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# Measuring LF Hz with multimeter

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#### BruceC

##### Newbie level 4
I was about to build a simple RF probe to connect to my old multimeter, but I see where the newer multimeters can measure frequency. But, before I invest in one, I am wondering how good a hand-held meter is for frequency measurements. I am going to build my first simple oscillator and I want to have a way to test whether it is oscillating, etc. I am assuming I can do this by looking at the voltages in the circuit with an RF probe.

This is just a hobby circuit, so I don't need super accuracy. I am trying to build a Colpitts oscillator to give me about 20-50k Hz. Thanks.

There's just no substitute for a scope.

Oscillator circuits like the Colpitts can be difficult to get going. Even if you use a proven schematic. Coils are the type of component that cause you to have to make adjustments around them.

You may get the circuit adjusted to the point where it operates for a few cycles after power is connected. Then a capacitor builds to a charge, yet doesn't discharge, the oscillations die out and the circuit stagnates. Nevertheless you're genuinely close to victory.

Will an ordinary meter show those first few oscillations? Alas, no. You won't know you've almost achieved victory.

I don't have a sound card on my PC, but I have thought about the scope programs. An oscilloscope would be ideal, but not in my budget.

Those hand held meters are relatively accurate both for voltage and frequency measurement, especially at low frequancy. The ones I've used have resonators to set the gate period so they won't be super accurate but at frequencies up to 50KHz they should measure within a few Hz. Another alternative to consider is a simple PIC based counter, there are several designs on the Internet, some measuring as high as 50MHz.

Brian.

BruceC

### BruceC

Points: 2
Those hand held meters are relatively accurate both for voltage and frequency measurement, especially at low frequancy. The ones I've used have resonators to set the gate period so they won't be super accurate but at frequencies up to 50KHz they should measure within a few Hz.

Can you recommend a model or will any do? I have seen the Amprobe brand at Lowes and the Klein brand at Home Depot that both claim to measure frequency.

The PIC based frequency counter is interesting. I hadn't thought of that and I do have a little experience with PICs.

I don't think there's much difference between cheap models, they are all probably the same machine in different disguises. The one I'm using is labeled "Taitan" but it has exactly the same functions as many other models. Try searching on Ebay for "VC99" it will show you a model almost identical to the one I have.

Brian.

I don't think there's much difference between cheap models, they are all probably the same machine in different disguises. Brian.

Good point. I have been reading user manuals for some of the multimeters with frequency capability and I see there are some differences in capability. I am primarily interested in the low end of the scale from 0-200K Hz, and not all go that low.

One question I have is whether you are limited to only measuring low voltage sources for frequency. For instance, I found a manual for the VC99 and it says under frequency measurement:

"The meter can still work if the input is higher than 10Vrms, but the accuracy is not guaranteed"

I have seen similar numbers listed for other multimeters, but I am not sure how to interpret it. I have done some modeling of my circuit in LTSpice and I should expect about 20 volts out of the oscillator (assuming I have modeled it right). So, I wonder if a multimeter can measure a frequency at that level.

Many years ago when I had no money and had to get by without much in the way of test equipment, what I would have done was to use a 10K resistor on the end of my live lead and measure all the DC voltages. Then stick a screwdriver into the coil (this stops the circuit oscillating) and re-measure the voltages. if the figures are different then it was oscillating! Now that solid state diode have been invented, use a germanium diode to earth, a .1 MF coupling capacitor to the live RF and again a 10K to measure the voltage or very low currents (50 micro A), this actually measures the peak RF voltage. With these sorts of circuits it is important that hooking your test gear onto the circuit does not stop it working.
Frank

BruceC

### BruceC

Points: 2
You're showing your age Frank - you must be nearly as old as me!

I remember using your scredriver trick on a coil in a power amplifier stage many years ago. I figured I would see a change in current if I detuned it far enough. Instead, I melted the screwdriver !

BruceC, I would guess the high voltage limit is because above a certain point the waveform will overload the input circuit and the resulting harmonics could give a wrong reading. If the voltage is too high, you can always drop it with a potential divider, the voltage will drop but the frequency should remain the same.

Brian.

BruceC

### BruceC

Points: 2
Many years ago when I had no money and had to get by without much in the way of test equipment, what I would have done was to use a 10K resistor on the end of my live lead and measure all the DC voltages. Then stick a screwdriver into the coil (this stops the circuit oscillating) and re-measure the voltages. if the figures are different then it was oscillating! Now that solid state diode have been invented, use a germanium diode to earth, a .1 MF coupling capacitor to the live RF and again a 10K to measure the voltage or very low currents (50 micro A), this actually measures the peak RF voltage. With these sorts of circuits it is important that hooking your test gear onto the circuit does not stop it working.
Frank

That is an interesting way of doing a test, but I am not sure it would work with my coil given it is a foot in diameter. I was thinking I could just put an AM receiver near my coil to see if it would pick anything up, but I am not sure I could tune it below the AM band. Will an off the shelf AM radio pick up in the 20k-200k band?

