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Old 10th Sep 2021, 11:44 am   #21
ms660
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Default Re: Ratio and Foster-Seeley detectors

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Originally Posted by Radio Wrangler View Post
Be wary of the oft repeated claims that the output of the ratio detector is independent of the signal amplitude. There are two flies in the ointment.

1) If you don't put any signal voltage in, even if that lack of voltage is modulated around the right frequency range, then there is not going to be any audio coming out. So the independence of IF signal level has to break down beyond limits. Conservation of energy insists.

2) If you put in a comfortable IF level and the ratio detector is working happily, If the IF level is not setting the scale of the output audio voltage... then what is? Are you getting a voltage reference out of nowhere?

Dimensional analysis of the parameters frequency, inductance, capacitance and resistance does not allow an absolute voltage to be established.

Alarm bells should be ringing.

David
No alarm bells, it was a good choice for AM immunity, one of the reasons it became popular.

Lawrence.
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Old 10th Sep 2021, 12:31 pm   #22
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Default Re: Ratio and Foster-Seeley detectors

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Originally Posted by Restoration73 View Post
Another method of FM demodulation uses a PLL - Phase Locked Loop.

Certainly in consumer designs, the ratio detector and the quad detector are most
prevalent. The quad circuit generates a higher level of white noise in the absence of a
carrier, but as this type uses a custom IC these usually contain a muting or "squelch"
circuit to prevent it being heard.
On some earlier TV's that used custom intercarrier sound IC's such as the Motorola MC1351 there was no muting or squelch and had a very high level of white noise in the absence of any signal.
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Old 10th Sep 2021, 4:59 pm   #23
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Default Re: Ratio and Foster-Seeley detectors

I was thinking more of radios, but that is true - TBA120 another early one for TV.
Hi-Fi tuners also have muting when the carrier is offset from the passband centre which could cause distortion. They also have a centre zero meter to prove it !
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Old 10th Sep 2021, 7:47 pm   #24
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Default Re: Ratio and Foster-Seeley detectors

Hi!

I'll add, as a interesting side–point, that the Mullard EABC80 "World Series" triple–diode–triode valve can be used with both ratio and Foster–Seeley discriminators, (altho' the Foster–Seeley needs to be the "unbalanced" type with this valve) and can also work with the "pulse–counter" integrating type FM detector, this valve has one complete independent diode, which is all these circuits need, so it's not a "done deal" that the use of an EABC80 always means a ratio–discriminator has to be used!

Several designs of "pulse counting" FM tuners have appeared in the construction magazines, PW October 1965 for one example!

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Old 10th Sep 2021, 8:28 pm   #25
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Default Re: Ratio and Foster-Seeley detectors

So if I understand this correctly, which I probably don't, because the tertiary is closely coupled to the primary it closely matches the values of the primary - so even though current may lead or lag voltage on the primary depending on where it is outside resonance, this same leading or lagging current will roughly be replicated on the tertiary?

If that last paragraph is correct, does this make the tertiary a reference point for the diodes, if you get my meaning?

Let's now imagine the primary tank is above or below resonance - because the primary and secondary are loosely coupled, e1 or e2 will be more or less in phase with ep BECAUSE voltage lags or leads BECAUSE the secondary is loosely coupled? And because the secondary forms a tank, above or below resonance signals on the secondary this helps to push e1 or e2 more or less in phase with ep?

Presumably next, e1 and e2 pass across the diodes and their values together compared with the ep reference is what determines the audio output?

Thanks for your help so far with this. It is going in!!

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Old 10th Sep 2021, 10:08 pm   #26
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Default Re: Ratio and Foster-Seeley detectors

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Originally Posted by AidanCroft View Post
because the tertiary is closely coupled to the primary it closely matches the values of the primary
The phase of the voltage on the tertiary will match the phase of the voltage on the primary, yes.

The magnitude may be different if the turns ratio is not 1:1, but that's a minor detail.

Quote:
Originally Posted by AidanCroft View Post
so even though current may lead or lag voltage on the primary depending on where it is outside resonance
Ignore current, it's a red herring! (For completeness, primary current will be 90° out of phase with primary voltage, because the primary is an inductor. That's irrespective of resonance).

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If that last paragraph is correct, does this make the tertiary a reference point for the diodes, if you get my meaning?
Sorry! No, I don't!

