User's Manual

10.2.7 TX FREQUENCY TRIM
This setting is used to trim the transmitter to frequency. This value should not normally need adjustment.
However, as the unit ages and/or if the transmitter power or the Aux In gain is changed significantly, slight
corrections may be prudent. Note: Any adjustments must be made at a unit temperature of 25 +/- 2 C (77
+/- 1.8 F). Due to internal heating, this adjustment must not be made after the unit has been transmitting
unless it has been allowed to cool to the correct temperature. Likewise, the adjustment itself should be
made as quickly as possible.
The unit should be set to a channel which is at an output power which is close to what will be used the
majority of the time. The RF output of the unit should be coupled to a frequency counter through a suitable
attenuator or coupler. Ensure that no modulation source is connected to the MIC IN or AUX IN. The PTT
should be activated and the TX Frequency Trim value adjusted for the correct frequency. The value can be
changed while the unit is transmitting.
10.2.8 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 AUX IN input. The output waveform should be sine, the
level at zero, and at a frequency of 500 Hz. Confirm that the Aux 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 Aux
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 1.2 kHz for a
very narrow channel, 2.3 kHz for a 12.5 kHz channel or 4.6 kHz for a 25/30 kHz channel. 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.