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Old 18th Mar 2017, 11:16 am   #21
kalee20
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Default Re: Permeability Tuning

If the loss resistance is independent of frequency - ie mostly the DC resistance of the coil, then:

Q = (1/R)[√(L/C)]

We also have:

Q = fres / BW (bandwidth)

and fres = 1/[2π√(LC)].

Putting these together, we have:

BW = R/(2πL)

Thus for constant bandwidth, you should vary C because bandwidth is independent of C!

In practice, as Al Skywave says, DC resistance is not the whole story, there is some parallel loss resistance as well as frequency-dependent series resistance. But this analysis is revealing, nonetheless!
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Old 18th Mar 2017, 11:53 am   #22
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Default Re: Permeability Tuning

Just as an aside, the problems associated with varying the resonant frequency of a tuned circuit (i.e. the variables) point to the main advantage of the superheterodyne set, namely that it's the I.F amplifier (a fixed frequency amplifier) that determines most of the important characteristics of a receiver.

The design of the I.F. amplifier is greatly simplified by the fact that it a fixed frequency amplifier. The designer has much greater control over things like gain, bandwidth, bandpass shape and stability etc.
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Old 18th Mar 2017, 1:17 pm   #23
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Default Re: Permeability Tuning

Indeed it is. The requirement for selectivity in the rF path is greatly eased, but not eliminated.

Considering a classic entertainment LW/MW/SW banded radio; on longwave the RF stage selectivity needs to be kept wide enough that it doesn't limit the channel bandwidth more than the IF does, or you get a peaky response in the audio. On this band the RF frequency is less than half that of the IF, so it is easy to get it narrow. And with two tuned circuits which are narrow, alignment and tracking difficulty is exaggerated

On medium wave, you need enough selectivity to block signals at the image frequency, usually around 910kHz higher. This isn't too onerous.

On shortwave, that 910kHz image is a lot closer, in ratiometric terms, and the filter narrowness available for a given Q of tuned circuit has got a lot wider. Rejection of the image has become poor, and gets worse up the frequency range. The classic shortwave sets like the AR88, HRO and CR100 were well mad and designed to do about the best feasible at the time, and they barely got 3dB attenuation of their image frequency at the top of their tuning range.

Tuned stages can be done with either permeability or capacitor tuning and similar bandwidth results obtained, if designed appropriately. Confusion is created because it isn't simply the Q of the naked esonator which counts, it is the in-circuit Q. This is carefully planned in the design of filtering, and the Q is managed by controlling the coupling factors into the impedances of preceding and subsequent stages. Capacitor coupling is popular through cheapness, but it gives tighter coupling at high frequencies and this gives lower Qs. A filter maintaining constant Q would get wider proportionately to frequency (simple scaling) But a filter with 'top capacitor' coupling couples stronger at higher frequency and gets even wider than scaling would predict as frequency goes up.

A filter containing 'Top Inductor' coupling reduces coupling factors ar higher frequencies and tries to combat the widening through scaling effect. These can be ade to have a much more uniform bandwidth across a tuning range.

Other coupling methods can be used such as taps in inductors, or tapped capacitance which are closer to simple scaled variation in bandwidth.

The choice of coupling also determines whether a filter rolls off faster on the top side or the bottom side. So even if you're doing fixed-tuned coupled-resonator filters , you want to be choosy about what sort of coupling yo wish to use. Would you like symmetrical skirts, or would you like it to fall faster on one side to dodge a problem frequency?... even if you want symmetrical skirts, do you want them symmetrical on a linear frequency plot or on a logarithmic one?

There's a whole menu to choose from, but it isn't explained in its entirity anywhere very visible. Most computer programs for generating filters only do the very basics. There's a lot more buried in 'Zverev' but that isn't easy reading.

So the way permeability tuners are implemented often is good for more constant bandwidth. But it isn't implicit in permeability tuning and it can be done with tuning capacitors too, if you're not penny-pinching.

An advantage to permeability tuning that is rarely mentioned is the avoidance of the sliding contacts on tuning capacitor rotors.

