User's Manual

10.2.6 DEVIATION AND BALANCE
The deviation adjustments are used to set the maximum limiting deviation of the transmitter. This
must be set properly to ensure that the unit will meet the regulatory spurious emissions
requirements, in particular, occupied bandwidth. The balance adjustment is used to ensure a
proper relationship between the modulating signal to the reference and to the VCO. If the ratio
i.e. balance is not correct, the transmit audio frequency response will not be correct which could
result in a distorted data waveform.
The optimum values for deviation and balance vary in a predictable manner as a function of
carrier frequency. In order to relieve the user of having to adjust deviation and balance each time
a transmit frequency is entered or changed, the radio calculates the required values based upon
the correct values for two special alignment frequencies. These required values have already
been determined at the factory and are stored in the unit. As transmit frequencies are entered or
changed, new calculated values will appear in the per channel boxes at the bottom of the screen.
These values can be changed on a channel by channel basis, if desired.
The procedure detailed here is for setting the deviation and balance at the special alignment
frequencies so that the deviation and balance will be correct at any programmed frequency. This
same procedure can be used to set any given channel values in the per channel boxes.
An FM demodulator should be connected to the RF output of the module through a suitable
power attenuator or coupler. The demodulator filters should be set for no de-emphasis, as low a
highpass cutoff as possible (<50 Hz, preferably down to DC), and a lowpass cutoff of
approximately 15 kHz. The demodulator output should be connected to an oscilloscope so that it
can be observed.
An audio oscillator should be connected to the AUDIO IN input. The output waveform should be
sine, the level at zero, and at a frequency of 500 Hz. Confirm that the Audio In Gain value is at
least 10.
On the channel drop-down menu, select lower band edge. Activate the PTT, and while observing
the demodulated waveform on the oscilloscope, begin increasing the audio oscillator’s output
level or the Audio In setting. The waveform should begin as a sinewave and at some point show
clipping. The clipped portion may not necessarily be flat. The audio oscillator level should be set
so that a substantial portion of the waveform is clipped, at least 50 %. Adjust the balance value
so that the clipped portion is flat i.e. horizontal rather than tilted. Although the programmer can
change values while transmitting, it is better to unkey between value entries. After the balance is
set, the deviation should be set to a value of 2.3 kHz. Select the upper band edge on the channel
menu and repeat. As a result of this procedure, the per channel balance and deviation values
may have changed.