Forums

Tuned Circuit Selectivity

Started by Cursitor Doom May 5, 2019
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


-- 
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 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
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
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
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 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.
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
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
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&nbsp; 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.
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