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Old 16th Mar 2017, 1:14 pm   #1
ms660
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Default Permeability Tuning

I've often read that using permeability tuning results in a more constant selectivity when tuning across the band, I've always accepted that for what it said but as yet I can't seem to find any explanation as to why, I would be interested to know.

Anyone know the reason?

Lawrence.
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Old 16th Mar 2017, 1:24 pm   #2
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Default Re: Permeability Tuning

Possibly more stable thermal properties?
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Old 16th Mar 2017, 2:25 pm   #3
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Default Re: Permeability Tuning

They most likely shape the slugs and lay the turns on the coils at design time.
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Old 16th Mar 2017, 2:44 pm   #4
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Default Re: Permeability Tuning

Both a variable inductor and a variable capacitor can be "shaped" to give a desired rate of change of resonant frequency.

I don't think that is what Lawrence is asking, he talks of "a more constant selectivity" which I take to refer to the bandwidth of the resonant circuit. There is no clue to this in the formula for resonant frequency as changes in L and C have equal effect on resonant frequency, but the answer will be elsewhere and is likely to be a mathematical one.

No doubt someone out there will be able to educate us.
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Old 16th Mar 2017, 2:46 pm   #5
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Default Re: Permeability Tuning

That's what I'm hoping, I have a feeling that R might come into it somewhere.

Lawrence.
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Old 16th Mar 2017, 3:11 pm   #6
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Default Re: Permeability Tuning

With capacitor coupling, varying L leaves the coupling factor relatively fixed and so the filter gets somewhat narrower at lower tuned frequencies. Varying C would leave the coupling getting less at low freq, so the overall filter gets disproportionately narrower at lower frequencies.

With tuning capacitors, the same trick is available if inductive couplers are used. It isn't something exclusive to permeability tuners.

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Old 16th Mar 2017, 3:29 pm   #7
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Default Re: Permeability Tuning

It was often used for the older car radios, the vibration can 'twang' air spaced capacitor vanes. Later varicaps were used, problem gone.
 
Old 16th Mar 2017, 3:34 pm   #8
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Default Re: Permeability Tuning

Thanks for the replies so far but I'm still unclear as to what's what selectivity wise.

Lawrence.
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Old 16th Mar 2017, 3:36 pm   #9
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Default Re: Permeability Tuning

Quote:
Originally Posted by merlinmaxwell View Post
It was often used for the older car radios, the vibration can 'twang' air spaced capacitor vanes. Later varicaps where used, problem gone.
A significant number of the midget "All-American Five" radios imported to the UK during WWII [Lend-Lease] used 'slug-on-a-string' permeability-tuning: at the time aluminium was better used to make B17 bombers than tuning-capacitors.
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Old 16th Mar 2017, 4:02 pm   #10
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Default Re: Permeability Tuning

As a 'top of the head' response, I looked at the formula for the 'Q' of a tuned circuit:

Q = 2 x pi x f x L/R

where f is resonant frequency, L is inductance in tuned circuit and R is series resistance of inductor.

If the circuit is tuned by varying capacitance, L and R remain constant. So as resonant frequency is reduced by increasing C, Q will also reduce along with f.

However, if the circuit is tuned by varying L (e.g. by permeability tuning) then as f is reduced, L is also increased, keeping Q more constant.

In reality, the resonant frequency is inversely proportional to the square root of L whilst Q is directly proportional to L, so Q isn't constant, but permeability tuning in principle maintains more constant Q. I guess that its main advantage would be in a TRF set.

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Old 16th Mar 2017, 4:08 pm   #11
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Default Re: Permeability Tuning

Tuning frequency goes like 1/sqrt(LC), so you can vary either C or L to change the frequency. Bandwidth goes like Rs/L or f^2 RsC - this assumes Rs is in series with the inductor (this could be the resistance of the inductor itself).

Sometimes, Rs is approximately constant with frequency. To get constant bandwidth you then need constant L - so vary C.
If Rs increases with frequency then constant bandwidth is not easily possible.

Now consider the different situation where bandwidth is dominated by a parallel resistance Rp - this could be partly due to the circuit coupled to the inductor. Bandwidth then goes like f^2 L/Rp or 1/(C Rp).

If Rp is constant then constant bandwidth requires constant C - so tune by varying L.
Rp may reduce with frequency so constant bandwidth is not easily possible.

So, as often is the case, the answer is "it all depends". It is likely that solid-state circuitry will impose a greater load (effectively in parallel) so permeability tuning may help there. Valve circuits may be better with capacitor tuning - but both these statements are generalisations so may not always be true.