I agree that using your diode circuit suggestion would be cheapest, and I have seen several good descriptions of the technique on the web. But then I saw that the newer multimeters were capable of frequency measurements and it wouldn't hurt me to upgrade my old meter. One thing I am still not sure about is what the frequency measurement on the cheap multimeters is indicating. I have seen the use of "Hz%" in a few descriptions, so they may be giving duty cycle and not straight Hz?

Your point about not upsetting the circuit and killing the oscillation during measurement also has me wondering if there is a way to design a measurement point into an RF circuit to prevent that?

---------- Post added at 14:03 ---------- Previous post was at 13:50 ----------

If the voltage is too high, you can always drop it with a potential divider, the voltage will drop but the frequency should remain the same.

Brian.

Yes, I was thinking the same thing. If possible, I would like to isolate my measurement point from the circuit enough that it won't kill the oscillation, so maybe I could put a divider in at the same time. Not that I have any idea how to do it, but then learning electronics is the main point of this exercise.

Given the size of your coil, all you need is a much larger piece of metal, or a short circuit loop of copper wire. The copper wire will alter the frequency a lot, say from 100KHz to 5 MHz, but if your loop gain is high enough it will continue to oscillate. To stop upsetting fragile circuits use a 10:1 scope probe on your counter - 10 M ohms shunted by 10 Pf or less, if it upsets your circuit, redesign the circuit, its not a good design.
Frank

Given the size of your coil, all you need is a much larger piece of metal, or a short circuit loop of copper wire. The copper wire will alter the frequency a lot, say from 100KHz to 5 MHz, but if your loop gain is high enough it will continue to oscillate. To stop upsetting fragile circuits use a 10:1 scope probe on your counter - 10 M ohms shunted by 10 Pf or less, if it upsets your circuit, redesign the circuit, its not a good design.
Frank

Good suggestions, thanks. I am thinking I may just substitute an inductor with known inductance close to what I hope my larger coil inductance should be. Then I can manipulate it a little easier and learn about oscillators before I try for the final design

The scope probe looks like the way to go with measurements. It saved me a lot of time on Google knowing the terminology, thanks.

Wow that's a neat little scope.. Why no FFT

I bought the DSO 062 from Jytech and it's pretty hard to look at.. It's a board level OEM with a tiny LCD screen but it has FFT.

I was about to build a simple RF probe to connect to my old multimeter, but I see where the newer multimeters can measure frequency. But, before I invest in one, I am wondering how good a hand-held meter is for frequency measurements. I am going to build my first simple oscillator and I want to have a way to test whether it is oscillating, etc. I am assuming I can do this by looking at the voltages in the circuit with an RF probe.

This is just a hobby circuit, so I don't need super accuracy. I am trying to build a Colpitts oscillator to give me about 20-50k Hz. Thanks.

Go on google and type in ELF sensor by Steve Rouch, I had one made, this measures hz freqency from 1hz all the way to 1kHZ, if the C1 capaciter in the sensor is taken out, it can measure all the way into 9MghHZ. The Sears model multi meter that measures hz frequency is model #82139 it is only @29.00 It is extremely accurate and really cheap. This is the best multi-meter in the US right now. This sensor was used by a life after death researcher and can be used in that way as well. My unit can measure the 60Hz frequency from almost a mile away from all power lines, that is how good this thing is.

---------- Post added at 21:24 ---------- Previous post was at 21:20 ----------

Sears model # 82139 This is the best there is in the US for measuring Hz frequency, from 1HZ all the way to 9mghz and it is only \$29.00. Go on google and type in ELF sensor by Steve Rouch, all the instructions are there for you to make on. It is a bit challenging if you do not know how to solder. I had to bring it to a electrical engineer to have made.

---------- Post added at 21:26 ---------- Previous post was at 21:24 ----------

Yes, the Sears model # 82139 can. Check out ELF sensor by Steve Rouch on google. His information is outdated on the multi meter, I just gave you the correct one. This measures from 1hz all the way to 9mghz.

BruceC

### BruceC

Points: 2
Thanks, I have been studying the Sears mutimeters and had seen the capabilities of their model 82139. I have also been looking at their model 82344 which also does frequency for around the same price. I will probably end up getting one of these two when I figure out which is more accurate.

The manual on the 82139 says for the 99.99Hz range and below its accuracy is +/-(1.5% reading + 5 digits) and above this range it is +/-(1.2% reading + 2 digits).

The manual for the 82344 says its accuracy from 1MHz down is +/-(1.0% reading + 4 digits) with a sensitivity of 5.0Vrms. I don't understand what the sensitivity parameter means and am looking for an explanation now. No sensitivity is given in the manual for the 82139.

So, I am not sure which is more accurate yet and need to educate myself on interpreting the accuracy statements. I have also seen an Ideal brand multimeter (model 61-312) for about the same price that states its frequency accuracy is +/- 0.3% + 4 digits with a sensitivity of 1V rms from .01Hz-100kHz, 5V rms from 1MHz - 10MHz. So maybe it is more accurate than the Craftsman models?

It would be interesting to test these multimeters with the output of a PC sound card using SoundArb to see how well they do. Have you ever used your Sears meter to test an oscillator circuit?

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