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 Post subject: Demodulator probe for oscilloscope question.
PostPosted: Dec Sun 11, 2016 3:26 pm 
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Have seen a couple you tube vids that use demod probes. One in particular is a BK. Couldn't see model number though. Found a new BK pr32a. Frequency range is perfect but the voltage spec is only 200 volts.
Would this be acceptable for doing the am/fm alignment? Probe should block the dc which in most of the vintage radios I have, the plate voltage is below this.

Suggestions welcome.

Jim

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 Post subject: Re: Demodulator probe for oscilloscope question.
PostPosted: Dec Sun 11, 2016 8:12 pm 
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Most of the early RF/demod probes contained 1N34 diodes that were only good for about 50 volts. The later ones with video detector diodes were rated up to 200 volts--if you were lucky. Thing to understand is, they were never meant to be used on the plate circuits of tube circuits. Even with a DC blocking capacitor it is still possible to store up enough of a charge in the cap to blow the diode if you ground the tip after touching the tip to B+. These probes were meant to be used on signal voltages only.

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 Post subject: Re: Demodulator probe for oscilloscope question.
PostPosted: Dec Sun 11, 2016 8:24 pm 
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Hi Jim,

In most cases, a demodulator probe is not needed for AM or FM alignments. If the AM or FM detector in the radio is alive, one generally connects the voltmeter or scope in a manner that allows the built-in detector (demodulator) to do its job.

Demodulator probes were more popular in the past when a typical scope did not have the bandwidth to look at RF signals directly. For example, a lot of typical radio/TV shop scopes only had 1 MHz of bandwidth. This was fine for looking at sync signals for TVs (the main use of the scope in a typical radio/TV shop) but wouldn't really allow looking at RF signals, except possibly an AM IF at 455 kHz.

Lots of scopes today that are affordable offer 10 - 100 MHz of bandwidth, so you can look at both AM and FM IF signals directly if you want to.

Anyway, a detector (demodulator) probe allows you to see the amplitude of an RF signal without trying to look at the waveform directly. If you want to check the amplitude of an RF signal at stages prior to the radio's built-in detector, or if the built-in detector isn't working, you can do that with a detector probe.

If the probe itself doesn't have a high enough voltage rating for the DC voltage at the point you want to probe, you can always add another capacitor in series between the probe and the circuit you want to check. For example, a .01 uF 630 V capacitor will handle most cases you might want to check.

Although I had a detector probe with the first scope I bought back in the 1980s (a typical 1 MHz TV shop scope from the 1950s), I don't even have a detector probe now that I use for anything. For AM and FM alignments, I simply probe the AM or FM detector output with a 10X AC or DC coupled scope probe, and if for some reason I want to see the RF (or IF) directly, I use the 10X probe with AC coupling to look at whatever part of the circuit is of interest. In most cases, for probing RF, I prefer not to actually touch the scope probe to the point of interest, since parasitic capacitance in the probe tends to shift the alignment or load down the signal. So I simply put the probe very near the point of interest, and unless the signal level is pretty low (as it would be in a front end stage), I can see the signal without disturbing it. This is especially useful for checking oscillator waveforms and frequency with a scope.

Although some old alignment instructions may call for a detector probe (because they assumed you had a scope too slow to see a signal directly), in most cases today you can probably accomplish what you need without a detector probe.

In general, for typical AM and FM radio alignments, I never need to probe the RF or IF directly. I simply look at the detector output, and that is fine (no detector probe needed).

If you have a specific case you're trying to troubleshoot or align with a detector probe, feel free to describe it here in more detail, and various people here can give advice on how to use a detector probe, and also how to get the same job done without a detector probe.

By the way, the caution in the post above about being careful not to probe too large a signal with a detector probe is important. Although a blocking cap gets rid of the DC concern, you want to be careful that the amplitude of the RF you're looking at is not more than a few tens of volts at most, or an older detector probe may meet its demise.

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 Post subject: Re: Demodulator probe for oscilloscope question.
PostPosted: Dec Sun 11, 2016 10:25 pm 
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Thanks you guys that helps a great deal.

Tom,

Your explanations are always simple and precise. Much appreciated.

Jim

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 Post subject: Re: Demodulator probe for oscilloscope question.
PostPosted: Dec Mon 12, 2016 1:06 pm 
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Location: Long Island NY
It should be pointed out that the purpose of a demodulator or detector probe is the same as the detector in a receiver; it removes the RF portion of a modulated signal, leaving a time-varying DC voltage which carries the information contained in the modulation. For AM, this will be the recovered audio; for FM it is possible to get some semblance of the original signal through what is known as slope detection. So a detector or diode probe is useful for signal tracing, in particular with audio signal tracers where demodulation is necessary before you can hear anything.

The real use of detector probes is in connection with sweep alignments. There, the frequency sweep is assumed to have constant amplitude which is "modulated" or modified by the bandpass of the circuits you are trying to align. In conjunction with suitable marker frequencies, the resulting trace on an oscilloscope screen gives you a pretty good idea what the center and edge frequencies are. Now as Tom mentioned, these procedures are usually done using the detector stages already built into AM and FM receivers. However, there are times when you want to look at individual stages upstream of the detector. This puts demodulator probes into the "nice to have" category, but they're not "must-haves."

One thing to be aware of, when buying a used demodulator probe is, a great many of them turn up with blown diodes due to past misuse. Do not pay a whole lot for one unless you can send it back if it doesn't work. It only takes a few parts to put a probe together and there are lots of schematics available online, so making your own is an easy project.

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 Post subject: Re: Demodulator probe for oscilloscope question.
PostPosted: Dec Mon 12, 2016 3:03 pm 
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Thanks Chris,

Did purchase a new one. Was wanting it to play with seeing the signal at each stage wth the sweep generator and markers. Made a simple detector for tracing but doesn't seem to work. Made from drawing online. Was trying to make a signal tracer with pc speakers. Probably try again sometime.

Jim

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 Post subject: Re: Demodulator probe for oscilloscope question.
PostPosted: Jan Sun 22, 2017 5:59 pm 
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Joined: Sep Tue 20, 2016 8:19 pm
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I broke down and bought an older active FET probe. The FET probe was cheap...the power supply was not. I am a beginner, but my understanding is that passive probes introduce too much load on RF circuits. The FET does not. In my case, my passive probes never showed me a correct picture when looking at IF sweeps. My FET shows it perfectly. As was mentioned, most scopes have plenty of bandwidth to look directly at the 10mhz range of IF, if you can't see what others keep showing online (pretty peak curves), then start with a better probe. I also have an old Fluke RF probe that I adapt to my scope and it also works fine and shows the correct picture. (Also, I bought an IF can 42IF 123 green, just so I could work out my equipment outside of a real tuner).


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