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Old 29th Apr 2019, 9:28 pm   #1
Avid_Nerdlinger
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Default Simple amplitude modulator for radio alignment.

I'm starting to think about building an audio signal generator and mixer. My TG102 signal generator (0 - 2MHz) has a sweep input which might be useable for FM but nothing for AM.

I can't find any cheap Chinese things I can buy off the shelf nor can I find a likely project in this or other forums. My skills are rusty (they were never that shiny) so I might be missing the right search terms. Can anyone point at an existing solution or a cheap product?
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Old 29th Apr 2019, 10:13 pm   #2
paulsherwin
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Default Re: simple amplitude modulator for radio alignment

The easiest solution is to use software on a computer or smartphone.

e.g. https://play.google.com/store/apps/d...pqd1fAP6OWEuAY
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Old 30th Apr 2019, 2:30 pm   #3
Avid_Nerdlinger
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Default Re: Simple amplitude modulator for radio alignment.

Nice app thanks. I've been looking at TG102's schematic and I'm now wondering about modulating it by injecting a sine wave at the output level potentiometer. There doesn't seem to be any heavy load or filtering there. I could test it with that app on my phone but I've just ordered a Chinese function generator kit for 1.44...
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Old 30th Apr 2019, 3:15 pm   #4
MrBungle
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Default Re: Simple amplitude modulator for radio alignment.

TG102 isn't exactly going to be a stable signal source for that sort of stuff. They drift around all over the place. Might be worth doing things about face and building a little transmitter such as the ludens one here and using the TG102 as the modulation source for it:

https://ludens.cl/Electron/amtx/amtx.htm
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Old 31st Jul 2019, 10:41 am   #5
Avid_Nerdlinger
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Default Re: Simple amplitude modulator for radio alignment.

The output of that Chinese 1.44 function generator was too weak so I bought an LM358 (d'Oh! - not enough bandwidth for the CW) kit for 3 to buffer it... Wired this directly to the output of TG102 and was able to get a decent looking AM waveform on the 'scope. After then connecting a length of screened cable with a 470R and a 10" 3-turn wire loop the waveform was quite distorted but good enough for the RF alignment procedure in the Hacker RP35 service manual.

Maybe I could work on this a bit more - sort out the loading, fix the distortion, add an AF amplitude adjustment pot (currently fixed and variation achieved by moving further away...). It might be worthwhile because I'm struggling to get a suitable sig gen off ebay, either thay are going for silly money or I have totally lost touch with consumer price inflation.
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Old 31st Aug 2019, 4:10 pm   #6
Oldcodger
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Default Re: Simple amplitude modulator for radio alignment.

Avid - found this simple ,called cheapo oscillator from an old copy of ETI CIRCUITS . I apologise for the quality but it's dated Nov 1976 and suffered from one dog I had using my stack of mags as a bed to escape from a brood of pups. No frequency given, but it's most likely an RC MIX on both of the gates. No doubt someone knows the how & why.
As for the mixer- something simple like a bridge ring modulator ( othr ex GPO/PO blokes will remember the theory better than me ), as shown in this circuit
http://www.eeeguide.com/diode-bridge-type-modulator/
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Old 31st Aug 2019, 4:37 pm   #7
Avid_Nerdlinger
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Default Re: Simple amplitude modulator for radio alignment.

Thanks Oldcodger. I had similar thoughts about a mixer and found some transformer and diode combos available on ebay but the costs were getting out of hand as was the cumbersome prototyping with all of the wires and power supply complications (I used batteries). I ended up buying an Advance E2 for 41 delivered so I don't need to pursue this Heath-Robinson project any further.

Andrew.
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Old 1st Sep 2019, 9:32 am   #8
lesmw0sec
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Default Re: Simple amplitude modulator for radio alignment.

Why not just use a voltage controlled amplifier chip, and vary the gain of it either by an audio osc. or some other audio source?
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