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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 10th Oct 2010, 4:48 pm   #1
vinrads
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Default Product detector circuit

I have been an avid SW listener for years , and would like to hear from people who have built a successful product detector .

I am using a Eddistone 640 , and a GEC BRT400 receiver.
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Old 10th Oct 2010, 5:37 pm   #2
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Default Re: Product detector circuit

Valve or transistor? There are some designs on the Web which look as if they can be built in to existing receivers. I once built a 12 transistor shortwave superhet with a product detector and it worked extremely well. That was nothing more than a simple two transistor circuit (using OC45's I think) so that gives some idea of how long ago it was....!

The only valve circuit I can recall now is in the RSGB handbook that uses an ECC82. I reckon you could build that into the Eddystone without too much trouble and according to the text, can also be used as an 'outboard' item. There are no special parts required and most of the parts should be found in the average scrapbox. Not sure about reproducing the circuit here because of copyright issues.

Remember you'll still need to retain the normal AM demodulator because the product detector will not demodulate AM so some switching will be necessary.


SB
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Old 10th Oct 2010, 5:42 pm   #3
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Default Re: Product detector circuit

I have heard of many detectors, but not this. Please could somebody tell me what a product detector is? I know that using the Base to Emitter junction on a transistor (I have used a OC45) gives an amplified signal at the collector.

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Old 10th Oct 2010, 5:56 pm   #4
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Default Re: Product detector circuit

Product detectors are mainly found in Communications receivers. They are particularly useful when listening to SSB transmissions. Very simply, they are a mixer type demodulator where one input is fed directly with the I.F signal and the second input from the BFO (or the carrier insertion oscillator). The valve (or transistor) then mixes the two inputs to provide an AF output. It won't reproduce AM because, as the name implies, it requires two inputs to provide the output. The circuit is quite simple and just requires a small amount of RF filtering on the output.

This is a very simplistic description and no doubt someone else can give a more detailed description if necessary.


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Old 10th Oct 2010, 6:23 pm   #5
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Default Re: Product detector circuit

hmm, if i am not all wrong that means about same effect as a slightly oscillating regenerative receiver. please tell me if i am wrong.

more or less a stable oscillation and then add in the modulation as a product or difference depending on what part of the signal is picked up with the LC circuitry.
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Old 10th Oct 2010, 6:42 pm   #6
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Default Re: Product detector circuit

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Originally Posted by Sideband View Post
It won't reproduce AM because, as the name implies, it requires two inputs to provide the output.
SB
?
http://en.wikipedia.org/wiki/Product_detector

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Old 10th Oct 2010, 8:09 pm   #7
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Default Re: Product detector circuit

It will do AM, but unless very stable local oscillators you'll easily get a hum to whistle. (the product of AM carrier and injected local osc). Note that a stable oscillator (BFO) doesn't solve it, you need the L.O. for the Superhet to be stable too. It will also do FM as well as SSB or Morse. The main original purpose of BFO was for Morse. To deliberately offset by about 800Hz from the carrier, not SSB.
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Old 10th Oct 2010, 10:50 pm   #8
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Default Re: Product detector circuit

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Originally Posted by Sideband View Post
Product detectors are mainly found in Communications receivers. They are particularly useful when listening to SSB transmissions. Very simply, they are a mixer type demodulator where one input is fed directly with the I.F signal and the second input from the BFO (or the carrier insertion oscillator). The valve (or transistor) then mixes the two inputs to provide an AF output. It won't reproduce AM because, as the name implies, it requires two inputs to provide the output. The circuit is quite simple and just requires a small amount of RF filtering on the output.

This is a very simplistic description and no doubt someone else can give a more detailed description if necessary.


SB
Your description is fine.
A few added points.

The level of the BFO (Beat Frequency Oscillator) injection into a normal diode AM detector has to be high for this method to work. This upsets the operation of the sets AGC circuit. The AGC sees the BFO as a very strong signal and turns the sets gain down accordingly. This is why most sets of this type tell you turn the AGC off and use the RF gain control. A product detector, which is usually tapped off from the same place as the diode detector,does not cause this effect.

A set fitted with a product detector is easier to use. There is usually a switch marked LSB(below 7 Mhz), USB(above 7Mhz) and AM. Select the correct one and use the main tuning until the signal is intelligible.

A product detector is just a variation of the sets mixer circuit.

Sorry if this is a bit wordy

Al
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Old 10th Oct 2010, 11:06 pm   #9
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Default Re: Product detector circuit

I'm currently toying with a product detector for an RA17L and would also be interested in any designs going (especially the coils). I was planning to use the BFO on-off switch to switch between ssb and am. As I understand the BFO control would operate at the same settings as before (ie +/- 1.5 - 2.0 KHz for LSB / USB depending on filter setting).
Rob.
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Old 10th Oct 2010, 11:35 pm   #10
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Default Re: Product detector circuit

Quote:
Originally Posted by Sideband View Post
Valve or transistor? There are some designs on the Web which look as if they can be built in to existing receivers. I once built a 12 transistor shortwave superhet with a product detector and it worked extremely well. That was nothing more than a simple two transistor circuit (using OC45's I think) so that gives some idea of how long ago it was....!

The only valve circuit I can recall now is in the RSGB handbook that uses an ECC82. I reckon you could build that into the Eddystone without too much trouble and according to the text, can also be used as an 'outboard' item. There are no special parts required and most of the parts should be found in the average scrapbox. Not sure about reproducing the circuit here because of copyright issues.

