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Old 17th Feb 2009, 9:23 am   #1
jcaines
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Default Measuring Resonant Frequency

I want to check that the approximate resonant frequency of a LC circuit I've got is about 7MHz, it consists of a home made coil in parallel with a variable capacitor, not having anything specific to use, i.e. dip meter, I connected the LC circuit to the output of an Avo oscillator, set it to 7MHz and connected the input of my 'scope to the other side of the circuit, by rotating the variable capacitor I get three definite points, at maximum and minimum capacitance the 'scope trace is maximum and when the capacitor is about halfway the trace goes to more or less a straight line, am I right in assuming this point is the resonant frequency, or am I making some basic errors, would anyone advise please?

John.
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Old 17th Feb 2009, 9:54 am   #2
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Default Re: Measuring Resonant Frequency

Your overall approach is about right but there are errors in your method of its implementation giving you a false result.
At resonance, a parallel LC cct. will give a rise in the voltage developed across it. This is because at resonance the impedance of this cct. will be high. If I read your Post correctly, you have connected the sig. gen. directly across this LC cct. This gen. will have a low impedance output, which will severely reduce the high impedance of the LC cct. at resonance (impedances in parallel). Thus, the voltage developed across it will be small. To prevent this, insert a high-value R (say 100 k-ohms) between the sig. gen. and the LC cct. Although this will reduce the voltage appearing across the LC cct., it will also reduce the shunting effect of the sig. gen. You'll obviously need to increase the gain at the 'scope to compensate. Similarly, the 'scope will reduce the voltage across the LC cct. if connected directly. The best preventative for this is to use a 10X high-impedance probe.

HTH

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Old 17th Feb 2009, 9:58 am   #3
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Default Re: Measuring Resonant Frequency

John

A parallel resonant circuit is a low impedance off resonance and a high impedance at resonance. I would apply the AVO oscillator output across the parallel LC circuit via a 4.7k resistor, to prevent the low output impedance masking the resonant point. The scope probe should be connected across the LC circuit and you should observe a voltage maximum at resonance. The probe capacitance will affect the resonant frequency to some degree.

Ron

Apologies to Skywave for duplicated post.
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Old 17th Feb 2009, 10:17 am   #4
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Default Re: Measuring Resonant Frequency

Thanks for your reply Al, I originally connected the LC directly across the signal generator, but as I didn't seem to be getting any clear results, presumably due to the reasons you and Ron said, I tried it in series, this gave the results I listed, I've attached a very crude sketch showing the layout.
Thanks to you as well Ron, I'll re-run the test both ways to see how it goes, I'm not after anything too accurate, just to make sure the variable cap' is able to tune it.

John
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Old 17th Feb 2009, 11:21 am   #5
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Default Re: Measuring Resonant Frequency

By coincidence I was reading this article last night. It's "Suggested Circuit No. 224" by G.A.French in RC July 1969. If you substitute your LC circuit in place of "inductor" and the 620pF capacitor it should indicate resonance for you. TR1 can be any high gain silicon tranny and D2 any silicon diode, 1N4148,etc.R1 should be chosen to give a collector current of about 4 to 5 mA and R4 selected to limit the meter current to 100uA. Resonance will be seen as a pronounced dip in the current.
Jim
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Old 17th Feb 2009, 12:20 pm   #6
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Arrow Re: Measuring Resonant Frequency

John,

Just for what it's worth, the technique I referred to earlier is a method I have used on many an occasion to determine the res. freq. and bandwidth of IF transformers, 465 kHz & 10.7 MHz, & crystal filters. The 100 k-ohm R is an estimate; some experimentation is usually required.

Al / Skywave
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Old 17th Feb 2009, 12:28 pm   #7
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Default Re: Measuring Resonant Frequency

A simpler way is to wind a two turn coil and connect your signal generator across that. Place it near the coil/cap you want to test. Make another two turn coil and connect to the scope and place this also near the circuit. These will not affect the resonant frequency very much and more important you will not be affecting the Q of your circuit so you should get a sharp maximum on the scope at resonance,
Pat G3IKR
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Old 17th Feb 2009, 12:42 pm   #8
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Default Re: Measuring Resonant Frequency

John,

According to your circuit, you have connected the parallel LC circuit in series with the signal generator's output.

With this connection, the frequency that the 'scope flat-lines is the resonant frequency (because the LC circuit is putting maximum impedance between the sig-gen and the 'scope).

To be honest, I wouldn't have thought that you'd get a very sharp null, because with normal 'scopes, the input impedance is itselt high so you'll need a massively effective LC circuit to seriously attenuate the signal. But, if you shunt the 'scope input with 10 kilohms or so, you may sharpen it up. Don't worry about the 'scope's input capacitance, because it won't affect the null frequency (as it would if you connected the LC circuit as suggested by ronbryan).
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Old 18th Feb 2009, 8:07 am   #9
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Default Re: Measuring Resonant Frequency

Or you could just use one of the many online calculators......

http://www.deephaven.co.uk/lc.html

Not sure what inductance you have?

http://my.athenet.net/~multiplx/cgi-bin/airind.main.cgi


I use these quite a lot, and they seem quite accurate.

