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Old 31st Aug 2019, 2:07 pm   #1
djsbriscoe
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Default Fisher TFM1000 pulse count detector-How does it work?

I'm trying to understand how the switch and pulse counter stage of the Fisher TFM1000 tuner works. I have read a copy of an article by A H Seidman from Electronics World Jan 1967. Attached is an LTspice file I have created in an effort to try to understand how the circuit works.
The first 3 limiter stages are straightforward. It is the switch, delay line and pulse counter that I don't understand (centre around Q5,Q6 and L3 in the LTSpice schematic). I have cross referenced the article above with the service manual and L3 is just marked 500 (it does not indicate nH,uH, or mH). However the photo of the unit shows a coil mounted on a ferrite rod. This could be the delay line. Could someone help me to figure this out? I'm just curious as to how this novel detector works. Thanks.

PS Please change the file extension on the attached txt file to .asc .It is the LTSpice schematic. The pdf file is a page from the service manual with the limiter/detector schematic. The article mentioned above is too large to upload. Please do a search of American Radio History to read it.
Attached Files
File Type: txt Fisher_TMF1000_PulseCountDetect1 - Copy.txt (11.3 KB, 17 views)
File Type: pdf FisherTMF1000_LimitDetectSchematic.pdf (1.56 MB, 31 views)
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Old 31st Aug 2019, 3:35 pm   #2
ms660
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Default Re: Fisher TFM1000 pulse count detector-How does it work?

So far as I understand it the delay line is the main timing element that sets/determines the pulse width.

Lawrence.
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Old 31st Aug 2019, 3:47 pm   #3
djsbriscoe
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Default Re: Fisher TFM1000 pulse count detector-How does it work?

The article is here on page 32

https://www.americanradiohistory.com...ld-1967-01.pdf
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Old 31st Aug 2019, 7:13 pm   #4
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Default Re: Fisher TFM1000 pulse count detector-How does it work?

Here is the 1N34A spice model for use with the LTSpice file above.
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File Type: txt 1N34A.txt (90 Bytes, 6 views)
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Old 31st Aug 2019, 10:59 pm   #5
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Default Re: Fisher TFM1000 pulse count detector-How does it work?

In a pulse count discriminator either a monostable or a delay line time-offsetting the same input (the IF signal after limiting) going into the two inputs of a exclusive-OR gate is used to create fixed-length pulses.

The delay line one created one pulse on each edge of the limited IF (rising or falling), so it's better than the plain monostable version.

A filter averages the total area of the pulses. If there are more pulses per second, the average is higher. if there are fewer pulses per second, the average is lower. in this way FM in the IF is converted to a varying voltage.

The averaging filter must pass audio and multiplex frequencies, and stop the IF frequency

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Old 1st Sep 2019, 9:40 am   #6
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Default Re: Fisher TFM1000 pulse count detector-How does it work?

Looking at the receivers schematic the delay line, L902, looks to be configured as an open circuit transmission line, that would fit in with the description in the Electronics World article.

The 500 written underneath L902 in the schematic might be the delay lines DC resistance....

Lawrence.
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Old 1st Sep 2019, 12:03 pm   #7
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Default Re: Fisher TFM1000 pulse count detector-How does it work?

Quote:
Originally Posted by ms660 View Post
The 500 written underneath L902 in the schematic might be the delay lines DC resistance....
Or capacitance?

Lawrence.
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Old 1st Sep 2019, 2:09 pm   #8
djsbriscoe
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Default Re: Fisher TFM1000 pulse count detector-How does it work?

Quote:
Originally Posted by Radio Wrangler View Post
In a pulse count discriminator either a monostable or a delay line time-offsetting the same input (the IF signal after limiting) going into the two inputs of a exclusive-OR gate is used to create fixed-length pulses.

The delay line one created one pulse on each edge of the limited IF (rising or falling), so it's better than the plain monostable version.

A filter averages the total area of the pulses. If there are more pulses per second, the average is higher. if there are fewer pulses per second, the average is lower. in this way FM in the IF is converted to a varying voltage.

The averaging filter must pass audio and multiplex frequencies, and stop the IF frequency

David
Thank for your reply.

It's trying to understand which component does what that I'm having problems with. I do not understand ANYTHING about transmission lines or which components IS the transmission line. It seems from the article that the first transistor (Q905 in the manual schematic) is just called a switch. I'm assuming that this just means it turns a sine wave into a square wave? The second transistor is the actual pulse counter. How does this work? Which parts form the averaging filter? What is the relevance of the 25ns delay? Is it JUST the coil L902 that causes this delay. Does a transmission line have to be used for this (You can get delays in IC form now Rhombus for example)?My idea of a transmission line is a length of coaxial cable or the traces on a PCB.

I'm going to try breaking this area of the circuit down a bit further. It's an excuse to learn a bit more about LTSpice as well.
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Old 1st Sep 2019, 2:18 pm   #9
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Default Re: Fisher TFM1000 pulse count detector-How does it work?

