Question on Q of toroid coil

You can often tell by looking at the function of the resistor. For example, R1 and R3 set the bias voltage of V1 at approximately VCC/2. They are labeled 4k7, so that means 4.7k. That makes sense since the network has to supply the base current for V1.

BTW, you might need a 50 ohm termination at the input when working at higher frequencies, say >10MHz.

Resistor R15 grounds the gate of the jfet, V4. Here, the leakage current is very low. It is labeled 4m7, so a 4.7meg resistor would be appropriate.

So he uses 4k7 to mean 4.7k, and 4m7 to mean 4.7megohm. That should help identify any other components in question.

Yes, it does look very nice. I'd do the low impedance driver made up of V1, V2, and V3 a bit differently, but it should work the way it is shown. You might want to put that part in LTspice and see how it can be improved. You want the lowest possible driver source impedance.

I particularly like the calibration method. It should help to minimize the effect of unavoidable gain variations from one end of the frequency range to the other.

Regards,

Mike Monett

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I thought I had this written up in a word file but couldn't find it, probably lost in a harddrive crash or upgrade. First you need to resonate the inductor at the frequency you want to know the Q. You should use a good low loss capacitor, I usually use polystyrene. This method uses the difference between the upper and lower 3db points. The less loading of you resonate circuit the more accurate the result. Let's use a 10uh inductor and a 1000pf capacitor as a parallel resonate circuit. This should resonate at about 1.59Mhz, however when you put the (X10) scope probe across the circuit it will slightly change the resonance and loading. {More on that later.} You need to lightly couple some energy from your signal generator into the resonate circuit. This can be done by placing the generator wires near the resonate circuit close enough to get the scope signal level you need but as far away as possible so you don't load the circuit. You can couple it with high value resistors if desired, but this increases loading on the inductor.

Then I adjust coupling and signal generator output to get 7 units on the scope. Why 7 units? I'm glad you ask! You want to move the signal generator frequency up until the voltage on the scope drops to 5 units.

Some explanation; We want to measure the upper and lower frequency points where the voltage drop is 3db or .707 times the resonate voltage. So, back to the 7 units, .707 times 7 units equals 4.949 units or 5 units when I'm looking at my scope.

So we adjust the signal generator frequency to peak the waveform on the scope. Let's say the waveform peaks at 1,596,200hz. To get the 7 units sometimes I adjust the generator drive and sometimes I change the scope variable attenuator.

I move the frequency up until the scope reads 5 units, Record this frequency. Lets say it's 1,600,200hz Now move the frequency down until the scope reads 5 units, Record this frequency. Lets say this is 1,592,219hz Do the math 1,600,200 - 1,592,219 = 7981 then using the resonate frequency of 1,596,200 / 7981 = 200 The Q of your inductor is 200

{More on that later.} Ok this is later, I have been known to isolate the scope probe with a

100k or 1meg ohm resistor. This helps reduce the loading on the resonate circuit, But this can also induce some 60hz into the scope wave form. I think I got this all correct, it has been a few years since I did this. I used it a lot when I was trying methods to improve the Q of some potcore inductors. Let me know if you have questions. Mike

Wow. Totally cool! Thank you very much, Mike. This is going to be a most engrossing experiment. Thank you, thank you, very much.

Dave

I came to the same conclusion, RE: 4k7 and 4m7 labeled resistors, but the rest I hadn't gotten to yet. Thank you.

This is definetly going to help...

Dave

Hi Dave, Something I always wanted to do but never go to, is build a board to fix things in position. Always seemed to get the info I wanted just laying things on the bench. A board should be made of low loss material. It should have pins to solder the capacitor and pins to solder the inductor to. I might add a permanent air variable capacitor to fine tune the resonate frequency. It would need an input for the signal generator. Maybe two parallel lines to couple signal into the circuit. One of these lines would run from the cap to the inductor, the other would be from the signal generator. Hmm, Just thinking out loud. Let me know when you try this. Mike PS. Proud owner of a Boonton 260A Q Meter

amdx ha scritto:

Due to frequency drift a good idea would be to use a variable high Q air capacitor in series with the inductance to fine tune the LC tank to resonance.