Sean
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Old 18th Feb 2009, 10:52 am   #10
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Default Re: Measuring Resonant Frequency

Thank you all for your very helpful suggestions, I've quickly re-run the experiment, I wasn't too far out initially, but as a number of you pointed out I needed a resistor in there somewhere, the output of the signal gen' is now connected across the LC via a 82K resistor, the only one lying on the bench at the time, this as suggested will stop damping the signal gen', and the scope is connected across the resistor which damps the 'scope's input, I've attached another crude sketch showing this, this seems to work very well, with the variable cap' either fully open or fully closed the 'scope reads 2V peak to peak, and with the variable cap' about halfway the 'scope reads 10mV peak to peak, by varying the input frequency I can find resonance at different positions for the variable cap', its quite easy to find this resonant point, I might rig up something more permanent in the future, either a more elaborate version of what I've done or like jim_jobe or oldtimer74 have suggested. Yes Sean, I've used similar sites to the links you posted, there excellent, I always used to get lost with all those 10 to the -12's at tech'.

John
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Old 19th Feb 2009, 9:29 pm   #11
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Lightbulb Re: Measuring Resonant Frequency

Here's the germ of an idea that you might like to investigate to develop your work into a useful piece of test gear.

You have an approximate idea (at least) of the variable capacitor's max. capacity. From this, estimate the mid. value of its capacitance. Now replace the var. cap. (at this setting) with a fixed value cap. of close tolerance and re-measure the resonant freq. (Should be very near to that found previously). Since you now know (a) this freq. and (b) the cap. value, you can calculate the value of the inductor. Now replace the fixed cap. with the variable C. By selecting various freqs. and adjusting the variable cap. for resonance, you can calculate the capacitance of the capacitor for any particular angular setting of this capacitor (since you know the inductance of the coil). Thus, you have calibrated the variable capacitor.

Pop the var. cap. into a box with a scale showing the capacity at various angular settings. With the same test set-up (sig. gen. & 'scope) you can measure, by calculation, the inductance values of a range of coils - always a useful facility.

Al / Skywave.
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Old 20th Feb 2009, 8:28 am   #12
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Default Re: Measuring Resonant Frequency

Quote:
Originally Posted by Skywave View Post
Here's the germ of an idea that you might like to investigate to develop your work into a useful piece of test gear.
That's an interesting little project and something I might do Al, I've already found the circuit very useful, in addition to its initial purpose, which was to check that the var' cap' I had would resonate at the right frequency with the coil I had just wound, I've been using it to check out the value of a couple of chokes, I've got a few 1% silver mica's and using a 100pf and 390pf in turn across the chokes to double check, I measured the two resonant frequencies, putting these values into the site that Sean listed http://www.deephaven.co.uk/lc.html gave me the inductance, but doing it with a calibrated var' cap' as you suggest would make it easier.

John
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Old 20th Feb 2009, 7:28 pm   #13
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Default Re: Measuring Resonant Frequency

I usually use a version of Oldtimer74's approach, it works great even at tank Q above 1500.
2-4 turn coils for both output from gen and input to scope.
this is also good way of measuring Q without loading tank much.
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Old 21st Feb 2009, 12:01 am   #14
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Default Re: Measuring Resonant Frequency

I might add it is a good idea to put a 39ohm resistor in series with the coil on the generator, this to give it bit more like a 50 ohm load at lower frequencies and not just a near dead short.
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Old 21st Feb 2009, 8:31 am   #15
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Default Re: Measuring Resonant Frequency

Quote:
Originally Posted by OErjan_S View Post
I usually use a version of Oldtimer74's approach, it works great even at tank Q above 1500.
2-4 turn coils for both output from gen and input to scope.
this is also good way of measuring Q without loading tank much.
I presume you need to wind a pair of coils to suit the diameter of every coil under test,
if so, its not a big job and they could be kept for future tests.

John
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Old 21st Feb 2009, 10:57 am   #16
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Default Re: Measuring Resonant Frequency

No, one set of coils is enough, diameter is no matter as long as it is in say 20-80mm and frequency is low enough to prevent them being good radiators.
the coils will act as a mini loop antennas both transmitting and receiving, luckily not very good transmitter antennas at 7MHz which is as we want.
with high Q tanks I have had to use above 250mm to any of the loops as they where noticably increasing bandwith otherwise (loading the tank down)
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Old 22nd Feb 2009, 5:17 pm   #17
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Default Re: Measuring Resonant Frequency

I'll have a go with the coils this week and see how I go on.

Thanks

John
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Old 24th Feb 2009, 2:59 pm   #18
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Default Re: Measuring Resonant Frequency

Well I got around to doing the tests as recommended by Oldtimer74 and OErjan_S, using a 4t coil for the 'scope and a 4t coil with 39Ω series resistor for sig' gen', I tested a home wound coil, first with a 100pF and then with a 390pf capacitor in parallel with the coil under test, the resonance peak was sharp on the 'scope and very easy to determine, when I put the capacity and frequency values into the on line calculator I got results of 151μH and 147μH, as a further check I put the coil details, diameter, length and turns into another on line calculator and it gave me an approximate inductance value of 144μH, not bad results for a quick check, certainly an excellent way of doing it,

Thanks

John
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Old 24th Feb 2009, 9:10 pm   #19
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Default Re: Measuring Resonant Frequency

Very interesting discussion.

I recall that back in the 60`s and 70`s there were SigGen G.D.O. adapters
- also some DIY, very simple ones - that provided accurate resonant freq.
measurements and GDO like features.

"GDO-like" adapter Pros: Accurate (SigGen accuracy), stable and constant
calibrated output level. Deep indicator was a Hi Z analog V-Meter.

Con: need to be hub-connected by coax to the SigGen.

IGT
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