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Old 30th May 2022, 10:48 am   #1
DeeBee1
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Default AM Suppression in SW Receiver

Hi, my apologies if I have posted this in the wrong sub-forum.

I've been building direct conversion short wave receivers from various circuits in books and magazines over the past year or so. Some have worked well and others not so well, but I feel I have learned a lot about RF electronics!

My latest project is for a 20m DC receiver for CW and SSB. It seems fairly straightforward except for the addition of a preset resistor in the tuned RF circuit that is used to 'suppress AM stations' (see picture). I have had AM breakthrough in other designs so this interests me, but I can't work-out how the addition of resistance here achieves this. It clearly adds to the R of the RLC circuit but how would that specifically suppress AM?

Can any kind person please shed some light on how this works if they have seen this before?

Thank you.
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Old 30th May 2022, 3:05 pm   #2
commie1
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Default Re: AM Suppression in SW Receiver

Quote:
Originally Posted by DeeBee1 View Post
Can any kind person please shed some light on how this works if they have seen this before.
Looks like a balance mixer to me, have you tried it and if so, does it suppress AM break through like it claims?

I doubt very much it will be any different to any other balanced mixer but it's a simple circuit so why not build it on breadboard and see.
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Old 30th May 2022, 3:26 pm   #3
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Default Re: AM Suppression in SW Receiver

The potentiometer creates an adjustable centre-tap on the coil, which allows you to balance the detector for any slight variations in things like the characteristics of the diodes.

Some similar designs will also include a small adjustable trimmer-capacitor from one side of the pot to ground, so you can balance out any capacitive inequalities. Which side of the pot would need the capacitor will depend on circuit layout.
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Old 30th May 2022, 10:44 pm   #4
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Default Re: AM Suppression in SW Receiver

The mixer is a "Single-balanced' type.

There are also unbalanced mixer types and double balanced types (and some exotic jobs!)

The balancing of this mixer nulls passage of signals from the RF port straight through to the audio amplifier. Any strong RF signal can overload the audio amp and cause self rectification in the semiconductor junctions, thus demodulating any AM they may have.

So the resistor does NOT help remove AM, it helps remove all signals trying to get straight through. It's only the AM ones that you'll hear.

Correct adjustment won't give you dramatic cancellation, but every bit helps.

David
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Old 30th May 2022, 11:27 pm   #5
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Default Re: AM Suppression in SW Receiver

Full schematic attached.
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File Type: pdf Elektor-1998-04.pdf (2.12 MB, 85 views)
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Old 31st May 2022, 6:15 am   #6
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Default Re: AM Suppression in SW Receiver

Ah, there is only a variable capacitor resonating the mixer transformer primary and no serious filtering ahead of the mixer to reduce out-of-band signals. There's nothing to stop them tenderising the FET RF amplifier, it's completely broadband.

It's a straight forward small direct-conversion receiver, designed around parts likely to be in a junk box. Inevitably there are limitations. If you run into overload problems, using a receiver type ATU ought to help.

In the 1980s, the Jandek stand at a lot of radio rallies had one of their little DC receivers running and people were rather surprised at how good this sort of receiver sounded.

There is no sideband selection, you hear whatever lies above your LO superimposed with whatever lies below, so it can get some frequency-inverted and offset audio appearing in a congested band. Find signals in clear areas and it'll be great.

It is possible to make a DC receiver select sidebands, it has to have two signal paths with two mixers running from LOs which are 90 degree phase shifted. The trouble is in doing 90 degree phase shifting for the audio, the phase shift has to be accurate not only across the audio band you want to hear, but across the full audio range the audio section can pass. Some SDRs use this technique, but pass their two audio paths into a stereo soundcard for the software to do the phase shift with a Hilbert-transformed digital filter pair.

So this little DC receiver is the tip of an iceberg!

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Old 31st May 2022, 7:42 am   #7
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Default Re: AM Suppression in SW Receiver

There were a number of kits available, Lake, Howes to name two selling kits for DC receivers among other items. I built two or three and had lots of pleasure from them with a QRP CW rig from the same people.
What I liked about them was their simplicity, just had to be aware of their limitations.
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Old 31st May 2022, 9:24 am   #8
DeeBee1
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Default Re: AM Suppression in SW Receiver

Thanks for your replies.

I've had mixed success with a number of direct conversion circuits from books and magazines, the most successful being RA Penfold's receiver from 'Simple Shortwave Receiver Construction' and some circuits from 'Experimental Methods in RF Design'. Limited success with a couple of receivers published in magazines.

Great fun, and I've built up a good stock of components!
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Old 31st May 2022, 10:15 am   #9
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Default Re: AM Suppression in SW Receiver

Quote:
Originally Posted by Nuvistor View Post
What I liked about them was their simplicity, just had to be aware of their limitations.
About 20 years ago, I decided to investigate direct conversion for 80,40 and 20m. Using a mc1496 multiplier preceded by bandpass coils, on 80m I could achieve about 1uV sensitivity but on 40 and 20, it took off with audio oscillations. The only way I could get 40 and 20m stable was to reduce the audio gain, which meant it would have compromised sensitivity on 40 and 20m.

I wonder what the sensitivity of this 20m dc receiver is? They don't seem to have published it in the article.

I have found the best method and with minimal complexity radio design is to go for a single conversion superhet, spec. is good throughout.
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Old 31st May 2022, 11:44 am   #10
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Default Re: AM Suppression in SW Receiver

The detector circuit looks very similar to the one that was published by Spaargaren.

Built quite a few DC receivers over the years, solid state and valve, single balanced mixer (trifilar wound coil) with the balance pot at the detector output, nulled out AM very well, managed to receive a transmission from the Falklands once, oscillator input was from a bench signal generator.

Lawrence.
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