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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 21st Jun 2018, 10:26 pm   #21
Radio Wrangler
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Default Re: How do IF transformers work?

You've not seen our scales of fees

Like any complicated subject, it's really a lot simpler tham it seems to be.... once you strip off any fripparies and look at it from the right direction.

Top coupling, bottom coupling, magnetic coupling, inductive coupling, capacitive coupling, acoustic coupling by mechanical rods, acoustic in quartz, link windings, ports in waveguides... enough yet?

They are all just types of coupling. A way for adjacent resonators to share a little of each other's energy. Don't worry about the different types of coupling.

Set the Q of the end stages and then apply couplings from stage to stage to stage in a chain. Make the strengths of the couplings in a pattern (from tables in a book) and voila! your filter!

You can choose alternate ways of coupling to make life easy.

If you part-filled milk bottles with water as your resonators, and taped on lengths of garden hose as your couplers you could make a filter meeting a planned shape this way. Plastic rulers positioned along a school desk, coupled by rubber bands, anyone?

Spot the resonators, spot the couplers, consider the couplers. Look for the way of setting the Q of the end resonators. Look for the pattern in the strengths of the couplers. You can now read many important types of bandpass filters.

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Old 22nd Jun 2018, 9:34 am   #22
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Default Re: How do IF transformers work?

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Originally Posted by turretslug View Post
kalee20 and Radio Wrangler really ought to get a consultancy whip-round from the rest of us for this
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You've not seen our scales of fees
Thinking of Rex Garrod and Tim Hunkin, the second!

If you think of a mechanical counterpart, you can make a pair of pendulums (pendula? Spell check doesn't like this) with cords and weights. It is pretty easy to fine-tune the lengths so that they are the same frequency.

Now, instead of suspending both pendula from a fixed support, consider what happens if you suspend them from something slightly moveable, another cord strung between two fixed points like a washing line. Setting one pendulum swinging (transverse to the line) will move the line very slightly, which will excite the second pendulum.

If the line is taut, there will be very little movement, so the second pendulum will hardly move. And the movement that it does see will be very sharply tuned.

If the line is less taut, there will be more movement and the second pendulum will suck noticeable amounts of energy from the first, so the second pendulum will be damped rather more.

If the line is less taut still, the second pendulum will suck still more energy from the first, and its amplitude reduces quite fast. But a curious thing happens - the first pendulum's swinging reduces BELOW the second, which then backfeeds to the first. The energy slowly oscillates between them.

This is what happens in the double-tuned IFT with a lot of coupling. Free-running, it will do this. With a forced input, there will be two peaks, with the difference between the peaks just about equal to the slow frequency that the energy shuffles backwards and forwards in the free-running case.
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Old 22nd Jun 2018, 10:55 am   #23
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Default Re: How do IF transformers work?

Thank-you once again for a most illuminating discussion.

I've always hankered after my own copy of Zverev, but I've used it mostly for the low pass prototypes - never really paid the k-Q tables much attention - I see now that they were for this sort of design problem.

So sumarising:

- the coupled resonators have low coupling coefficient - i guess I wasn't expecting as little as 10% (is that k=0.1 for mutual coupling coefficient?). When they say 'critical coupling' in the tuning instructions, they mean for minimum bump in the middle.

- in terms of shape of IF filtering - is it fair to say that the filters in a domestic radio (i'm thinking more of '50s and '60s sets here) are Q limited and so there is no benefit in trying to produce a particular overall shape, hence the tendency for each stage to be synchronously tuned?

-coupling controls - fascinating to have a mechanical cam for shifting the windings! so the winding spacing is derived by trial and error resulting in the spacing between the two coils. Presumably there are rules of thumb for the dimensions of the windings and former - looks like the link to RDH has some of these. More study required..

In the SABA IF transformers - there seems to be a core for adjusting the coupling - is this an intermediate winding for coupling and the core used to adjust the coupling factor?

As an aside - I'm very grateful that people here feel able to spend their time discussing these subjects - I'm a practicing analogue/RF engineer and I'm finding that fewer people even in my own discipline have good analogue skills - the art of such techniques needs to be preserved so that others may 'stand on the shoulders of giants'. I'm also grateful to learn new things and it brings much satisfaction to get to grips with things and to appreciate the cunning of our forebears.