Cheers

Charles

Hey Mike,

I have a couple of questions on the proceedure you outline below for calculating the (approximate) Q of a coil. In your example you used a fixed value capacitor along with the coil under test, and resonate these at the appropriate frequency (1.59 MHz in your example.) Once that is done, you move the frequency from the signal generator first up from resonance to get the lower reading on the Oscope (.707 x the original voltage level displayed on the scope, or five units rather than seven), and then down from resonance for the same purpose, jotting down the starting and stopping frequencies each time. Then, subtracting the higher frequency from the lower provides a value which is noted. The Q of the coil is found by dividing the resonant frequency by the difference between the two formar values (the previously mentioned "noted value.") In my case, I ran the signal from the generator into the actual circuit I am using, which consists of a variable cap and a fixed value inductor. Bottom line is, 4.0 MHz to 4.5 MHz minus 4.0 to 3.5 MHz is 1 MHz. Then dividing 4.0 by 1.0 gives me .25. This indicates that the coil under test has a Q of 25. Is that right? Am I doing this correctly?

The coil involved is an Amidon T-50-2 toroidal core, wound with #36 wire, and I expected something more along the lines of a Q around 200, as indicated was supposedly typical for this frequency according to their catalogue. Now, granted, this is a lot better than I would have gotten with my original coil, which consisted of #24 wire wound around a toilet paper core, but it is still a little disappointing. If I am doing this right, do you have any ideas on how to increase the Q of this coil?

Many thanks for your help in the original posting. Without that, I would be a lot more lost than I already am. :)

Cheerfully yours,

Dave

Nevermind. After a minute considering the numers, I see that I had a Q of

1. Also figured out that my freq counter was loading down the circuit something terrible. The addition of a 100K resister helped that, and I now come up with: 6.4 MHz - 6.1 MHz and 6.4 MHz - 6.3 MHz equals a Q of 16, and the numbers are still somewhat suspect. All of this is spit and bailing wire, but the latest results seem more reliable (at least to me.) Suspect I need to rewind the coils with more attention to spacing, and resolder all of the connections. I *know* the Q has been much higher because at one point I had to add a fine tuning knob to the device after realizing that those little clicks and pops weren't static, but were shortwave stations going by at the speed of light. The fine tuning knob allowed the addition or subtraction of capacitance one pF at a time and allowed me to pull in very, very weak stations. Not doing that now though, so something has changed. Much has been done since that time, so I have a lot of solder joints to re-work. Still, it's looking better.

Thanks again for your original post. If you don't see this and reply I'll email you and report, in view of your request to let you know when I tried to put all of this into action.

Regards,

Dave

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Just as a side comment, Q about 200 seems not unreasonable for this coil. A rewind/altered spacing should make little difference to that value . It seems your tuned circuit is somehow being loaded with about 9kohm across it, giving the measured Q=16 value. (either that, or the coil resistance is reading about 30ohms!). If your only loading is a 100k resistor feeding the circuit and a 1Mohm scope across it, then you should be able to see upto Q=150. Sure that 100k isn't a 10k?.

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The highest Q calculattion I was able to obtain was 32, and the most frequently repeated value was on the order of 1 or less. I don't believe the coil resistance is anything like 30 ohms, but it now occurs to me that a poor solder connection with the very fine wires I am using might look something like this. And the 100 K resistor is actually 89K, but it is not limiting the feed, it is limiting the load that the freq counter puts on the circuit. Should Ihave another 100K resistor feeding the circuit? What about a 1 Mohm,or a 1.8 Mohm? One more thing: I am using surplus (used) switches to switch between the inductors, and I now wonder if they are the problem.

Thanks for the feedback, and the ideas. Your input is much appreciated.

Dave

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The setup should be similar to this ...

89k, 100k, 1Meg etc ___ o----------o------|___|------o-------o------, | | | | | | | | | | .------o-. .-----o-. | | | .--------. | o/p| | i/p| || | | | .-----.| | | | | C| --- | | | || | | | | C| --- | | |-----|| |Sig-gen | |Counter| C| | | | '--- -'| | | | | | | | | -Scope-| | | | | | | o---o i/p | | 0V| | 0V| | | o---o 0V | '------o-' '-----o-' | | | '--------' | | | | | o----------o-----------------o-------o------' Tuned circuit of interest

(created by AACircuit v1.28 beta 10/06/04

I'm seeing a small part of another thread about a frequency counter, so will wait to see how that pans out. john

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