Zero Beat Help & Harmonics
I've been doing research on Dip Meters hoping to put together a comprehensive resource since most of the books on the subject are long out of print. The information is from a variety of sources & compiled over some years.
One of the topics is improving a Dip Meter accuracy using Zero Beating.
I have a few questions.
1. How do you know that you are Zero Beating the fundamental frequency & not a Harmonic?
2. Are you likely to have to worry about Sub-Harmonics? How can you be sure you aren't detecting a Sub-Harmonic?
3. I have seen comments that you can use a Volt Meter for accurately detecting Zero Beat. Anyone know how to do that?
4. Some manuals describe a Dip Meter used with the earphone / headphones & with oscillation turned off as a Tuned Detector, Heterodyne Detector, Absorbtion Meter. What is correct? What is the difference between the various terms.
5. I am including my draft of this section for comment by those familliar with this practice.
More accurate frequency reading may be obtained by checking the Dip Meter?s frequency with a Frequency Counter or by ?Zero Beating? it using a Radio Receiver with accurate calibrations or digital frequency readout, or with an accurate variable frequency source like a Signal Generator.
Alternately, a small coil may be used as a pick up for a frequency counter, or the Dip Meter may be modified by adding a simple buffer amplifier to use for a connection for an external frequency counter, or by fitting a small frequency counter module internally.
Zero Beat. Zero Beating is a method of frequency determination either using a fixed frequency source with a radio receiver to check receiver calibration or a variable frequency source such as a Signal Generator & a tuned detector which is normally a radio although it can be the Dip Meter used in Tuned Detector or Heterodyne Detector mode with Headphones or an Earphone.
As the Radio Receiver is tuned around the fixed frequency source or the Signal Generator frequency is tuned around the fixed radio frequency you will hear a whistle that changes in pitch. As you approach the frequency from one direction, the pitch will decrease. When the two frequencies are exactly the same the pitch will disappear completely, then as you go past the frequency the pitch will increase again. The point where the pitch totally disappears is called the ?Zero Beat.? Hearing Zero Beat will not be possible more accurately than 10Hz at the best so normally you will take note of where the pitch disappears when approaching the frequency from one side, where it disappears approaching from the other side & then split the difference.
It is important to be sure that you are detecting the actual or Fundamental Frequency & not a Harmonic which is a multiple of the actual frequency. You check this by tuning up or down checking for other signals.
With some radio receivers it is possible to see fluctuations of the S Meter close to Zero Beat ? again it is normal to approach the frequency from both sides.
Zero Beating with a Radio Receiver. A radio receiver with accurate frequency readout is used with the frequency directly read from the radio.
1. A radio receiver is placed nearby & the Dip Meter used normally. A short length of wire may be required on the antenna connection but it only needs to be a short length & it doesn?t need to be very close to the Dip Meter.
2. After the Dip Meter has been used, then the receiver is tuned for Zero Beat as per the Zero Beat section above. Although the Dip Meter output will be very weak, the beat note should be clearly heard on the receiver.
3. The degree of coupling to the circuit under test & having other items nearby will affect the Dip Meter frequency so the Dip Meter should be retuned if it is moved or an antenna wire added because the changes may affect its tuning.
Zero Beating using an Accurate Frequency Generator. It may seem strange to say that you can zero beat with a frequency source but a Signal Generator with accurate frequency output may be used. The frequency is read from the Signal Generator.
1. The Signal Generator is placed nearby & the Dip Meter is used normally. The Signal generator normally will not require any connection to its output. Coupling is just by stray capacitance.
2. After the Dip Meter has been used normally, it is left in position & the oscillation turned off to use it as a Heterodyne Detector with Headphones or an Earphone.
3. The Signal Generator is now switched on & tuned for Zero Beat as per the Zero Beat section above. Although the Signal Generator output will be very weak, the beat note should be clearly heard.
4. Although the Signal Generator will not normally require any connection to its output, if the output is too low, then a short length of wire may be connected to the Signal Output terminal (not earth). This will normally be the centre pin if a coaxial connector is used. The wire does not have to be close to the Dip Meter.
5. The degree of coupling to the circuit under test & having other items nearby will affect the Dip Meter frequency so the Dip Meter should be retuned if it is moved or the wire is added because the changes may affect its tuning.
I was making and have used various Dip Meters for various purposes.
THe tuned circuit o the Dip Meter oscillator is open and therefore using the Zero Beat techniques to exactly find the frequency is in my opinion not impossible but very flawed.
A RF oscillator with the open tuned circuit generates a suitable signal to find resonance in other coupled circuits, to estimate inductance and capacitance when the other kind is known. One can test receivers, transmitters and other oscillators and use the Dip Meter as a detector to hear zero beats and estimate distortion in RF modulators, etc.
The open resonant circuit frequency is easy affected by hand and other objects around, so using such device to measure EXACTLY its frequency is rally a bad idea. Try for yourself; no way to explain this by writing.
If you need to EXACTLY measure a RF frequency, use a counter. Some Dip Oscillators were designed with a digital frequency display but a good and reliable frequency source must use a well designed and stable resonant circuit. Try to look inside of good signal generator.
This is why the Dip Oscillators are favored by radio amateurs who can use it well for various purposes, by training the operator how to handle it. No writing can explain why and how, one ,must simply try it to see how it really works.
Hi,
Thanks for the reply. You are right that I really wasn't thinking about the inherent inaccuracies with the dip meter when I put that draft together. My other notes specified that a Frequency Counter is by far the preferred option but many sources state the Zero Beating as an option so I put that section together. I should have added some of the notes from the Frequency Counter Section in the beginning & made a greater emphasis on inaccuracies & practical problems of the method.
I do have a couple of Dip Meters that I have used but I'm not a HAM & so I don't do much higher frequency work.
I have been amazed at some of the troubleshooting tricks I have een over the years. I watched one tech who often troubleshooted using a couple of different analog multimeters by using their impedance on different ranges as a troubleshooting tool. It really was amazing to watch.
I would still like info on Zero Beating because I think it a handy tool to keep in reserve for other purposes.
How do you ensure that you aren't on a harmonic?
Are sub-harmonics ever much of a problem?
Thanks,
Brian.
Dear Brian:
I am glad that you have a certain experience with Dip Meters and other instruments.
Concerning the Zero-Beat principle as a tool to determine a frequency, any two signals injected to a non-linear element (mixer , detector, oscillator) can generate an audio beat. Therefore some Dip Meters have an earphone plug and one can listen to the output when the device detects a modulated signal, or, the beat if another signal source comes at a frequency coincides with that of the oscillating Dip Meter.
In my experience, the open resonant circuit of the Dip Meter is strongly affected by hand and other object movement. Therefore one can use this effect for a "fine tuning" but then the accuracy of the dial showing the frequency is poor. If you have a quartz-stabilized frequency from another signal source, then you can check the dial accuracy of the Dip Meter.
People often have 100 kHz or 1 or 10 MHz quartz oscillator for such purpose. Then only one's experience is used in being sure which harmonic is being detected. Usually one has a calibrated dial and the zero-beat is only used to verify there was no significant shift of Dip meter frequency against the dial indication.
Subharmonics may be a problem in poorly designed oscillators. Some quartz resonators are "overtone", operated on harmonics, mainly above 20-30 MHz.
Modern instrumentation like frequency counters, spectrum analyzers and oscilloscopes are costly but require less experience in RF design to get good results.
Good luck, Jiri