RF detector/demodulator probe questions.

[xwarp]

1. Why so many different designs.
2. Which designs are used for FM. For example, see the following attached images. The "yellow" background images are from Eico. The last is for a Sencore AM/FM signal generator for AM/FM radio alignment.

[Fank1]

There is no huge difference in any of them.
All you are doing is rectifying the RF and filtering it so you get a dc voltage out.
They are not "instruments" just a way to detect RF and get a relative value out.

[w2aew]

These probes don't care whether the signal is unmodulated or if it is FM - they'll still read the same since FM doesn't change the RF amplitude.  Depending on the value of the capacitors used to "hold" the RF amplitude, you may be able to view the baseband on AM signals.  I did a video on this:

[gyropilot]

Sorry to resurrect such an old thread, but my question dovetails in...

Can anyone please explain to me when and why an RF demodulator probe would be appropriate to use with a scope?

Now I get why you'd want to use an RF demodulator probe with a DMM, but I'm having trouble understanding the use case for a scope.

Thanks!

[CJay]

They're not that useful now as it's a lot easier to lay your hands on much better spec test gear for cheap (Spec An, multi hundred MHz 'scope etc.) but the demodulating probe is used to check the modulating signal and can show you if there's distortion happening in an RF stage, it won't give you an indication of carrier level with a DMM.

Diode peak detectors and demodulating probes are both useful in their own way and can give you very accurate RF measurements if used appropriately

[PA0PBZ]

but the demodulating probe is used to check the modulating signal and can show you if there's distortion happening in an RF stage

I agree, but since AM modulation is normally used at lower (HF) frequencies it is equally easy to just feed the signal into the scope directly*

^{*Of course properly scaled}

[CJay]

but the demodulating probe is used to check the modulating signal and can show you if there's distortion happening in an RF stage

I agree, but since AM modulation is normally used at lower (HF) frequencies it is equally easy to just feed the signal into the scope directly*

^{*Of course properly scaled}

Absolutely, but  back in the day where a 5MHz 'scope was a luxury item that cost many months of salary a demod probe was an exceptionally useful addition to the test gear armoury, these days a 200MHz scope can be had second hand for less than a couple of days wage and has removed a lot of the need for such tools, I'd still contend that the probe can be useful but it's far from essential.