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Originally Posted by AidanCroft View Post
Let's now imagine the primary tank is above or below resonance - because the primary and secondary are loosely coupled, e1 or e2 will be more or less in phase with ep BECAUSE voltage lags or leads BECAUSE the secondary is loosely coupled?
Yes, I think you've got it. At resonance, e1 will be 90° out of phase with ep in one direction; e2 90° the other direction. So above resonance, one will swing to less than 90° thus becoming more in-phase, the other will swing to more than 90° becoming less in-phase. Below resonance, the reverse applies.

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And because the secondary forms a tank, above or below resonance signals on the secondary this helps to push e1 or e2 more or less in phase with ep?
It's a bit complicated, but yes, the secondary forming a tank is essential to this phase shift, together with the looseness of the coupling.


Quote:
Originally Posted by AidanCroft View Post
Presumably next, e1 and e2 pass across the diodes and their values together compared with the ep reference is what determines the audio output?
The diodes don't care about phase, they just peak rectify. One diode sees e1+ep, the other sees e2+ep. But because of the shifting phase between ep and e1 or e2, (ep+e1) might grow in peak value while (ep+e2) shrinks, one side of resonance, and vice versa below resonance. And the output is just the difference of the two peak rectified voltages.
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Old 10th Sep 2021, 10:32 pm   #27
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Default Re: Ratio and Foster-Seeley detectors

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If that last paragraph is correct, does this make the tertiary a reference point for the diodes, if you get my meaning?
I was asking if the tertiary (with its ep) produces a reference point? Later you talk about either e1+ep or e2+ep so I wondered if ep was producing a reference? Kind of like a ground, but not a ground!!

On a related note, a ratio detector has diodes pointing in opposite directions so presumably they don't conduct when the input goes negative?

A Foster-Seeley has diodes pointing in the same direction so how does a Foster-Seeley respond to the input signal going negative?

Thanks so much for breaking down my response. It makes understanding the mess in my head so much easier. I nearly threw my 'phone across the room trying to understand it yesterday!!

Aidan.
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Old 10th Sep 2021, 10:45 pm   #28
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Default Re: Ratio and Foster-Seeley detectors

A Foster-Seeley is almost the same, but with the diodes pointing in the same direction you get two peak-rectified voltages of the same polarity.

The output is taken as the difference of the two.

If they are both 10V at resonance, then off-resonance you might get one decrease to 11V, the other to 9V - the difference will be 2V. Off-resonance in the other direction, you'd get 9V and 11V with a difference of 2V of opposite polarity.

Whereas the ratio detector produces two rectified voltages of opposite polarity, say again 10V and 10V, because of their polarities they appear in series giving 20V. Above resonance one might increase to 11V the other decrease to 9V, but the sum is still 20V (and you take your output from the junction point, so you get 1V), Below resonance they'd go to 9V and 11V, still 20V sum, but the junction point is now -1V.

The ratio detector thus broadly gives only half the output of the FS, but it does have this constant DC voltage present, irrespective of frequency deviation, which charges up a big electrolytic capacitor. If the signal suddenly tries to increase in amplitude, the big electrolytic (which takes a lot of energy to charge) makes the diodes clamp the increase giving tremendous loading on the circuit - and that's what limits the amplitude increase.
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Old 10th Sep 2021, 11:51 pm   #29
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Default Re: Ratio and Foster-Seeley detectors

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Quote:
Originally Posted by AidanCroft View Post
If that last paragraph is correct, does this make the tertiary a reference point for the diodes, if you get my meaning?
I was asking if the tertiary (with its ep) produces a reference point? Later you talk about either e1+ep or e2+ep so I wondered if ep was producing a reference? Kind of like a ground, but not a ground!!
If you look at your original circuit, your first post - you could think of point A as a reference, and ground it - and take your output from Point B instead. It's quite arbitrary! And ignore R3, that's just a subtlety.

Once you do this, you could think of ep as a reference phase, I suppose, which'd be the same as the primary phase. And then the e1, e2 pair are at ±90° to this reference phase at resonance, moving to perhaps -70° and + 110° one side of resonance, and -110° and +70° the other side.
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Old 11th Sep 2021, 2:50 am   #30
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Default Re: Ratio and Foster-Seeley detectors

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There's also the gated beam demodulator as invented by Zenith in the early 1950s using the 6BN6 valve, which is rather cunning!!
The 6BN6 gated beam valve was one of several valves developed especially for use as FM quadrature demodulators, some oscillatory (injection locked) and some passive, as it were. An outline was provided here: https://www.vintage-radio.net/forum/...6&postcount=15.

