulation
13. To set the analogue modulation carry out the following procedure:
a) Set LK3 of the VOX keying module to AC to select the ANALOGUE mode.
d) Set the audio generator output to 1.25kHz at a level of approximately 11mV
peak-to-peak as measured on the oscilloscope. Monitor the deviation on the
modulation meter.
c) Refer to Table 2(a) and adjust the generator output level to give give a
deviation corresponding to the channel spacing.
d) Increase the signal generator output level by 20dB and adjust RV4 to give a
deviation coresponding to that given in Table 2(b). This should be the larger of
the Peak + or Peak - readings on the modulation meter.
e) Reduce the signal generator level by 20dB and re-adjust its level to give a
deviation corresponding to that given in Table 2(c).