[ARC5] Selectivity improvement in ARC-5 receivers - another question.

[Kenneth G. Gordon]

On 2 Jun 2014 at 15:56, Brian Clarke wrote:

> Hello Ken,
> 
> All is not lost, except access to the synaptic connections to your long 
> unused memory cells.

Ha! You mean I have to clean out all the cobwebs and detritus? Big job! :-)

> The matter of which you speak is covered well at a quasi-conversational 
> level in the RSGB (1987) 'Radio Communication Handbook' 5th edition, section on
> 'Interstage coupling' (pp 6.20 to 6.25). Fred Terman (1955) 'Electronic and
> radio engineering' Ch 3, 'Properties of circuits with lumped constants' gives a
> more rigorous treatment with more formulae in 38 pp. The selectivity curve, ie,
> the slope of the skirts, shown in the RSGB handbook appears as Fig 3-11 in
> Terman.

I've got two different Terman's and an RSGB (3rd edition, I think)

> Fritz Langford-Smith (1960) 'Radio design handbook' supplies a 49 pp
> chapter - Ch 26 'Intermediate frequency amplifiers'; it is similar to Terman in
> terms of level and design detail; there is an appendix devoted to calculating
> the coupling coefficient, and there is a major section on selectivity and the
> calculation thereof.

Don't yet have Langford-Smith, but will get it asap.

> The slope of the skirts, in dB per decade, depends on the frequency, the Q of
> the reactive elements and the coupling coefficient between the coils.

Ah! So all three. I was mistaken.

> Q is not
> only that of the reactive components, but also includes generator and load
> impedances. The coupling coefficient is related to coil spacing and other
> matters of geometry, such as the polarisation of the windings and their
> co-axiality.
> 
> Terman's and Langford-Smith's books are for designers; the RSGB handbook is for
> talented amateur technicians. I suspect the ARRL Handbook would be similar in
> level to the RSGB handbook - they are not really design reference manuals in the
> way Terman and Langford-Smith are.

No.

> In short, it ain't just frequency. And ultimate stop-band attenuation will
> depend on shielding as well.

OK. Excellent. Thank you, Brian.

Another question (to which you may not have the answer): in the ARC-5 IFTs 
there is often a small circular grounded metal plate between the two coils. My 
first impression on finding one of these was that its purpose was to reduce 
the magnetic coupling between the two coils.

However, after further investigation, and thinking about it, I am not at all sure 
that is its real or only purpose.

Moving the coils further apart, rather than adding a second "plate" does far 
more to improve the selectivity of those IFTs than adding that second plate, 
spacing those plates so that one is nearer one coil, and the other nearer the 
other coil.

In fact, it seems that removing that metal plate entirely while in the process of 
moving the coils further apart results in no apparent ill effects.

This apparent result, however, may be simply because my limited test 
equipment did not show the effect.

In any case, not knowing whether or not my removing the plate had any 
effect, I put it back in, but centered it between the two coils.

I have yet to complete my own "research" on this selectivity improvement 
idea. I am FAR behind Jeep's work on it.

Your thoughts?

Ken W7EKB