Operating instructions

If a message, similar to this is seen, it indicates a potential fault condition. If the final

text indicates that the channel and mod reference errors are within specification (the last

text output), then the unit is able to be used, but nonetheless, it is advisable that an e-

mail be sent to RF Technology indicating the problem. Such a problem may be caused

by a crystal having aged to such an extent, that it is getting close to the region where it

may soon, no longer be adjusted, or that, with further degradation, the low frequency

performance may be compromised.

As no other calibration procedure requires the top cover to be removed, you should

replace the cover, should you have had to remove it for this procedure.

6.2.3 The “Deviation” Calibration Procedure

T50> cal dev

This procedure should not normally be invoked as part of any field maintenance. The

only conceivable time that it might ever be

used, would be if a non standard maximum

deviation was required.

Note that the nominal output power of the exciter’s output is 1W. Whilst the power

level is not set to the maximum level, the deviation meter should either have an input

power rating of at least +30dBm, or suitable attenuation is required between the exciter

and the deviation meter. Note that for this test the nominal RF output frequency is

37.5MHz.

The first stage begins with the following message:

This procedure sets the Balance and Max Deviation Levels

Connect an audio signal generator to the External Tone input

Set the output to be a sine wave, 3.87 V rms and 120Hz

Disconnect the RF connection to the PA

and connect a deviation meter to the exciter's BNC output,

and use a CRO to monitor the Audio output from the deviation meter.

The Channel VCO Bias is 2.5V

The Modulation VCO Bias is 2.3V.

Enter + or – to increase or decrease the Balance, until the

CRO signal is as symmetric as possible.

Here the user can hit the +, p, or P, keys to increase the Balance, or -, m, or M keys to

decrease it. In response, the firmware, which has opened up the maximum deviation

digital POT (U303) to maximum gain, will adjusted the Balance VCA accordingly.

This test applies a very high level to the External Tone Input which becomes “saturated”

by the limiter for the Tone input. The result is a square wave with a slight ring on the

edges. The ringing is caused by the elliptical filter used as a 3kHz low pass filter.

If the modulation balance is not working properly, the top edges of the resulting

modulated signal will be slewed.

Then the firmware will make the following request.

change the signal input to Line 1, and adjust the frequency till the

deviation peaks

When this has been done, the User needs to hit the Enter key, and the following will

appear again.

Enter + or - to increase or decrease the deviation,

and <RET> when the deviation is 5kHz