25Feb00

Frequency Questions

A few hams have posted messages on the bulletin board about problems associated with frequency, either accuracy or stability. This section discusses them and reports on how a couple of hams have dealt with them. If you choose to follow any of the steps shown, you are on your own; I cannot vouch for their effectiveness and have not done them myself.

Frequency Accuracy

Ideally, your FT-847 would be dead-on accurate with respect to frequency. The truth is, however, that there will be small deviations from the true frequency, and this is normal. Do not panic. In addition, the higher the frequency, the more pronounced a small error becomes. For example, an inaccuracy of 1 ppm in frequency translates to a 440 Hz inaccuracy at 440 MHz.

One ham reported that his rig was 50 Hz off at 7 MHZ, 100 at 14, 300 at 50, etc. The error was about 7 ppm. One solution here is to adjust the Reference Output Adjustment to bring the radio closer to the true frequency to begin with. This frequency forms the basis for accuracy of reception on all bands. If you find that your frequencies are off, and the change is somewhat linear across the bands, it could be that your reference oscillator is slightly off frequency.

Tweaking The Reference Oscillator Frequency

How does one do this? David G7LMT reported that his rig was about 2k off frequency on the 70 cm band. He opened up the radio, located and adjusted TC1001 to produce 45.250 MHz (+/- 10 Hz) on TP1002. Of course, you must ensure that your frequency counter is accurate enough to do this correctly!

Denis F6CRP reported that he simply tweaked TC1001 until his radio read the frequency he knew another radio or beacon was on, and is pleased with how easy it was to do. Your results may vary!

Both TC1001 and TP1002 are on the AF-CNTL board. This board is on the bottom of the rig and the two points are located on the "front" (i.e., nearest the front panel) half. TC1001 is located just to the right of and below the ceramic filter that's parallel to the front and back of the rig. TP1002 is located between the ceramic filter that's perpendicular to the front panel and the front panel. While this is not the complete alignment for this test point, it may be good enough to tweak this basic oscillator. Click here to see a sketch of the AF-CNTL board with TC1001 and TP1002 marked.

Note that you cannot always rely on other indicators for this setting. For example, you could tune to WWV on FM and check the discriminator setting. This assumes, of course, that your discriminator is accurately positioned. You could flip back and forth between USB and LSB on WWV, but again, that assumes that your carrier points are accurately set. The bottom line is to be sure that what you're using to set or measure frequency is more accurate than your radio!

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Frequency Stability

The specifications call for an frequency stability of +/- 2 ppm from 0C - 40C. This translates to +/- 28 Hz on 20m, but is 880 Hz either side of "true" at 440 MHz. This is a worst case number; Yaesu reports that the actual stability is generally much higher than this. Without a temperature stabilized oven for the crystal, this is comparable to specs for other radios in the same class. Given this, a radio could vary over time from 880 Hz below nominal to 880 Hz above, and still be in spec.

There have been suggestions or questions that the frequency varies by how warm the rig is and whether the fans are running or not. In many hours of on-the-air operations with another ham running another FT-847, I have not noticed any variances in frequency over time. We're both in relatively stable ham shacks. Your mileage may vary.

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