Using a Precision Frequency Service

The information below applies to Precision Frequency Transmissions of all types, but is of maximum use to Radio Amateurs if the transmissions are made on the Amateur bands. So, we should therefore encourage the development and use of an Amateur Precision Frequency Service! Other suitable services are not widely available as they either have insufficient carrier stability, have unsuitable modulation, or are not on any one frequency for more than a few hours at a time. Of these the standard frequency transmissions of WWV, WWVH, CHU etc are the best, and propagation permitting, are available throughout the HF spectrum.

There are four main Amateur uses for standard frequency transmissions. They are:

  1. As a reference for calibration of receivers and frequency standards.
  2. As a monitor signal for oscillator or receiver drift measurement
  3. As practice frequencies for Frequency Measuring Contests.
  4. As stable transmissions for propagation research.
Receiver Calibration
Receivers can be calibrated by receiving the transmissions, adjusting the receiver display to read exactly 1kHz lower (in USB) or 1kHz higher (in LSB), and then adjusting the receiver calibration until a 1kHz tone is received. The tone frequency can be measured with a frequency counter or sound card software such as a Spectrogram or PSK31 program. This technique is good to better than 1 in 107, although dependent on the quality of the signal received. As much as anything else, this depends on the ionospheric propagation conditions prevailing at the time. Results are best with a ground wave signal.

Reference Calibration
Calibratiion of a local reference (such as a crystal calibrator) can be achieved by tuning in a reference transmission on an AM receiver, and adjusting the local reference for zero beat (good to about 1 in 106 if care is used). For higher precision adjustments, use a Spectrogram program and an SSB receiver. Adjust the local reference so that the two traces (received signal and local reference) coincide. This technique is good to better than 1 in 107, dependent on conditions, as explained above.

Drift Monitoring
To monitor drift, tune in the transmission with the receiver cold, and start a Spectrogram program to monitor the tone frequency. Use a very slow speed trace, and you will record the warmup drift and stability performance of your receiver. You may need to experiment to get the best results.



Warmup performance of a good receiver (Kenwood TKM-707)

Frequency Measuring Contests
Those interested in Frequency Measuring Contests will find these transmissions great for practice measurements, and of course also for calibration. The quality of the signals will be as good as any used during the contest. No secrets are given away here - FMC participants are fiercely competitive!

Propagation Research
All HF signals are affected by the ionospheric propagation medium, which consists of charged particles in the upper atmosphere which interact with the radio signals and cause refraction - bending of the radio waves so they eventually come back to earth. Since the refractive properties depend on the earth's magnetic field, the amount of ionization generated by the sun, the altitude of ionozation, the range and the radio frequency, there is much to be learned about the behaviour and results of these effects. The section on the Ionosphere gives some information on studying these effects.

Your Own Standard Frequency Station
Despite the fact that high precision Amateur Transmissions are rare (and perhaps unheard of until the author started development in this area a few years ago), it is actually quite straightforward to put together a QRP (low powered) Precision Transmitter, quite good enough for use on 80 - 30m. It is known from experience that a 2W transmission will give quality coverage over a wide area. ZL1BPU has been monitored regularly in Australia and around the Pacific on 3840kHz, while VK2BLR, using the same transmitter design, is exceptional copy in New Zealand on 3602kHz- the signal can even be received during daylight hours!

There are two reasonably simple approaches to take. One is to use a GPS or TV locked crystal oscillator (well described under Build a Reference). The other approach, which is much more versatile, is to build a DDS or PLL synthesizer with a precision reference which can in turn be locked to TV or GPS. To be useful, the exciter needs to achieve about 1 part in 108 accuracy or better, and have low phase noise.

A really cool way to put a superb signal on the air would be to use a small Rubidium source such as the FEI 5650A (available second-hand over the internet). These units are smaller than your fist, and programmable in 1Hz steps from 0 to 20MHz!

A simple 2W power amplifier will put the signal on the air. Keeping the power low will prevent complaints of "unwanted carriers", and also makes keeping the signal stable much easier - from experience, the more power you run, the harder it becomes to maintain accuracy. The Spectrogram measurement technique is very sensitive, and low power is all that is required.

Copyright © M. Greenman 1997-2005. All rights reserved. Contact the author before using any of this material.
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