I suspect that the idea that permeability tuning gives constant bandwidth arises from one of two causes (and then got repeated without these restrictions attached):
1. under certain circumstance, it can give constant bandwidth
2. it was adopted in some cases for some other reason, but someone guessed at the reason and got it wrong
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Old 16th Mar 2017, 5:33 pm   #12
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Default Re: Permeability Tuning

Is that for constant Q or constant bandwidth?
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Old 16th Mar 2017, 6:11 pm   #13
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Default Re: Permeability Tuning

I've just done a bit of number crunching for a parallel tuned circuit and Q values ignoring Rp but including Rs, Rs was entered as constant, I chose Fmin as 0.55MHz and Fmax as 1.6MHz, the ratio of min Q to max Q was more or less the same for C tuning and L tuning using Q = XL/Rs, not sure what all that means yet

Lawrence.
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Old 16th Mar 2017, 8:20 pm   #14
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Default Re: Permeability Tuning

As was mentioned earlier, many car radios used permeability tuning. Did not many older car radios have an RF preselector stage?
This would perhaps lead to the conclusion that this is a better method?
Les.
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Old 16th Mar 2017, 9:37 pm   #15
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Default Re: Permeability Tuning

Most Car Radios of which I know had RF Stages and Permeability Tuning, although some Blaupunkt models didn't have RF stages, and a few cheaper Philips AM sets even used Plastic dielectric tuning gangs in the late 70s/early 80s.
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Old 17th Mar 2017, 9:20 am   #16
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Default Re: Permeability Tuning

The tuners may have more sly features than meets the eye. The patent
number shown refers to a device for broadcast receiver use that changes
capacity, as well as inductance, and can specifically improve performance at
the lower end of the band.

http://www.google.st/patents/US2531231
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Old 17th Mar 2017, 10:08 am   #17
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Default Re: Permeability Tuning

Hartley118 is on the money.
Art Collins of American fame had a fix on permeability tuning and used it to good effect.
I have a couple of his sets and they perform exceptionally well.
I have 2 51J4 receivers and 3 R390A receivers. Both are full permeability tuned with the R390As particularly complicated in the gearbox area. Not good "band spinners" with their mechanical design.
Sorry for being a little off topic but sort of relevant.
Mods please delete if you feel the need.
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Old 18th Mar 2017, 2:44 am   #18
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Default Re: Permeability Tuning

To elaborate on G8HQP Dave’s comments, which I think rudimentarily may be summarized as “the devil is in the detail”, it is possible that RF tuning arrangements that were designed to provide constant bandwidth (in other words variable Q) across the tuning range may have been more common with permeability than with capacitor tuning. One reason for this may be that permeability tuning arrangements were often bespoke, designed to meet the needs of specific receivers, whereas capacitor tuning arrangements often used standard third-party capacitors and coilsets. In a bespoke design, constant bandwidth tuning would be just another of several application-specific requirements to be addressed.

An example of a complex and I imagine highly bespoke ganged permeability tuning arrangement, and one which had to work across a couple or more separate and screened compartments, was found in the Eddystone 880 series receivers. Whether it was tailored to provide constant bandwidth I don’t know, but it does not seem improbable that it would have been. For most of its other valved HF receivers, Eddystone used it own four-compartment variable capacitor, with three or four gangs as needed. But for the 880, which I think had six gangs in all, purpose-built permeability tuning was probably much easier from a mechanical viewpoint.

Variable RF bandwidth across the MF band was an issue with wideband receivers and tuners, which needed to be flat across say a ±12 kHz bandwidth throughout, but which with standard circuitry would have missed this target at the lower end of the band. Various solutions were applied. With capacitor tuning these included a bandpass tuned input with mixed coupling; switching in a small resistance in series with the tuning coil in wideband mode, this having greater Q-reduction effect at the lower end of the band; and stagger tuning of the RF and mixer coils at the lower end of the band, but not at the higher end. The only example I know of with permeability tuning was the AWA AM3, which as I understand it used a modified car radio front end. Whether this was done for performance reasons or because the car radio circuit was readily available I don’t know. But there was a good chance that of the readily available circuits and modules, only that for the car radio came with an RF amplifier, hence it was chosen, and with it came permeability tuning as a consequence and not a cause. Unfortunately I have not retained a copy of the schematic, so I don’t know what, if any features were added to keep the RF bandwidth wide enough at the low end of the MF band. Anyway, the available evidence suggests that in a case where constant bandwidth was a desideratum, there was not widespread use of permeability tuning, suggesting that it had no inherent advantage.

So as G8HQP Dave suggested, there is a risk in drawing conclusions from simple analysis of the empirical evidence. The choice between capacitor and permeability tuning may not always have been made because one was distinctly superior to the other, but for availability or mechanical design reasons. Apparently in the very early days of FM receivers and tuners in the UK, suitable three-gang variable capacitors were very scarce, with the two-gang type being a standard product. Thus if a three-gang front end was required, permeability tuning was necessary. That might explain for example why the Pye FenMan II (3-gang) had permeability tuning, whereas the FenMan I and the Mozart (both 2-gang) had capacitor tuning.


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Old 18th Mar 2017, 11:47 am   #19
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Default Re: Permeability Tuning

I have always understood that with permeability tuning, a tighter degree of coupling between the aerial coil and the first tuned cct. can be achieved compared to fixed inductance and variable capacitor tuning. Hence, the overall gain is higher with permeability tuning.

Al.
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Old 18th Mar 2017, 11:56 am   #20
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Arrow Re: Permeability Tuning

Quote:
Originally Posted by ms660 View Post
. . . the ratio of min Q to max Q was more or less the same for C tuning and L tuning using Q = XL/Rs, not sure what all that means yet
That's because your equation for Q is for the inductor alone. For a tuned circuit, containing L, C and R, you need Q = (1/R)[√(L/C)]. Note that that R is the combined d.c. resistance and the H.F. resistance, the latter being somewhat variable with freq.

Al.
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