The first integrated quadrature demodulator appears to have been developed by Fairchild, with a fairly simple modulator section. It was soon followed by a Sprague IC that used a double balanced modulator based upon the six-transistor tree, this then becoming the norm. Initial development of this IC species was along the vector of improved functionality for TV sound applications, including internal voltage-operated volume controls and AF amplifier drivers, valve as well as transistor). RCA was the first to develop a complete radio receiver IF subsystem with quadrature demodulator, namely the previously mentioned CA3089 of 1971, which became an industry standard. In the early 1980s, National added a feedback circuit to the modulator section to provide very low distortion with a single-tuned quadrature circuit.

Before then, RCA had developed an integrated form of the ratio detector (CA3013/CA3014 of 1966). In the late 1960s, when others were adopting the quadrature demodulator, RCA developed what it called its differential peak detector in integrated form.

Perhaps the wideband ratio detector should be mentioned as a subspecies. It was used to get distortion levels down to or below the Foster-Seeley discriminator level, but required extra IF gain and limiting. Thus it was not so common in the valve era, although Radford used it in the FMT1, which had a four-stage IF strip. But it was a common choice in the early solid-state era, where abundant IF gain and limiting were easier to obtain, particularly with ICs, and before the quadrature type took over. Examples of early solid-state tuners with wideband ratio detectors were the Rogers Ravensbourne (1st version) and the Leak Stereofetic.

Cringeworthy though it may be, I suppose it should be recorded that the slope detector had a brief showing in the early IC era, with the TAA350.

And the integrated PLL type does not seem to have had a major impact; some applications were mentioned recently here: https://www.vintage-radio.net/forum/...2&postcount=39



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Old 11th Sep 2021, 7:18 am   #31
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Default Re: Ratio and Foster-Seeley detectors

In the ratio detector transformer, the two halves of the centre-tapped secondary need to be closely coupled together.

The primary could be closely coupled to them, but this would reduce the selectivity of it also acting as a double-tuned IFT as well, so the primary is usually loosely coupled to the secondary.

The tertiary winding has to be loosely coupled to the others, otherwise it would force its output to be in phase with them, and all the phase shifting vector addition would not come about.

The reason for the diodes LOOKING to point in opposite directions is an artefact of the way the circuit is traditionally drawn. You can also view them as pointing in the same direction around a circuit loop. Effectively in series though in different places around the loop. This amounts to a sampling bridge. You could look at it as a synchronous rectifier, where the on/off state of the diodes is dominated by the secondary voltage from the transformer. The ratio detector with the phase shift between the path being rectified and the switching drive of the synchronous rectifier/sampler makes the ratio detector a close relative of the quadrature detector, just done without transistors or ICs. Quite clever, but a small circuit with a lot going on in it, so difficult to understand the first time you think about it.

It becomes a lot clearer if you are familiar with diode sampling bridges, with synchronous rectification, and with the quadrature detector. Then a number of things suddenly fit together.

To understand quadrature detectors, it helps to first understand the phasor diagram for AM, and then the much more complex one for FM.

The AM phasor just has to make a vector increase and decrease in length without wagging. Two sideband components woill do this.

The FM phasor has to model motion around a circle without changing length. This takes an infinite series of vector terms (= sidebands), but it shows you the phases of the components, and the Bessel series gives you the amplitudes. This vector addition phasor also explains the carrier nulls you get under some conditions... something that looks bizarre if you're used to AM.

The ratio detector is small, cheap, usually saves one stage of IF amplifier/limiter, and for a while it took over the world. But do look out for the limitations on what it can do. The claims made for it, without those limitations being stated, really do break a couple of fundamental laws of physics. I just happen to think that's important if you want to understand the thing.

David
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Old 11th Sep 2021, 8:31 am   #32
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Default Re: Ratio and Foster-Seeley detectors

That can't be right David - let's look at each bit in turn:

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Originally Posted by Radio Wrangler View Post
In the ratio detector transformer, the two halves of the centre-tapped secondary need to be closely coupled together.
Agree - you need a well-defined pair of antiphase outputs from the transformer, just like the drive transformer for a push-pull AF amplifier.

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Originally Posted by Radio Wrangler View Post
The primary could be closely coupled to them, but this would reduce the selectivity of it also acting as a double-tuned IFT as well
If the primary be closely coupled (if in doubt, take an extreme case: k=1) then primary and secondary will be in-phase or opposite-phase, and being on- or off-resonance will make no difference to that.

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The tertiary winding has to be loosely coupled to the others, otherwise it would force its output to be in phase with them
But that's where we run into problems. The primary and secondaries being closely coupled, they share the same magnetic flux. Put another winding in, closely coupled, it too will be in-phase or 180 degrees out of phase. Reduce the coupling to this tertiary winding, and the winding voltage will still be in-phase or out-of phase - it'll just be smaller in magnitude and it'll start to have leakage inductance in series with it. But it'll never see a phase shift with respect to the others.

