There is an odd fault that can stump even a seasoned radio technician and that is a module that is off-frequency. All transceivers use crystals to determine the operating frequency (including synthesised units) and over time these can drift significantly away from the operating frequency.
The fault usually manifests itself as unreliable comms between two units that decidedly (but not necessarily) have a good comms path. What will happen (as Murphy will have a hand in this) is when you are on site all will appear ok but once off site things will go awry. Temperature will be a factor (warm during the day when testing but cold at night when fault appears) and must be taken into account.
The speed at which telemetry systems operate (the amount of time the transmitter is on) makes it extremely difficult for typical two-way radio test equipment to detect this error. Even if the dials could respond at the speed needed the human eye does not have time to respond.
The best method of determining a frequency error problem, without the fancy tools, is to do a path test between the two units and then replace each unit separately (don't replace both) and note if the problem disappears. If it does, send the suspect unit to the distributor for inspection.
For the more technically minded the best method to test for frequency error is an oscilloscope attached to the receiver's discriminator output (before any AC decoupling and signal conditioning). This will allow direct reading of any frequency errors that may exist on the network. If possible have the comms taking place on the channel relayed to a terminal to help identify the culprit. Oh, I know this sounds like the obvious but, if every unit is off frequency by an average then maybe the receiver you are using needs calibration!
If you are going to embark on this practice it is strongly advised you take a newly calibrated unit, bring out the discriminator test point, and conduct the following tests with a two-way radio company.
Couple the output to an oscilloscope set to DC input (preferably the one you'll use in the tests). Inject a signal into the module on the working frequency at -70dBm and no modulation. Note the voltage relative to ground as this will be the reference for in-field measurements.
Now adjust the modulation to ±5kHz. Note the minimum and maximum points. They should be equidistant from the reference point measured earlier. The output level (difference between the minimum and maximum) should also be noted as this gives an indication as to the sensitivity of the discriminator and can be used as a means to determine the frequency error and modulation levels of transmitters. E.g. should the output be 2.5Vp-p this would indicate 0.25V/kHz (as ±5kHz=10kHz total shift).
Now that you have both the 'zero frequency error' point as well as a means to gauge both the amount of error and modulation depth, one last item remains but there is little one can do to test for it, this being signal quality. It would be handy, however, to slowly shift the incoming frequency by ±10kHz and observe how it affects the quality of the output signal. This will give you a solid understanding of what to look for as well as seeing how it affects reception.
As was initially pointed out, this part was for the technically minded and should such testing be desired further training in this should be sought. A superb area for training in radio matters is Amateur Radio clubs where eager students are taught the basics of radio and radio transmission.
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