Tuned Circuit Selectivity

Positive feedback works wonders for Q. Just arrange it so the feeding back circuitry doesn't alter the tuning of your LC.

NT

That is going in the wrong direction.

How are you coupling the signal gen into the resonant tank? Are you using a 10x probe on the scope?

A Q of 50 should be easy at that frequency, and that would make a very sharp peak.

What are your L and C values?

Here's my LC program.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

What type of inductor, air, ferrite, rod, toroid, potcore? How much inductance? If your capacitor is small, adding 15pf of scope capacitance will mess with you. It will change your resonant frequency, just how much is the concern. Standard radio values for 1.35MHz would be, 240uh 58pf, adding 15pf from the scope probe is a large change in your resonant frequency. I have some toroids and air caps that could probably get a Q up around

1200, maybe even 1400. Easy to see the peak on a scope, but hard to adjust the generator knob to be right on the peak. You need to lightly couple your input signal to the LC and connect the scope for minimum loading. I've been known to put a 1 Meg resistor before the scope, but often then you get 60Hz interference. Shorten up the leads, Ground lead too. I have always used parallel LC circuits. Mikek

when you calculate the resonant Q_t of a parallel-tuned shunt LC tank you have to include the impedance of the source as well as the load, and the ESR of the inductor, at the resonant frequency, that ESR times Q_u^2 of the inductor, the inductor unloaded Q at the resonant frequency, all in parallel. If you're driving it with a voltage source of too low impedance it's like the "water" you're trying to fill the tank with is draining right back out thru the pipe you're filling it with.

Try inductively coupling the signal in or connect them like this:

feed from a low Z source thru a large DC blocking cap and find the resonant frequency with a dual-trace by seeing where the phase shift flips from -90 degrees to +90

But with an unknown inductor how do u know precisely the inductor unloaded Q at the resonant frequency of the tank if you need to know what the inductor's unloaded Q is at that frequency to calculate precisely what the resonant frequency of the tank is? Yes it's a bit of a conundrum just do your best. It's probably about one hundred and fifty...ah....two.

What?? Measure f_res. Ping it & observe oscillation. Or do it actively with pfb. That approach does have nearly a century of use behind it.

NT

In a real circuit where the inductor has ESR pinging it will give the damped resonant frequency, while driving it will give the driven resonant frequency, which are different.

Usually what you're interested in is the driven resonant frequency but if you want to calculate that exactly _on paper_, for a mystery inductor, you need to know the unloaded Q of the inductor at the driven resonant frequency, but you can't work backwards from the damped resonant frequency response to get it because the damped and driven resonant frequencies aren't exactly the same.

it was a bit of a joke cuz IIRC the frequency discrepancy is only significant for pretty low unloaded-Q inductors like less than 10, maybe.

Anyway I'm thinking about ordering one of those HP5819As "vector impedance analyzer" or whatever from the 80s. that figures all this stuff out automatically. 35 years later they've come down in price a lot all things come to those who wait I guess

that's why I said the unloaded Q of OP's inductor is probably 152. or

150. or probably something, whatever.

Maybe CD would actually try to use a low Q inductor in a tank circuit and then complain that it behaved like a low Q tank circuit what do you think.

On a sunny day (Mon, 6 May 2019 02:33:37 -0400) it happened bitrex wrote in :

A grid dip meter was a useful instrument long ago:

Build one once. Ebay is full of those, from 15$ upwards.... But after winding so many RFcoils I just have some turns and capacitance references in my head you can then find the turns and C for other frequencies easily. Something with square root...

This is a cheap home made LC meter:

If you want real precision, inductors aren't linear anyway, so measure rather than just calculate.

NT

Yes, 10x/1x switchable. And directly coupled.

I'd have thought so, yes.

33uH & 385pF

I find it's easiest just to use the full features of a programmable scientific calculator, TBH. YMMV of course.

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Yes, I think this is the area where the problem is.

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I still have a selection of them and I built one once. NEVER had any luck with *any* of the damn things for some reason! I suspect they're only useful if you are testing really physically large combinations of C and L.

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I believe there must be a simpler solution out there somewhere. Most probably what amdx said.

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Xc = Xl = 292 ohms at 1.4 MHz. If you connect a 50 ohm signal generator across that parallel tank, Q is about 0.16. Not much of a resonant bump. Couple gently from the generator into the tank, with a big resistor or a tiny cap, or just proximity.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

So what type of inductor? Is there a good way to couple to it? Sometimes I just hang a clip lead from the generator near the coil.

Mikek

On a sunny day (Mon, 6 May 2019 13:13:35 -0000 (UTC)) it happened Cursitor Doom wrote in :

You have internet access so:

gives 1.41 MHz for your values,

The lossy part is likely the L, so measure its resistance R. Q = w.L / R = (2 * pi * f * L) / R

3dB bandwidth B = f / Q. If I remember my school days correctly.

Examples:

Are you wanting the loaded or unloaded Q of your network?

On a sunny day (Mon, 6 May 2019 13:18:12 -0000 (UTC)) it happened Cursitor Doom wrote in :

Yes a bit tricky to use, oh what is large, in the few MHz range and lower it works.

With your raspi as signal generator and a simple scope or diode voltmeter you can find out the resonance too, or make the LC oscillate and use a frequency counter. Usually there is some signal, else there would not be an LC, scope it. here a nice 25 MHz parallel LC, tunable, I like that, real silvered wire..

It is easy. When scoping it, the few pF scope probe lowers frequency a bit. Any loading increases bandwidth. This one is just a test to filter out junk from an incoming weak signal, before it becomes permanent on a board.