# Tuned Circuit Selectivity

Started by May 5, 2019
```On Monday, 6 May 2019 07:33:42 UTC+1, bitrex  wrote:
> On 5/6/19 12:40 AM, tabbypurr wrote:
> > On Monday, 6 May 2019 03:59:39 UTC+1, bitrex  wrote:
> >> On 5/5/19 10:49 PM, bitrex wrote:
> >>> On 5/5/19 6:13 PM, Cursitor Doom wrote:
> >>>> Gentlemen,
> >>>>
> >>>>
> >>>> I'm just trying a number of combinations of L and C to find the right
> >>>> values for resonance at around 1.35Mhz. The problem I'm having is that
> >>>> the resonance point is far from clear. It's as if the Q of the components
> >>>> is very low (even though they actually aren't). I'm trying to think of a
> >>>> way to make it more 'peaky' on the oscilloscope display to take the guess
> >>>> work out of finding that sweet spot.
> >>>> ATM the two components are in parallel, but I'm thinking maybe I'd have
> >>>> more luck if I wired them in series and increased the Zo of the signal
> >>>> generator by placing a highish value resistor into the genny's central
> >>>> output pin and feeding the tank via that.
> >>>> Would that work or has anyne got any better ideas?
> >>>>
> >>>> CD
> >>>>
> >>>>
> >>>
> >>> 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.
> >>
> >> 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

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

NT
```
```On Sun, 05 May 2019 16:59:19 -0700, John Larkin wrote:

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

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

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

I'd have thought so, yes.

>
> What are your L and C values?

33uH & 385pF

>
> Here's my LC program.

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

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```
```On Sun, 05 May 2019 20:24:11 -0500, amdx wrote:

>   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

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

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```
```On Mon, 06 May 2019 07:28:29 +0000, Jan Panteltje wrote:

> A grid dip meter was a useful instrument long ago:
>  https://en.wikipedia.org/wiki/Grid_dip_oscillator
> Build one once.

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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```
```On Sun, 05 May 2019 15:36:13 -0700, tabbypurr wrote:

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

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

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```
```On Mon, 6 May 2019 13:13:35 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

>On Sun, 05 May 2019 16:59:19 -0700, John Larkin wrote:
>
>> How are you coupling the signal gen into the resonant tank? Are you
>> using a 10x probe on the scope?
>
>Yes, 10x/1x switchable. And directly coupled.
>
>>
>> A Q of 50 should be easy at that frequency, and that would make a very
>> sharp peak.
>
>I'd have thought so, yes.
>
>>
>> What are your L and C values?
>
>33uH & 385pF
>
>>
>> Here's my LC program.
>
>I find it's easiest just to use the full features of a programmable
>scientific calculator, TBH. YMMV of course.

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

```
```On a sunny day (Mon, 6 May 2019 13:13:35 -0000 (UTC)) it happened Cursitor
Doom <curd@notformail.com> wrote in <qapbtv\$52e\$1@dont-email.me>:

>On Sun, 05 May 2019 16:59:19 -0700, John Larkin wrote:
>
>> How are you coupling the signal gen into the resonant tank? Are you
>> using a 10x probe on the scope?
>
>Yes, 10x/1x switchable. And directly coupled.
>
>>
>> A Q of 50 should be easy at that frequency, and that would make a very
>> sharp peak.
>
>I'd have thought so, yes.
>
>>
>> What are your L and C values?
>
>33uH & 385pF
>
>>
>> Here's my LC program.
>
>I find it's easiest just to use the full features of a programmable
>scientific calculator, TBH. YMMV of course.

You have internet access so:
https://www.daycounter.com/Calculators/LC-Resonance-Calculator.phtml
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:
```
```On 5/6/2019 8:15 AM, Cursitor Doom wrote:
> On Sun, 05 May 2019 20:24:11 -0500, amdx wrote:
>
>>    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
>
> Yes, I think this is the area where the problem is.
>
>
>
>
>

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 5/5/2019 5:13 PM, Cursitor Doom wrote:
> Gentlemen,
>
>
> I'm just trying a number of combinations of L and C to find the right
> values for resonance at around 1.35Mhz. The problem I'm having is that
> the resonance point is far from clear. It's as if the Q of the components
> is very low (even though they actually aren't). I'm trying to think of a
> way to make it more 'peaky' on the oscilloscope display to take the guess
> work out of finding that sweet spot.
> ATM the two components are in parallel, but I'm thinking maybe I'd have
> more luck if I wired them in series and increased the Zo of the signal
> generator by placing a highish value resistor into the genny's central
> output pin and feeding the tank via that.
> Would that work or has anyne got any better ideas?
>
> CD

```
```On a sunny day (Mon, 6 May 2019 13:18:12 -0000 (UTC)) it happened Cursitor
Doom <curd@notformail.com> wrote in <qapc6k\$52e\$3@dont-email.me>:

>On Mon, 06 May 2019 07:28:29 +0000, Jan Panteltje wrote:
>
>> A grid dip meter was a useful instrument long ago:
>>  https://en.wikipedia.org/wiki/Grid_dip_oscillator
>> Build one once.
>
>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.

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..
http://panteltje.com/pub/25MHz_220pF_par_LC_IMG_6896.JPG

It is easy.
When scoping it, the few pF scope probe lowers frequency a bit.