Wednesday, August 03, 2005

Orion Frequency Calibration

The Ten-Tec Orion Transceiver is specified for frequency accuracy of + or - 3 ppm over the temperature range 0 - 50 C. That amounts to +/- 30 Hz at 10 MHz, or nearly +/- 90 Hz in the 10 meter band. In an SSB QSO, an error of 50 Hz is pretty noticeable. In a PSK31 QSO, a whole QSO fits into a 30 Hz band, so such an error could be quite serious. Fortunately, we rarely need to set our VFOs with such accuracy. We need precision, but we can just tune for best reception. It does matter near a band edge, but it would be unusual to have to work within 50 Hz of an edge.

The most likely case where good accuracy can help in the HF bands is when we operate in roundtable mode on SSB. Frequently, a group sets up on a specific frequency, like 3813.000 kHz. If we don't have good accuracy, and if we don't all tune up carefully on one station, we may have to use the RIT every time a different person is speaking. The better our absolute setting accuracy, the less trouble we will have in net operations.

So, how well can we do with a little effort on the Orion? I developed a technique for measuring my frequency error against WWV. (See note below.) My zero beat setting before adjustment was typically about 9.999987 MHz after warmup, 13 Hz low, corresponding to the master oscillator running high by about 1.3 ppm, well within Ten-Tec's spec.

The master oscillator in the Orion is a temperature-compensated crystal oscillator (TCXO) running at 44.55 MHz. It is a Siward series TXO32, apparently, with a mechanical fine adjustment. The following is my record of how I adjusted my oscillator.

Orion bottom
First, remove the bottom cover of the Orion. There are 4 screws on the sides and many little Torx head screws around the rear lip. The top cover can stay in place. After the cover comes off, you are treated to the spacious glory of the Orion underchassis. The TCXO (circled) is on the A10 synthesizer board at the top.


tcxo
This is the temperature compensated crystal oscillator (TCXO)


adjustment
Here we do the actual adjustment. Note the pickup loop for the Icom R-8500 receiver to monitor 44.55 MHz. Fortunately, the TCXO does not require a non-metallic adjustment tool. An ordinary jeweler's screwdriver works fine. The adjustment feels a little coarse if you are trying for exact zerobeat with WWV -- i.e., sub-1 Hz beat. Keep in mind that 1 Hz at 15 MHz is .07 ppm, much finer than the specified oscillator accuracy. Because the oscillator temperature will be substantially less than normal with the case open, you do not want to zerobeat WWV. (See text below.)

R8500
The Icom R-8500 communications receiver was useful to monitor changes to the TCXO. It has its own internal TCXO reference. Such a receiver is not required for the Orion adjustment, but it is a help if available.

sticker
At the end, we put it all back together and give ourselves a professional-looking (?) calibration sticker.


Procedure
Before attempting the adjustment, it is useful to study the warmup characteristic of the Orion TCXO by zero-beating WWV over several hours, as the operating temperature stabilizes. Room ambient temperature was about 78-80 F. Naturally, you need to do this with the transceiver in its operating position with the covers on. Here is a typical warmup run monitoring WWV on 15 MHz.

Time
(min)
FrequencyError
(ppm)
014.999 9801.3
1115.000 0000.0
3814.999 9950.3
7314.999 9890.7
8514.999 9860.9
10114.999 9841.0
13714.999 9831.1
17714.999 9801.3

It would have been interesting to measure temperature along with time, but I did not have a temperature probe. (The A10 board runs pretty hot to the touch, and it is in a poorly ventilated area under the chassis. I would estimate the operating temperature is around 50 C.)

After running the warm-up curve, we note that the TCXO ends up 1.3 ppm high. The dial reading is 1.3 ppm or 20 Hz low. Therefore, we want to trim the TCXO so the dial reading increases by 20 Hz. That should bring the TCXO very close after warmup.

Another issue is which WWV frequency to use: 2.5, 5, 10, 15, or 20 MHz. Other things being equal, the highest frequency gives the best setting precision. One Hz at 20 MHz is .05 ppm. Propagation is a concern, however. We need a strong and steady signal. If the signal has much QSB (fading), it is likely to have a lot of frequency dispersion. You won't be able to find a steady zerobeat. You may want to avoid periods of significant solar or geomagnetic activity for the same reason. (Check http://www.n3kl.org/sun/noaa.html for current data.) For the most part, for my path the 15 MHz transmission worked best. I compared my results at 15 MHz with 10 MHz as a check. They were in good agreement.

Conclusions
The Orion's TCXO can be adjusted to within a few times 0.1 ppm, and it will stay put if the transceiver's operating temperature is stable. Crystal aging is a factor, however, so the calibration may have to be repeated periodically.

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How to Zero Beat the Orion with WWV.

I have tried two methods of finding a precise zerobeat between the Orion's effective local oscillator frequency and a reference frequency transmission.
  1. Zerobeat using CW Spot tone. (easiest for me) Set up for normal CW reception with say 300 Hz bandwidth. Hold down the Spot button and tune until you hear the beat note when the signal frequency equals the spot tone. Again, you should find the zerobeat within +/- 1 Hz.
  2. Zerobeat on noise. If the IF passband offset (PBT) is set for say -100 Hz and the bandwidth is 200-300 Hz, set the AGC to "fast" and the mode to USB or LSB. The Rx audio deemphasis should be zero or positive. The tuning step needs to be 1 Hz. Find the beat note and adjust slowly for lower and lower audio tones. After the tone becomes sub audible (about 60 Hz depending on your speaker), carefully tune to zero. When you are within one or two Hz of zero beat, you should hear the receiver noise fluctuate up and down with the beat. (This is the effect of AGC.) It should be possible to locate the zerobeat to +/- 1 Hz. This method requires a fairly strong reference level.
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Note added (1/8/2006): An alternative procedure is given by K6SE at http://www.n5na.net/download/Orion_Freq_Calibration.pdf .

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