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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 14th Jan 2022, 12:06 am   #61
Radio Wrangler
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Default Re: Ratio and Foster-Seeley detectors

Perfectly simply. the two phases are opposites, 180 degrees apart. Just view it as one, centre-tapped winding. If the rate of change of flux produces one volt per turn at some particular instant, it produces one volt in each and every turn. Compared to the centre tap, one end of the winding goes to +N/2 volts, while the other end goes to -N/2 volts.

This is best when all the turns of the winding are closely coupled together, and centre-tapped windings are often two bits of wire, wound bifilar.

Now in the ratio detector transformer, we have to get a tertiary winding loosely coupled to the centre tapped winding, in order to make the necessary phase shift (nominally 90 degrees, so simply having turns in the opposite direction won't do here, we have to get leakage inductance from leakage flux... so this winding has to be a little way apart from the centre-tapped one

The primary can be close-coupled to either of the centre-tapped winding, OR the tertiary, but cannot be close to both at once.

David
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Old 15th Jan 2022, 2:56 pm   #62
kalee20
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Default Re: Ratio and Foster-Seeley detectors

Quote:
Originally Posted by Radio Wrangler View Post
The primary can be close-coupled to either of the centre-tapped winding, OR the tertiary, but cannot be close to both at once
I can't quite accept that, David - unless I've misunderstood!

If the primary and the centre-tapped secondary are tightly coupled (let's assume 100%) then the terminal voltages must be in-phase, identical except for scaling according to turns ratio. This applies whatever load exists on the secondary. Twiddling any trimcap which exists on either pri or sec will have the same effect, all voltages may peak at resonance, but phase relationships between them won't change.

A loosely-coupled tertiary which gets a sniff of the magnetic flux will also have a voltage induced in it which is in-phase (or 180 degrees out), albeit that it'll be much reduced in amplitude from what's expected from turns ratio - and it won't be a very stiff source of voltage owing to the large leakage inductance. All as circuit (a). So the discriminator won't work.

The way to get it to work is connect a capacitor across the tertiary, tuning the leakage inductance to resonance. The voltage across the cap (ie the available terminals of the tertiary) will then be 90 degrees out of phase at the centre frequency, and it'll then work as a discriminator, as circuit (b). I've not seen it done, but is this what you meant?
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Old 15th Jan 2022, 5:06 pm   #63
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Default Re: Ratio and Foster-Seeley detectors

Misunderstanding, I think, Peter.

THe ratio detector relies on one of the output windings to be loosely coupled to everything else to create some leakage inductance, then it needs that winding to be loaded to create the phase shift. There are many ratio detector variants and the choice of which winding gets the loose couple has been done either way. This allows the primary to be tightly coupled to one or the other of the output windings.

So many variants played, so it's difficult to phrase things to cover all the cases and not be misleading.

Sorry

David
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