The Well Adjusted Ham
There are better ways to calibrate your SSB transceiver.
As seen in February 2004 QST
By Tom M. Morton, W5TOM
"I'm on exactly 18.142.000 and you are still too high, tune down 10Hz." "No I'm reading 000 on the end, you are too low"...and so on. I am sure we all have heard the frequency dance many times. The advent of accurate transceivers and 1Hz readouts has made us all very conscious of frequency; especially since the "broadcast" sounding audio is also in the game. When a station is transmitting a SSB signal with low frequency content in the 50 to 100Hz range, a VFO change of 10Hz makes a noticeable difference. Much more now than in the past, being exactly on frequency has sent many Hams to their instruction books to find out how to zero in on those zeros at the end of their readouts.
The "Zero Beat" method for calibrating a SSB transceiver is known by all Hams; just tune in WWV and zero beat the carrier in SSB mode. In the old days, this was more than adequate, but not any more. Because most receivers and ears roll off the very low frequencies (below 50Hz), zero can't be heard and is very difficult to find. The problem with the zero beat method is the zero part. This is illustrated in Fig.1. (at the end of the page) Here you can see that the beat signal drops off dramatically as you approach the useful region. At best, under most conditions, this procedure will get you within + or - 20Hz.
A more precise method is the "Sideband Toggle" method. This uses the tones that are modulated on the WWV carrier. This modulation is double sideband AM and the tones appear on both upper and lower sideband equally. The ones we are interested in are the 600 and 500Hz tones that are transmitted on alternate minutes in the WWV hour. With WWV tuned in, you toggle back and forth from upper to lower as fast as possible and try to get the tone to sound the same on both. The tone change you hear is twice the frequency error, which helps your ability to discriminate. This is a good method, but not without it's pit falls. This requires your ear to remember exactly what the tone sounded like a second ago. This may not be so easy; depending on the speed you can toggle your transceiver. Some radios donít allow you to do this at all. There is a variation of this method called the "Dual VFO" method. You tune one 300 or 400Hz above and the other the same below. The offset produces the tone with the carrier, and the WWV modulated tone is not used. The lower the offset you use, the more accurate you will be, however a higher frequency is easier to discriminate with your ear. Now you toggle from one VFO to the other and try to match the tones as in the "Sideband Toggle" method. This is only useable with the modern radios that use the same main oscillator for both VFOs. If the second VFO is separate from the radio, it uses a different oscillator that introduces a new oscillator with no connection to the standard or the oscillator to be calibrated. If persistent, these procedures will get you within + or - 8 to 10 cycles.
These next two methods, as far as I know, are my original ideas. Until someone presents published evidence to the contrary, I will call the next one the "W5TOM Method". Had it not been for KV2AA's need to calibrate his Kenwood TS-690, I would have had the jumper used to calibrate my TS-50. I mailed it to Joe, thinking that it went to an old TS-440 I had sold years ago. Since these radios canít toggle sidebands there was no way to calibrate the radio unless I wanted to try to make a jumper with tiny connectors I didnít have. Along with my coverless TS-50, there was a battery-operated short wave RX on the bench playing the Oldies. It came to me in a flash. I could use WWV's tones to match the frequency. I tuned WWV on the TS-50 with upper sideband (the only choice) and the battery RX to the same WWV signal, but in AM (also the only choice). Frank Sinatra would have to wait. I now had the modulated 600/500Hz tones on both. The tone on the AM RX had to be accurate, since the AM mode uses no local oscillator, but the same tone heard on the 50 changed pitch as I tuned the VFO. I adjusted the volume and put the speakers next to each other noticed that I could beat the two tones together very dramatically. With these methods, the human ear and brain is the discriminating test instrument, for better or worse. But having two tones of the same frequency at the same level and direction coming at you in real time, made that instrument work much better. You have a zero beat setup that isnít zero and it's simultaneous. It is like syncing the engines on a B-24. With the "W5TOM Method" you can achieve + or - 1Hz with greater ease.
I have saved the most accurate methods for last. These require a computer with a couple of audio programs and the ability to send audio to your computer through the sound card. This next one is a variation of the W5TOM method, except it uses a computer based audio oscillator program to generate the 500/600Hz tone that will be used to beat with the WWV's tones. A good one is "NCH Toner" that is available free of charge from <nch.com.au/tonegen/index.html> It is more stable and both generated tones can be sent to a pair of earphones through your computer. After the levels have been adjusted, just simply beat the two tones as in the previous method. Since the tones are the same level and appear in the middle of your head, the sync is very precise and dramatic. I was able to hit it dead on many times in a row with the VFO readout covered with a cloth. The calibration accuracy is exact, and the overall accuracy is that of the oscillator in your computer.
