For my 70th birthday (I’m now 76), my wife gave me a TenTec Rebel 506 QRP transceiver. The Rebel is built around an Open Source Arduino-compatible controller. Both hardware and software are intended for user customization and I had great fun tweaking it.
The Rebel has a slightly higher than normal sidetone and frequency offset. Since I was not used to this and don’t have perfect pitch, I found it hard to precisely land on the frequency of the station I wanted to work. I needed one of those “tuning eyes” they used to put on radios in the 1950s! Actually, a beat indicator isn’t a new idea in amateur radio, but I at the time I couldn’t seem to find any modern circuits. After considering various options, on the way home from a NAQCC (North American QRP CW Club, http://www.naqcc.info) outing in the Virginia/West Virginia mountains, my friend Russ K3NLT, suggested a tone decoder.
I settled on the tone detector circuit based on the LM567 and described on the website below and built it into my Rebel (see photo above):
The only change was C6; I had to raise the value to get this into the audio frequency range I wanted, which I did by experimenting, ending up with 0.47uf. The LED lights when I have hit the frequency I want, so that is the only output I needed. Here is the schematic.
The unit is calibrated by adjusting R2. You could do this by using a frequency generator or even by ear to match the sidetone. However, I have an audio frequency counter app on my iPhone and that did the trick nicely. Mine seems to be accurate within about 5% or better.
Now, when tuning, the LED flashes when I have arrived at my offset frequency and I am aligned with the other station. No more guessing.
I’ve added these to a number of my rigs (both internally and externally) that don’t have built-in indicators, including a recently-purchased TenTec Century 21 digital. In fact, I sometimes use one with my TS-590sg that does have a tune button designed to match the received signal with the sidetone. The 590 tune button works well IF the CW isn’t too slow and/or you time the push when there is a signal. If not, the tune can go wandering off, causing me to lose the signal completely.
If you don’t want to build your own, or want a more sophisticated model, the “grandson of zero-beat” kit is excellent. It not only provides a visual indication when you are getting close on both sides of the signal but has a simple one-button push to set up the sidetone frequency initially.
(Note: A version of this blog originally appeared in "QRP Quarterly" Fall 2014.