The AM Forum

Can anyone point me toward plans for a homebrew RF Sampler and Demodulator for X-Y oscilloscope monitoring that will handle full legal power?

Thanks & 73,
Barry N1EU

To do this right, you will need to get an attenuated audio sample from the output of the modulator and feed the Horizontal plates of your display CRT.

The attenuated RF goes to the vertical plates.

The schematic is from the Radio Handbook, 13th ed., P. 618

You can use a regular O-scope by just using the H&V inputs, sweep and timing not needed.

73DG

This video shows a homebrew low power circuit and measurement using an oscilloscope:

http://www.youtube.com/watch?v=y4Zt_LJX1Tc&feature=related (http://www.youtube.com/watch?v=y4Zt_LJX1Tc&feature=related)

I'd like to be able to do this at full power and just hoping to see some suggested circuits for a high power sampler/demodulator.

Experience has shown that 'recreating' the audio waveform by demodulating it introduces errors not found in the actual modulator output sampling method.

The 'power' of the transmitter has nothing to do with it.

73DG

Here's a link to some inductive RF pickups and demodulators.
http://home.comcast.net/~msed01/RFpickup.html (http://home.comcast.net/~msed01/RFpickup.html)
You can adjust the level of signal sampled by winding more or less turns on the transformers.

However, beware, the described (Youtube) method of deriving a "trapezoid" by demodulating the RF and comparing the two is useless!  By simple identity the demodulated RF and the RF itself must be virtually identical (save for negligible diode voltages) and will always show a "perfectly linear" trapezoid pattern even under the worst signal conditions!  

For a trapezoid to have value it must compare differing signals before and after some stage or component. For example; comparing the modulating plate voltage to the RF waveform indicates if the final has sufficient drive or adequate loading, etc.  Most stations utilize a trapezoid to compare the input and output of a linear amplifier to determine if in fact it is being operated in a "linear" fashion; proper load, drive, bias, etc.  

Comparing the "RF output" to the detected level of the "RF output"...  useless.
In any event, being able to monitor your signal reliably is a valuable asset in the AM shack.  You'd be well served by building some RF sampling devices for that purpose.

73,
Mark
 

Trapezoid schmapezoid, real men simply just look at the outpoot waveform. that tells the tale of the modulation percentage, and the positive and negative peaks.................

I find the trap more useful -- i can see asymmetry with it more easily, and if something goes non-linear I can see that immediately. 

The RF pickup is easy--if the feed is unbalanced you can just insert a good feedline switch like a Tesso that has a lot of isolation and feed an unused position to the scope.  if the feedline is balanced it's even cheaper and easier--just mount a short pickup wire a few inches away from the feedline in a way that does not unbalance it.

getting the audio off the modulator is harder but in one of the threads here within the past few months someone posted how to do it with a voltage divider network between the mod. transformer secondary and the PA plate choke.

When tuning up an amplifier for the first time, or changing to a band I'm not familiar with, I like to use the trapezoid initially. It's especially useful (along with a Spectrum Analyzer) to find a satisfactory bias level.
After that, I never look at it again and find the scope pattern and a discerning eye quite sufficient for daily operation. 
I also find looking at RF phase relationships between inputs and outputs pretty interesting.  :) But that's just me.
YMMV