The last method, I call the "Hamalyzer Method". This requires a computer running Hamalyzer, a computer based, software driven, audio spectrum analyzer connected to your radio's audio through the sound card. It is available from <hamalyzer.com> for a nominal fee. You will have to set the following parameters on it: trace=free, FFT size=16384, sample rate=11025, and X range adjusted to present .6K in the middle of the field. With this setup, you will have a resolution of one cycle, a free running real time trace, and a line up the middle that is 600Hz. Tune in WWV on a sideband, and use the minutes in the transmission that modulate a 600Hz tone. You simply adjust your oscillator to put the trace exactly on the 600Hz line. See Fig2. Like shooting fish in a barrel. Although it appears that you are exactly on the money, the FFT engine generates no in between values. This makes your accuracy + or - one half a cycle, and the questionable human ear discriminator can be used for Frank Sinatra.
There is a significant difference between calibrate and adjust. If you calibrate your VFO by one of the previous methods, you determine the readout error of your dial. Let's say you are 23Hz low. That means to be right on, you must set your VFO 23Hz higher to be exactly on frequency. Just knowing this means your radio is now calibrated, however, those pesky numbers on the last digits of your readout are still there. Now you must adjust the readout to put those zeros in their place, and this may be easy or difficult depending on your radio. Some unpleasant time must be spent in the instruction book. Calibration, to be useful must have a "traceable" source of standard. In these cases, that is the WWV transmission, which is accurate within 10 to the minus 9 Hz, when you take distance and atmospherics into account...certainly good enough for any Ham. In the case of the W5TOM (AM receiver) method, Zero Beat method, and Toggle methods, WWV is the only source used. Therefore the traceable accuracy is WWV's accuracy, or 10 to the minus 9 Hz. In the other methods either the oscillator in your computer or sound card is used. Now, the overall accuracy will be that of the least accurate in the mix. Yep, you guessed it; it's the computer and the sound card oscillators. These typically are + or - 30 to 100 parts per million, depending on cost and quality. Even at 100ppm, the accuracy of your 600Hz tone or trace on the analyzer is within + or- 1/100 of a Hz. Certainly of no consequence for what we are doing here, and the traceable standard still is WWV.
The precision of these methods was determined by repeatability trying to calibrate to WWV with the frequency readout covered so the numbers didn't influence the brain used as the discriminator. Each time, after the attempt, the readout was uncovered and the frequency was recorded. This produced the accuracy figures presented. The radio used in these tests was the Ten Tec Pegasus (with integrated Ten Tec R320 receiver) and the computer was a Compaq Presario 1800 laptop running N4PY radio software <firstname.lastname@example.org>, Hamalyzer, and the NCH Toner. The WWV web site < boulder.nist.gov/timefreq/stations/iform.html> will give you the signal and time schedule for their transmissions.
Five years ago, I had a Kachina 505 computer-controlled transceiver that would self calibrate and adjust to within +or- one Hz. Armed with this novelty, and being an AM broadcast fan, I decided to check out some local Houston broadcast stations for frequency accuracy. Most were within a couple of cycles of the assigned frequency, however, KPRC, the Houston, 50kw, blowtorch was 32Hz high in frequency. Just for fun, I called the station and got the engineer on the phone. I told him that he was 32Hz high. He was incredulous and wanted to know how I knew. I told him what I was doing and he politely said he would look in to it. Obviously, from his tone of voice, he thought I was a nut. A few hours later, after lunch and some errands, I checked KPRCís frequency again, and they were dead on frequency. Being well adjusted cannot only eliminate the frequency dance, but it also can be fun.
Acknowledgements: I would like to thank Sam, KT4QW, for beta testing these methods and his help in developing the article. And of course, thanks to Joe, KV2AA, for needing the jumper which started the investigation.
Fig1: This trace was generated while tuning to zero beat from one side. It shows the drop in audio level as the critical point is approached.
Fig2: The trace of the WWV 600Hz tone is shown exactly on the mark. One Hz on either side is very noticeable.