Zero Beating - Keynote # 6, 1998
I have had several questions asking me how to zero beat signals with a particular QRP kit. Well, my quick answer is "I don't know." WAIT! Don't stop reading. I am going to try to give you enough information so that you can figure it out. Every kit has a slightly different method of accomplishing a good zero beat, so I can't just say do this or do that. That might work with one rig, but not another.
I am going to approach this article the same way as I did my power measurement article last month. That is, I'm going to try to keep it neither too simple nor too complicated.
The term zero beat comes from the fact that two signals mixing together will create a beat tone equal to the difference in frequency between them (and for the purists, a sum frequency also, but we are not interested in that). For example if you mix together signals of 7,058,000 Hz and 7,058,800 Hz you will get a third signal equal to 7,058,800-7,058,000 or 800 Hz which is in the audio range and hence can be heard. This is the only way a pure CW signal is made to be heard since it carries no modulation of its own (unless it's MCW, has a bad hum, etc.,). Although most all receivers convert the incoming signal to one or more intermediate frequencies before finally converting the signal to one in the audio range, I am going to describe direct conversion from the received signal to audio. The principle is still the same whether it's done at 7058 kHz, 455 kHz, or any other IF.
Back now to the zero in zero beat. When two signals are on exactly the same frequency, the difference is 0, hence zero beat. In a receiver or transceiver, there is an oscillator called a BFO or beat frequency oscillator. The output from this oscillator is mixed with the incoming signal to produce the audio frequency. For example, if we are trying to hear a CW signal on 7,058,000 kHz, our BFO will be tuned to, say 7,058,800. The difference between the two will produce an audio frequency of 800 Hz.
Originally, before transceivers, when separate receivers and transmitters were used exclusively, one method of zero beating a signal was to temporarily turn off the BFO in the receiver, and use the VFO in the transmitter as a BFO. You would tune the VFO until an audio tone was heard, then keep tuning carefully as the tone got lower and finally became zero and inaudible. Presto, your VFO is now set to the exact frequency of the station you are listening to. You turn the receiver BFO back on and call the station.
Nowadays, in many cases, it is either not possible or convenient to turn the BFO on and off. For separate receivers and transmitters, it is simplest to just turn on the VFO and tune so that the audio tone it produces is the same as the one being produced by the received signal. This will not produce a perfect zero beat, but unless you are totally tone deaf, you can get within a few dozen Hz or so. Of course if you are a musician with perfect pitch, you can do even better.
OK, now down to the real nitty-gritty, transceivers. When you use a transceiver, the VFO and the BFO are generally the same. When you are receiving that signal on 7,058,000 then the oscillator is acting as a BFO tuned to 7,058,800, and when the transceiver is keyed to transmit, the oscillator is offset, or switched 800 Hz lower in frequency so you transmit on 7,058,000 instead of 7,058,800.
So, to zero beat a signal in this case (an 800 Hz offset), you tune in the signal until it produces an audio tone of 800 Hz, and you will then automatically be zero beat when you transmit. Now this 800 Hz is not standard by any means, so you must read the kit manual or contact the manufacturer to find out what they use for this offset frequency, be it 700 Hz, 1000 Hz, etc. That is the figure you use when tuning in a signal on your particular transceiver.
Next question, how do I know when I'm hearing an 800 Hz tone. Well, if you have perfect pitch, I guess it's simple. For those of use who don't, we can settle on being fairly close, or use a frequency counter or an oscilloscope to measure the audio frequency. What happens if your offset is 800 Hz, and you tune for a tone of 600 Hz? Well, the signal you are receiving is 7,058,000. With that 600 Hz. tone, the BFO/VFO is set to 7,058,600, and when you transmit, the 800 Hz offset will take it down to 7,057,800 and you will wind up being 200 Hz lower than the station you are working.
Some transceivers that use a sidetone for monitoring keying set the frequency of the sidetone to the same as the offset frequency. So if you can get used to adjusting the tone from the received signal so it sounds the same as the sidetone, that will ensure a zero beat.
Please, if you have any tips for zero beating with your particular rig, let me know, and I'll share them here in the column. Zero beating is especially important for the QRPer. It can often make the difference between being heard and not being noticed at all. I have only used one transceiver, the QRP+, and do not have any manuals from other transceivers, so my knowledge is limited.
Don't forget that past editions of my column are available on my web site at http://home.windstream.net/johnshan/. My land mail address is 478 E. High St., Kittanning, PA 16201-1304. -30-