No, it's the primary and secondary being both tuned, and very loosely-coupled, that results in the 90 degree phase-shift between them (strictly, it's not quite 90 degrees but is very close).

Last edited by kalee20; 11th Sep 2021 at 8:32 am. Reason: Punctuation
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Old 11th Sep 2021, 6:59 pm   #33
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Default Re: Ratio and Foster-Seeley detectors

Here's another explanation for Aidan from Riders:

https://nvhrbiblio.nl/biblio/boek/F-...r%20Schure.pdf

Scroll down to page 18 for the description and page 19 for the circuit and references given in the description.

I use two tabs next to each other, it's less scrolling up and down which helps to reduce the brain/memory battle.....

Lawrence.
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Old 11th Sep 2021, 8:32 pm   #34
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Default Re: Ratio and Foster-Seeley detectors

In the ratio detector I'm familiar with, there is no other connection between the primary circuit (in the collector/anode of the driving device) and the circuitry of the secondary and the tertiary windings than the magnetic flux.

Therefore the phase between the primary and the other windings is unimportant and can be anything with no effect on operation. There is no other path for it to be compared against. The phase between the centre-tapped secondary and the tertiary winding is what makes the discriminator work.

The primary is usually lightly coupled to all the rest in order to make the transformer have double-tuned selectivity.

The tertiary winding needs to be lightly coupled to the centre tapped secondary, andnot much coupled to the primary. I'd wind a primary on a former ( with a resonating capacitor) and spaced away from it the centre-tapped secondary (with a resonating capacitor) wound as a bifilar pair for best coupling, then spaced away, on the opposite side of the bifilar winding to the primary, I'd wind the tertiary. Direct coupling from the primary to the tertiary will allow the less-well filtered primary flux to bypass the resonated CT secondary and get to the tertiary.

This component may look like a three-winding transformer, but it's really a complex bit of integrated magnetics design. It's the heart of a frequency discriminator and a two-pole filter, all in one can.

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Old 11th Sep 2021, 8:37 pm   #35
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Default Re: Ratio and Foster-Seeley detectors

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In the ratio detector I'm familiar with, there is no other connection between the primary circuit (in the collector/anode of the driving device) and the circuitry of the secondary and the tertiary windings than the magnetic flux.

Therefore the phase between the primary and the other windings is unimportant and can be anything with no effect on operation. There is no other path for it to be compared against. The phase between the centre-tapped secondary and the tertiary winding is what makes the discriminator work.

The primary is usually lightly coupled to all the rest in order to make the transformer have double-tuned selectivity.

David
The one in the link I referred to is a Foster-Seeley.

You can see a typical ratio detector transformer with the relative spacing and coupling on page 209 in here:

https://worldradiohistory.com/ARCHIV...w-1947-Jun.pdf

Somewhere I've still got my exercise book from the mid 60's with the theory etc of the Foster-Seeley discriminator, it was part of my City & Guilds course.

Lawrence.

Last edited by ms660; 11th Sep 2021 at 8:52 pm. Reason: link added
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Old 12th Sep 2021, 12:39 am   #36
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Default Re: Ratio and Foster-Seeley detectors

Here is the complete RCA Review article on the ratio detector, by its developers Seeley & Avins.
Attached Files
File Type: pdf Ratio Detector RCA Review 194706.pdf (1.57 MB, 48 views)
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Old 12th Sep 2021, 9:07 am   #37
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Default Re: Ratio and Foster-Seeley detectors

Thanks, that contains the missing explanation that seems to have been dropped in the passage of time.

The ratio detector does not have a limiting function without any fine print. It rectifies the IF signal, stores it on that large capacitor, and uses it to scale the audio output voltage. They suggest a time constant of 0.2 seconds for that capacitor and the load resistors.

So the ratio detector has no self-limiting effect for slow variations in signal amplitude. Weak signals will translate to weak audio. What it does do is correct for faster variations in amplitude. It will strip off audio frequency AM components to a good extent but you will still get reduced audio on weak stations and be able to hear aircraft flutter once it is slow enough. The reference voltage for the self-limiting function is the longish term signal level. The laws of physics remain intact provided this mechanism isn't forgotten.

This is the mechanism which explains why ratio-detector sets don't produce quite as much inter-station noise as do the more recent crop of quadrature detector sets with their strongly-limiting, high gain IF amplifiers.

Regarding phase shifting, The phase shift that matters, the phase shift that the ratio detector uses is that between the centre-tapped secondary and the tertiary windings.