Remember you'll still need to retain the normal AM demodulator because the product detector will not demodulate AM so some switching will be necessary.


SB
Mods can you help here?
I have the circuits from the copyright 1961 3rd edition and the copyright 1968 4th edition of the RSGB handbook in front of me. The circuits are basically the same standard circuit although component values change and an RFC turns into a resistor. Would we infringe copyright by sending the the cct and text to the OP?

Al
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Old 11th Oct 2010, 12:02 am   #11
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Lightbulb Re: Product detector circuit

Quote:
Originally Posted by glowinganode View Post
I'm currently toying with a product detector for an RA17L and would also be interested in any designs going
Rob.
This is an idea that has been in my 'one day I'll try this' book for simply ages .
If you do try it, Rob, please let me know how it works out.

http://www.vmarsmanuals.co.uk/newsle...t_detector.pdf

Al. / Skywave.
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Old 11th Oct 2010, 12:09 am   #12
Alistair D
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Default Re: Product detector circuit

Quote:
Originally Posted by glowinganode View Post
I'm currently toying with a product detector for an RA17L and would also be interested in any designs going (especially the coils). I was planning to use the BFO on-off switch to switch between ssb and am. As I understand the BFO control would operate at the same settings as before (ie +/- 1.5 - 2.0 KHz for LSB / USB depending on filter setting).
Rob.
The circuit(s) in the RSGB handbook rely purely in the IF passband for any filtering. The width of the passband does not alter the perfomance of the product detector. It does reduce the possibility of adjacent channel reception.

On my Yaesu FRG 7000(also a Wadley Loop receiver) the AM, USB, LSB switch does 2 things. Between AM and USB/LSB it switches crystal filters from 8khz down to 2.2khz then the USB, LSB positions switch different crystals into the BFO.

Al
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Old 11th Oct 2010, 7:56 am   #13
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Default Re: Product detector circuit

What's wrong with the existing if filtering?
At least adding a product detector would allow the s meter and agc to work properly.
Rob.
p.s. thanks for the link Al, I'd never thought of doing it that way.
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Old 11th Oct 2010, 8:07 am   #14
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Default Re: Product detector circuit

Quote:
Mods can you help here?
I have the circuits from the copyright 1961 3rd edition and the copyright 1968 4th edition of the RSGB handbook in front of me. The circuits are basically the same standard circuit although component values change and an RFC turns into a resistor. Would we infringe copyright by sending the the cct and text to the OP?
You probably would be infringing copyright, but I doubt anyone would care. Who would know in any case? Questions like this are best not asked, as knowing the answer you can't then plead ignorance.

Please don't turn this thread into a discussion about copyright.
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Old 11th Oct 2010, 11:03 am   #15
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Default Re: Product detector circuit

Quote:
Originally Posted by vinrads View Post
I have been an avid SW listener for years , and would like to hear from people who have built a successful product detector .

I am using a Eddistone 640 , and a GEC BRT400 receiver.
You have 2 receivers with different IF's (frequency), if you choose a design ensure it can incorporate switched IF frequency. You may be able to get some ideas from the Eddystone receivers that had product mixers fitted, the EA12 for instance , 940 , 840.

There used to be a kit sold in the states for the collins R390A which was quite succesful , this was based on modern components and had good reviews.

Mike
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Old 11th Oct 2010, 2:36 pm   #16
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Default Re: Product detector circuit

A product detector is a mixer, therefore any mixer circuit can be modified to act as a product detector. You just have to change the output to handle audio instead of RF, and probably low-pass filter the remaining RF away. One thing to bear in mind is that a product detector is probably handling larger signals than a front-end mixer (except in a direct-conversion receiver) so signal handling is more important than noise.

If you are familiar with broadcast receivers, then one thing to try is an ECH81 product detector. Input from IFT, oscillator triode uses single-tuned IFT (or half an IFT), output is a resistor instead of an IFT. Unusually, this product detector can even have AGC too!
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Old 11th Oct 2010, 2:43 pm   #17
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Default Re: Product detector circuit

As promised.
Attached Files
File Type: pdf Page1.pdf (363.6 KB, 149 views)
File Type: pdf Page2.pdf (498.0 KB, 126 views)
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Old 11th Oct 2010, 5:34 pm   #18
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Default Re: Product detector circuit

Those are the same circuits I have although my RSGB edition is slightly later dated June 1969. They give the value of the RF choke in the 12AU7 (ECC82) circuit as 1.5 to 2.5mH. Should be dead easy to build that into an exsiting receiver.



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Old 11th Oct 2010, 7:03 pm   #19
vinrads
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Default Re: Product detector circuit

Thanks for the circuits and comments i hope to be building them to see how they work.
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Old 11th Oct 2010, 9:21 pm   #20
Alistair D
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Default Re: Product detector circuit

Quote:
Originally Posted by Sideband View Post
Those are the same circuits I have although my RSGB edition is slightly later dated June 1969. They give the value of the RF choke in the 12AU7 (ECC82) circuit as 1.5 to 2.5mH. Should be dead easy to build that into an exsiting receiver.



SB
Have a look at page 4.20 of your edition there is a variation of the circuit that does not have the RFC. That was the one I found last night.

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