Tuned Circuit Selectivity

[Cursitor Doom]

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


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This message may be freely reproduced without limit or charge only via
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protocols constitutes acceptance of this condition.

 

On Sunday, 5 May 2019 23:13:38 UTC+1, 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

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


NT

 

[John Larkin]

On Sun, 5 May 2019 22:13:25 -0000 (UTC), Cursitor Doom
<curd@notformail.com> 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.

That is going in the wrong direction.

Would that work or has anyne got any better ideas?

CD

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.

https://www.dropbox.com/s/sghfwz72kehcnoj/LC7.EXE?dl=0

https://www.dropbox.com/s/v3yt0in63pm9bex/LC7.txt?dl=0



--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[amdx]

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 anyone got any better ideas?

CD

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

 

[bitrex]

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.

 

[bitrex]

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. 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:

<https://en.wikipedia.org/wiki/RLC_circuit#/media/File:RLC_parallel_band-stop.svg>

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

 

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

 

[bitrex]

On 5/6/19 12:40 AM, tabbypurr@gmail.com 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

 

[bitrex]

On 5/6/19 2:33 AM, bitrex wrote:

On 5/6/19 12:40 AM, tabbypurr@gmail.com 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.

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.

 

[Jan Panteltje]

On a sunny day (Mon, 6 May 2019 02:33:37 -0400) it happened bitrex
<user@example.net> wrote in <6zQzE.1130936$VB3.116246@fx45.iad>:

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

A grid dip meter was a useful instrument long ago:
https://en.wikipedia.org/wiki/Grid_dip_oscillator
Build one once.
Ebay is full of those, from 15$ upwards....
https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313.TR0.TRC0.A0.H0.Xgrid+dip+meter.TRS3&_nkw=grid+dip+meter&_sacat=0
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:
http://panteltje.com/panteltje/pic/lc_pic/index.html

 

[Cursitor Doom]

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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[Cursitor Doom]

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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protocols constitutes acceptance of this condition.

 

[Cursitor Doom]

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
the leads, Ground lead too.

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





--
This message may be freely reproduced without limit or charge only via
the Usenet protocol. Reproduction in whole or part through other
protocols, whether for profit or not, is conditional upon a charge of
GBP10.00 per reproduction. Publication in this manner via non-Usenet
protocols constitutes acceptance of this condition.

 

[Cursitor Doom]

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.



--
This message may be freely reproduced without limit or charge only via
the Usenet protocol. Reproduction in whole or part through other
protocols, whether for profit or not, is conditional upon a charge of
GBP10.00 per reproduction. Publication in this manner via non-Usenet
protocols constitutes acceptance of this condition.

 

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

 

[Jan Panteltje]

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.
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.

 

[John S]

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

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

 

[Jan Panteltje]

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:
http://physics.usask.ca/~angie/ep311/lab4.htm

 

[amdx]

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
the leads, Ground lead too.

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

 

[John Larkin]

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