Have a look at this video for a basic understand of a simple "pulse counting" detector:

https://www.youtube.com/watch?v=jQlN2fc7LJc

The delay line in the Fisher is not terminated, watch this video to see what happens when a transmission line is not terminated with anything:

https://www.youtube.com/watch?v=I9m2w4DgeVk

Then bear in mind that whatever is flowing in the delay line is coupled by the delay lines capacitance to the next stage.

Lawrence.

Last edited by ms660; 1st Sep 2019 at 2:38 pm.
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Old 1st Sep 2019, 6:20 pm   #10
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Default Re: Fisher TFM1000 pulse count detector-How does it work?

The Fisher is a poor example to learn from. There has been much bean-counting and reduction of component count.

Instead, have a look at the Revox B261 tuner. this is a pulse counter without shortcuts and is a lot easier to follow. Manual on electrotanya website.

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Old 1st Sep 2019, 6:52 pm   #11
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Default Re: Fisher TFM1000 pulse count detector-How does it work?

Quote:
Originally Posted by Radio Wrangler View Post
The Fisher is a poor example to learn from. There has been much bean-counting and reduction of component count.
I'd disagree: the Fisher design was well-designed for the market-sector it addressed - and it sold well, which to me says the 'bean-counters' got it right!

[Why are 'bean-counters' seemingly deprecated? I count myself as a bean-counter with tech-awareness. We seek to produce 'stuff' that matches the consumer-marketplace of the day and is also economical-to-produce so our businesses continue to return profits to those who have put your pension-funds into the business]
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Old 1st Sep 2019, 7:11 pm   #12
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Default Re: Fisher TFM1000 pulse count detector-How does it work?

In addition to my earlier post, if you watched the transmission line video, look at what's in the first link below, you should see some waveform similarities and also see how the delay line has electrical similarity with the classic electrical representation of a transmission line, but a difference to remember is that although one of the conductors (the coil) is long, the other conductor is shorter, much much shorter in comparison, it's only the length of the delay line itself, but in terms of looking at the delay lines coupling capacitor function, current flows via it at all points along it's length corresponding to a pulse travelling through the delay line both in the forward and in the reflected directions, that total time along with the delay incurred by Q906's storage time determines the width of the pulses that are to be integrated by R928 and C917 to produce the audio, so far as I can make out from the description in the Electronics World article.

http://web.hep.uiuc.edu/Engin/CDF/CM...sc/rhombus.pdf

Not too much on the web about constructing a cylindrical delay line, however here's one, used for luminance delay in a colour telly project from the late 1960's, it clearly shows how the capacitance is obtained and the length of those elements in comparison to the length of the delay line and the length of wire used in the delay lines coil:

https://americanradiohistory.com/hd2...h=%22hh2500%22

Lawrence.
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Old 1st Sep 2019, 9:55 pm   #13
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Default Re: Fisher TFM1000 pulse count detector-How does it work?

I know nothing about delay lines in this context, but was quite familiar with luminance delay lines in colour TV sets. I even met the first "bucket brigade" chips used in early (1990?) CTVs.
With oscilloscopes however, I had an old Marconi 2200A. In that there must have been around 100 individual coils and capacitors making up the delay line for viewing the start of a trace, by allowing the trigger pulse to get thro directly, followed by the delayed Y amp signal to the CRT. These filled the LH rear side, top to bottom, and I think halfway up, and it was a tall 'scope.
A couple of years later I had a newer Telequipment D43 with a type JD amplifier. That incorporated a neat little delay line "block" made from "contra rotating" spiral coils, with capacitance to earthed central foil, giving the same effect as the many components of the Marconi DL. That same block became common on the D67, D67A, many of the Y amps of the 63, 75 and 83 series.
Les.
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Old 1st Sep 2019, 10:32 pm   #14
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Default Re: Fisher TFM1000 pulse count detector-How does it work?

We're stuck with the speed of light, or so it seems, so all signal connections and cables introduce delay. So all lines are delay lines. The ones thus named are just the ones where the delay is the wanted function.

You can make pulses timed by resistor/capacitor or resistor/inductor, but using a length of cable as a delay element is rather elegant, and can be made very stable.

You can simulate the cable with a large array of inductors and capacitors (as in Les' marconi scope) but real cable can give better bandwidth and better phase linearity.

For FM tuners where cost is an issue, the ratio detector seems to be the demodulator of choice. A pulse count demodulator has a much higher parts count unless corners are cut and then the advantage over the ratio detector is lost.

For a matter of teaching the operation of a circuit, it's best to avoid cut-down versions where the boundaries between circuit functions have been made unclear as parts have been saved.

It may sound counter-intuitive, but the one with the most parts can be the clearest to understand.

David
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Old 3rd Sep 2019, 2:05 pm   #15
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Default Re: Fisher TFM1000 pulse count detector-How does it work?

Thanks everyone for the very informative links.
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