Kevin.
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Old 22nd Jun 2018, 11:11 am   #24
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Default Re: How do IF transformers work?

Which Saba is it?

Lawrence.
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Old 22nd Jun 2018, 11:56 am   #25
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Default Re: How do IF transformers work?

The 10% mentioned suggests over coupled or low Q if critical.

Lawrence.
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Old 22nd Jun 2018, 11:57 am   #26
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Default Re: How do IF transformers work?

Quote:
the coupled resonators have low coupling coefficient
It is worth noting that this does not necessarily equate to being inefficient as it would in the non-resonant mains transformer situation.

The way I like to think of it is this...
Because the coupling is poor it means that energy fed into the primary is not finding it so easy to escape to the secondary so builds up in the resonant winding until the higher level does result in it all (mainly) making it across.
So it changes the view the primary has of the impedance attached to the secondary. This can also be a very useful impedance matching device, e.g. to make a small loop antenna (which has very low radiation resistance) look like a nice 50R. I even have a formula for this somewhere.
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Old 22nd Jun 2018, 12:15 pm   #27
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Default Re: How do IF transformers work?

Hi Lawrence - its a Freudenstadt 8. I posted the tuning instructions translation on the thread for its restoration.

Kevin.
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Old 22nd Jun 2018, 12:20 pm   #28
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Default Re: How do IF transformers work?

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Originally Posted by brightsparkey View Post
...the coupled resonators have low coupling coefficient - i guess I wasn't expecting as little as 10%...

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The 10% mentioned suggests over coupled or low Q if critical.
Two opposing opinions!

Lawrence is nearer, obviously he has a feel for the figures - coupling between the circuits is actually really low for critical response. I gave 10% only as an order-of-magnitude, and yes, typical 470kHz IFT's are often much less (though 10.7MHz ones, where Q's are lower, may approach 10%. But I've not yet designed one).

With a high Q to the circuits, at resonance the second circuit puts quite a heavy load on the first, so killing its own peak rather effectively. You can think that if the first circuit has really high Q, it only takes a smidgen of resistance to clobber the peak. So equivalently, only a tiny bit of energy sucked out by the second, makes a massive difference.
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Old 22nd Jun 2018, 12:56 pm   #29
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Default Re: How do IF transformers work?

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Originally Posted by kalee20 View Post
Lawrence is nearer, obviously he has a feel for the figures - coupling between the circuits is actually really low for critical response. I gave 10% only as an order-of-magnitude, and yes, typical 470kHz IFT's are often much less (though 10.7MHz ones, where Q's are lower, may approach 10%. But I've not yet designed one).
For what I said before, I just referred to an example in my book which was for something approaching a typical AM IF frequency, primary and secondary inductances and resistances were different but their Q's were the same so critical k=1/Q which happened to be k=0.01 (approx.)

Lawrence.
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Old 22nd Jun 2018, 1:54 pm   #30
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Default Re: How do IF transformers work?

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Originally Posted by brightsparkey View Post
In the SABA IF transformers - there seems to be a core for adjusting the coupling - is this an intermediate winding for coupling and the core used to adjust the coupling factor?
As far as I can make out, according to the schematic (Freudenstadt 8) there are only the usual two slug (inductance) adjustments per transformer, the coupling adjustment (k) is probably mechanical similar to what's shown part way down in this link:

http://saba-forum.dl2jas.com/index.p...z-8-Automatic/

EDIT: And below is a link to another model Saba which looks like it employs the same system, it gives a full explanation and IFT internal diagram (Fig 2, page 4)....In English!:

http://85.144.192.2/nvhr/Saba_Konstanz8.pdf

Lawrence.

Last edited by ms660; 22nd Jun 2018 at 2:03 pm.
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Old 22nd Jun 2018, 2:54 pm   #31
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Default Re: How do IF transformers work?

Lawrence,

that's truly awesome! Now I understand how it works. A seriously over-engineered design, but elegant. This seems to be the way with SABA.

I hanker for a later stereo set with automatic tuning like the Freiburg 11, but they usually are outside of budget or collection only in Germany..

Thank-you once again for filling in the gaps.

Kevin.
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