A Couple of Methods for Adjusting your Rig’s
Frequency Accuracy without any Test Equipment
By W2AEW, 2002
Most of today’s modern, synthesized rigs use a single master oscillator to control the frequency of operation. It is very common for this master oscillator to be 20MHz. This frequency will be used as an example throughout this text. However, the ideas presented here apply equally well even if the master oscillator is a different base frequency.
The accuracy of this oscillator determines the frequency accuracy of the rig. The accuracy of the master oscillator is usually expressed in parts per million (ppm). For example, if the 20MHz oscillator in a rig has an accuracy specification of 10ppm, that means that it can be off by as much as 200Hz (20MHz*10ppm). It is important to note that this relative accuracy also applies to the synthesized frequency. For example, if your rig is tuned to 14.200MHz and the reference oscillator has 10ppm accuracy, then the rig could be off by as much as 142Hz. If you are tuned to 3.800MHz, then the rig could be off by 38Hz. You can easily see that the amount that the rig can be off depends upon the operating frequency – the higher the frequency, the more the rig could be off, and the lower the frequency, the less the rig could be off. This is why you might find that your rig may be off by 100Hz on 10 meters, but only off by 50Hz on 20 meters. The accuracy is relative to the operating frequency. We’ll use this to our advantage a little later – stay “tuned”.
The frequency “standard” that we will use is readily available to nearly all amateurs – the WWV time broadcasts. These stations broadcast on 2.5, 5.0, 10.0, 15.0, and 20.0MHz. The broadcast frequencies are very accurate. The only other “instrument” used is your ears!
This method takes advantage of a couple of facts: the WWV broadcasts are in AM, and there is a ~500Hz audio tone transmitted in the background during the first 40 seconds or so following the time announcement. The AM signal is double-sideband. This means that the modulation signal appears in both the upper and lower sideband.
The method involves first tuning in WWV on the highest frequency where it can be heard. This means tuning in at 10, 15 or 20MHz – use the highest one that you can hear depending upon propagation conditions. The reason that we want to use the highest receivable frequency is because the frequency error will be the greatest and easiest to detect.
Once tuned into WWV, switch to USB. Listen for the 500Hz tone that is transmitted in the background for the first 30-40 seconds or so after the time announcement. While listening, switch to LSB, and back and forth. If your master oscillator is accurate, there should be no difference in tone/pitch between listening to the signal on USB or LSB. However, if you hear a pitch change, this means that your oscillator is a little off frequency.
Don’t panic though, it may not be that bad. Let’s say you are tuned to 20MHz WWV, and you hear a difference in pitch of the 500Hz tone that is about 20Hz (510Hz on USB and 490Hz on LSB). This amount of frequency difference is moderately noticeable to hear. Assuming your master oscillator is 20MHz, this means that it is off by 10Hz – which is only 0.5ppm. This kind of accuracy is very good. However, if you are hearing a pitch change of 200Hz – very easy to hear – then the oscillator would be off by 100Hz, or about 5ppm. You can usually adjust this out.
Your ear can detect very small changes in pitch, particularly at low audio frequencies. The adjustment method is to simply adjust the master oscillator and repetitively switch between USB and LSB until you can’t detect a pitch change between the different sidebands. It is best to do this after the rig has warmed up so that any thermal drift has already stabilized.
The method for adjusting the oscillator, via a trimmer capacitor, menu setting, etc., will vary from rig to rig. Check the manual for your particular rig to find out how to make the adjustment.
This method also involves comparing two audio tones, but not the transmitted audio tone. This method is best suited for rigs that feature dual VFOs. The idea is to listen on sideband, and intentionally tune off frequency so that the WWV carrier creates an audible heterodyne.
Lets use WWV on 20Mhz and a heterodyne frequency of 300Hz as an example. Tune one VFO to 20.000.300MHz and set it to LSB, and tune the other VFO to 19.999.700Mhz and set it to USB. You should hear the 300Hz heterodyne on each VFO. Now, switch back and forth between the two VFOs. If the heterodyne tone pitch is the same, then the master oscillator is on frequency. If you hear a pitch difference, then the oscillator is off. As in Method 1, the pitch change that you hear is 2x the frequency inaccuracy. Simply adjust the master oscillator until the heterodyne pitch remains the same between the two VFOs.
The advantages to this second method are that you can change the tone that you use by simply changing your “offset” for each VFO. If you’d like to use 400Hz, then tune the VFOs to WWV+400Hz and WWV-400Hz respectively. The key is to use the lowest audio frequency you can. This is because the frequency error will be a larger proportion of the tone. For example, it would be a lot easier to hear a 10Hz variation on a 400Hz tone than it would be to hear the same variation on a 1500Hz tone!
Using nothing but your ears, it is easy to adjust the master reference oscillator in your rig to an accuracy of 1ppm or better by simply taking advantage of the WWV broadcasts. Use the highest WWV frequency you can hear to make the inaccuracy more pronounced. By using USB and LSB reception, you essentially down-convert the master oscillator frequency into the audio band, where you can “hear” the inaccuracy and adjust it out.
I didn’t invent these methods – they are simply taking advantage of the features of the WWV signals and your ears!