David
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Old 18th Mar 2017, 7:59 pm   #24
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Default Re: Permeability Tuning

From what I remember from college days back in the 60's Permeability Tuning was done in ( mostly ) car radios to make the components smaller. No need for a massive variable ganged air finned capacitor, when a smaller solid non variable would do the job, with the tuning done by coil variations and in a smaller space. Least ways that's what I remember, possibly modulated by several shocks from higher voltage kit.
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Old 19th Mar 2017, 8:56 am   #25
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Question Re: Permeability Tuning

Quote:
Originally Posted by Radio Wrangler View Post
The classic short-wave sets like the AR88, HRO and CR100 were well-made and designed to do about the best feasible at the time but they barely got 3 dB attenuation of their image frequency at the top of their tuning range.
"3 dB" attenuation of the image? Over the years, I've owned and restored all of those sets (and many others similarly with two tunable R.F. stages) and upon measurement have found the image rejection at 28 - 30 MHz to be a lot better than -3 dB. So was that a typo?

Al.
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Old 19th Mar 2017, 9:04 am   #26
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Default Re: Permeability Tuning

Typo. Should have written 30dB sorry

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Old 20th Mar 2017, 1:16 am   #27
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Arrow Re: Permeability Tuning

O.K. David: no need to apologise, we all make typos now and again. But your remark did cause me to do a bit of a double-take though! Anyway, no harm done.

Cheers,
Al.
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Old 20th Mar 2017, 8:12 pm   #28
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Arrow Re: Permeability Tuning

Quote:
Originally Posted by kalee20 View Post
If the loss resistance is independent of frequency - ie mostly the DC resistance of the coil, then:

Q = (1/R)[√(L/C)]

We also have:

Q = fres / BW (bandwidth)

and fres = 1/[2π√(LC)].

Putting these together, we have:

BW = R/(2πL)

Thus for constant bandwidth, you should vary C because bandwidth is independent of C!

This analysis is revealing, nonetheless!
"Revealing": indeed it is - and in all my years of fooling around with circuits (and studying RF theory), I've never seen that analysis before. Now not wishing to suggest for one moment that I had any doubts in your algebra, but out of sheer curiosity, I did the sums myself. For those who are interested, they go like this . . .

Q = [1/R].[√(L/C)], from which we get: C = L/(Q.R) . . . . . eqn. 1.
fo = 1/[2π√(L.C)], from which we get: C = 1/(4π.L.fo) . . . . . eqn. 2.

Equate the values of C from eqns. 1 & 2:
i.e.: L/(Q.R) = 1/(4π.L.fo), and simplify.

We get:
fo = (Q.R)/(4π.L).
So: (fo/Q) = R/(4π.L).

But: (fo/Q) = (BW), where BW = bandwidth, by definition.

Hence: BW = R/2π.L . . . . as derived and shown by kalee20.
And, moreover, it is independent of C.

My further thoughts . . . .
The 'problem' there is that that result implies that in an LC cct., L should be kept constant and C made the variable. (So much for the benefits of permeability tuning ). However, the fly in the ointment with a coupled LC cct. used for tuning is the loading effect of the primary on the secondary and that loading will affect the BW of that LC cct. (Usually to reduce the BW). In other words, the 'R' in Q = [1/R].[√(L/C)] needs to be taken fully into account and that R - as a consequence of the effect of the primary - will be frequency dependent, since the effective degree of the coupling will be also. And that tells us why in most radios where the aerial input coil is quite loosely coupled to the first tuned cct., why that coupling is usually quite low.

Al.
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Last edited by Skywave; 20th Mar 2017 at 8:17 pm.
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Old 21st Mar 2017, 12:05 am   #29
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Default Re: Permeability Tuning

I still think something is being missed. In an efficient filter the bandwidth of the resonators should be dominated by coupling to the real components of the source and load impedances. If resonator bandwidths are principally set by the resonators own losses, then the insertion loss is going to be high.

Just have a look in Zverev. He has the derivations for insertion loss versus the ratio of resonator Q (unloaded) to filter Q (filter centre frequency divided by filter bandwidth.

I agree with those derivations for a tuned circuit in isolation, but they are coupled to things in use, and that makes a fairly large difference.


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Old 21st Mar 2017, 11:30 am   #30
G8HQP Dave
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Default Re: Permeability Tuning

In broadcast receivers or older general coverage receivers the input filter is rarely efficient. This is because low noise figure is not needed, and the set must work reasonably well with a random antenna impedance - hence loose coupling to avoid too much detuning.
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Old 21st Mar 2017, 9:03 pm   #31
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Arrow Re: Permeability Tuning

Quote:
Originally Posted by G8HQP Dave View Post
In broadcast receivers or older general coverage receivers . . the set must work reasonably well with a random antenna impedance - hence loose coupling to avoid too much detuning.
Which is a summary of the closing conclusions in my post #28.

Al.
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