Easy frequency calibration check on modern transceivers

A common design architecture in modern transceivers is that all ALL receive oscillators and tx sidetone oscillator are synthesised from ONE master oscillator. If that one oscillator is correctly on frequency, everything about the transceiver frequency is correct.

(One small exception, it is not unusual that a supplementary crystal oscillator is used in FM tx because of issues with modulation of the signals from the DDS, so frequency accuracy on FM tx may depend on one further adjustment.)

When switched to CW mode, the tx sidetone frequency should be exactly the same as the sidetone from a carrier at the displayed frequency. So, for example, if the receiver is tuned to 10.000000MHz in CW mode, the beat note produced from WWV (provided that it is strong enough to be heard) should be at exactly the same frequency as the tx sidetone oscillator.

If the transceiver VOX is turned off, so that the transceiver does not transmit with key down, but produces sidetone, if the receive frequency is adjusted in increments of 1Hz, a point will be found where the sidetone oscillator beat with the beat note from WWV's carrier is less than 1Hz. The difference in the displayed frequency and the known WWV frequency is the error in the frequency calibration.

For a demonstration with commentary, listen here. The demonstration is performed on a TS2000 from the front panel without removing covers. The TS2000 can tune in 1Hz steps, but uses the RIT/XIT control for the fine steps. Note that some aspects of the demonstration (eg sidetone frequency, actual error) are dependent on the operating configuration, but the system works the same on any TS2000, regardless for instance of the sidetone choice.

Most modern Icom transceivers provide simple access to 1Hz stepping though the TS (tuning speed) button and direct readout, see your manual. Though Icom produces some radios that are difficult to calibrate (eg IC-910 which requires not only removal of covers allowing the unit to cool, but also removal of the 23cm module for access to the adjustment), the IC-7000 permits frequency calibration without removing the covers, and without tools.

The first step to using this technique is to understand your transceiver, how it generates the various oscillators. As time passes and we dumb down, user manuals are less likely to contain the necessary information which by its omission is deemed to not be of interest to users, you may need to locate a service manual (often available at no charge on the 'net).

Note that DDS do not have infinite frequency resolution, but most implementations should have sufficient resolution to support calibration to 1Hz resolution, although the calibration will not hold due to the short and long term drift of the oscillator.

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Changes

V1.01 18/02/09 13:56:33 -0700 .