Author Topic: RF sampler / demodulator for oscilloscope  (Read 7428 times)

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Offline Radio Tech

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RF sampler / demodulator for oscilloscope
« on: March 17, 2015, 06:47:42 AM »
So you went out and got you a 0-scope.
Now what can I do with this?

Plenty. One thing is monitor the purity of your carrier and you’re audio. And if you got yourself a multi-channel scope then you can do both at

the same time.  Simply run your RF through this simple device between your radio and watt meter. Connect the sample port to channel A and the

DEMOD port to channel B. Select channel A and view your carrier, now select channel B and view your audio that has been picked from the carrier

signal.

While monitoring the carrier you can look at the signal for distortion, also look for bright spots that indicate you are reaching 100%

modulation.

While looking at the demodulated audio you can view the scope to see if your modulation is flat topping or distorted.
Now view both signals at one time. Observe your modulation riding on top of your carrier.
This is a great little tool when working on transmitters.  Nothing worse than having a customer come back because folks tell him he sounds

distorted.
This is also great for SSB rigs if your scope has the feature to drive x and y and then you can view the trapezoid waveform on ssb. 

Parts list
Small case or 1 inch by 4 inch double sided circuit board
4- BNC chassis mount connectors (if you want you can use 2 SO-239 for RF in and out  and 2 BNC for sample and demod)
1-  0.10 uf cap
1 -  10 pf cap
1 – 1000 pf cap
2 – 1N34A geranium diodes (1N60 will work)
1 – 690K resistors
1 - 47K resistors
1 – 1K resistors

Resistors can be ¼ watt since we are only using this to monitor 100 watts and down.
Buddy kc4umo

Offline Paul M0BSW

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Re: RF sampler / demodulator for oscilloscope
« Reply #1 on: December 11, 2016, 02:27:19 PM »
I have been meaning to build one of these for some time, so I'll give yours a whirl,  Buddy.
73 Paul

Offline Radio Tech

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Re: RF sampler / demodulator for oscilloscope
« Reply #2 on: December 11, 2016, 03:20:51 PM »
Alan w2aew built this a while back. Works like a charm and he has a YouTube video on it.


Direct YouTube link
https://www.youtube.com/watch?v=y4Zt_LJX1Tc

Buddy kc4umo

Offline RJ

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Re: RF sampler / demodulator for oscilloscope
« Reply #3 on: December 11, 2016, 03:39:51 PM »
Cool device. I'm still looking for a box to build me one of these as well. I already have a RF sampler that I bought but I'm almost sure it has a choke in it. Never took it apart to see. Could have room in there to add the demod part but think I would rather just build a full unit like you did.

Offline Radio Tech

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Re: RF sampler / demodulator for oscilloscope
« Reply #4 on: December 11, 2016, 08:29:09 PM »
Yep this is one of the best samplers I have used to date. Good design.  As far as project boxes here is a thought, if you have some pcb material around you can make a box with it. Just solder it together.
Buddy kc4umo

Offline giboni

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Re: RF sampler / demodulator for oscilloscope
« Reply #5 on: May 23, 2018, 11:19:45 PM »
 :)

 Looks easy to build. Am going to use a steel electric conduit box to put it in.

Apparently. For lets say 1KW+ on AM. I need to use larger resistors. What value and watts could they be ?

Best John