Phase relationships with the primary are of no importance. The detector function operates only from the centre-tapped secondary and the tertiary. There is no other path from the primary for any phase comparison to be made. Just so long as the centre tapped secondary and the tertiary have the right phase/frequency relationship, the primary can do what it likes as far as FM demodulation is concerned.

It would be a shame to have an expensive IF transformer can in the discriminator and only get one pole's worth of selectivity out of it. The accountants would get edgy.

There is temptation to resonate the primary as well as the centre-tapped secondary. The temptation extends to saying that loose coupling to the centre-tapped secondary will allow a two-pole response where the coupling and Qs can be arranged to make a suitable M-shaped response, as if it was just a simple IFT. But with the tertiary close-coupled to the primary, then there is a difference in the bandwidth in amplitude terms of the tertiary voltage compared to the centre-tapped secondary voltage and you start to get increased distortion on deviation peaks. There is an uneasy tradeoff between having the secondary tuning giving the required phase shift and attempts to get a bit of cheap extra selectivity.

Re-jigging the transformer so that the primary is loose coupled to both the tertiary and the centre tapped secondary gives the freedom to have more selectivity without such bad behaviour at modulation peaks, but the centre-tapped winding and the tertiary still have to be loose-coupled between themselves, however the drive from the primary is arranged. The centre-tapped secondary has its parallel resonating capacitor, while the tertiary winding works into the C-R load circuits and thereby drives the centre tap of the other winding.

I had some fun developing an FM detector and had a number of alternative variants of the ratio detector breadboarded on the bench. I was looking to improve the large deviation distortion characteristic. I got some, but the sensitivity to IF input level dropped. What I was seeing was the normal insertion loss of a double-tuned IF superimposed on the FM demodulator function. It seemed only fair, I was also getting the selectivity of a double-tuned IF transformer superimposed on the FM demodulator function. So maybe the accountants wouldn't be too pleased to see the need for more IF/RF gain?

The ratio detector isn't a single circuit, there are several variations on it. Some, seen in this paper, others came along later.

I did like the early variant in the paper where they said they used a battery in place of the big capacitor to give a fixed self-limiting amplitude. That is exactly what the basic physics look at what people say a ratio detector does shows is lacking. The authors also note that with the fixed limiting value, low level sensitivity is lost. Batteries are inconvenient if embedded in circuitry, and the need for added gain costs money.

The FM tuner in my living room does not have a ratio detector. It's a pulse counter using a rolled-up length of coax as the pulse-width determining element. The linearity is excellent. However, it does need a further conversion to a lower IF than 10.7 MHz to get reasonable sensitivity in the discriminator. It also has entirely L-C IF filtering, a good linear phase design. No ceramic filters here.

Ratio detectors had their day in cost-critical domestic radios before ICs came along and made quadrature detectors an easier option. Japan's cheap labour of the period caused their radios to stay with discrete components for some time after European and US manufacturers had gone over to ICs, and this favoured the ratio detector in their products.

David
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Old 12th Sep 2021, 9:56 am   #38
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Default Re: Ratio and Foster-Seeley detectors

Thanks for the article Synchrodyne!

I'm pretty confident in my own mind that coupling between input winding and secondary has got to be loose for the required 90 degrees phase shift.

Agree with RW that what the primary actually does is of no relevance in practical receivers, it's what is on the secondary and on the tertiary which matters - however, the tertiary is coupled closely with the primary (article says so on page 208) so it is the same phase as the primary. The tertiary is only added because it would be impractical to use the actual primary voltage because it would need to be floating at AF, both ends (note that this is possible with the FS, hence no tertiary, but a coupling cap).

But I'll have to do some sketches to convince myself. Hoping to post later!

Last edited by kalee20; 12th Sep 2021 at 10:00 am. Reason: Clarify
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Old 12th Sep 2021, 10:15 am   #39
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Default Re: Ratio and Foster-Seeley detectors

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Thanks for the article Synchrodyne!
You must have missed Post#18......

Here's a couple of capacitively coupled (pri.-sec.) schematics, anyone recognize these (make/model)?

Lawrence.
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Old 12th Sep 2021, 11:30 am   #40
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Default Re: Ratio and Foster-Seeley detectors

The essential phase shift for these detectors is a result of the leakage inductance between primary and secondary. Loose coupling and a fairly low value of load resistance are needed to achieve this - easily demonstrated with a vector diagram.

The capacitive centre tap shown above was used by Murphy amongst others. It seems to work well enough on the sets I have re-aligned.

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