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 Post subject: Demod probe results
PostPosted: Apr Wed 17, 2019 10:49 pm 
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I put together this demodulator probe with a view to using it for an attempt at sweep alignment. The caps are 1 nf and the diodes are 1N34A's:
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Here is the output of my hobby grade sig-gen (Precision E-75) @ 300 KHz without the demod probe (not a very pretty version of a sine wave, is it?):
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Here's the trace using the demod probe with the sig-gen output set at unmodulated RF, 300 KHz:
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And finally with the sig-gen output set at modulated RF, 300KHz:
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Based on these results, does it look like this demod probe will be adequate for sweep alignment (which I have never done)?

Tom


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 Post subject: Re: Demod probe results
PostPosted: Apr Wed 17, 2019 11:05 pm 
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Is the sweep IF alignment for AM type radios. If your scope has enough bandwidth you don't really need a demod. I do mine without a demod.. But to answer your question, looks to be working, but I'll let others chime in.
Here is a 455 Khz sweep without a demod probe on a 50 Mhz scope through a 455 Khz IF transformer5.
.


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Last edited by pauls.ironhorse on Apr Fri 19, 2019 3:20 pm, edited 4 times in total.
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 Post subject: Re: Demod probe results
PostPosted: Apr Wed 17, 2019 11:07 pm 
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Put a reasonably clean sine wave into it.


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 Post subject: Re: Demod probe results
PostPosted: Apr Thu 18, 2019 8:29 pm 
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If I understand correctly, the scope photos posted only show what the demodulator does to the signal source, and tell us nothing about the demodulated output.

Looks as if the demodulator presents a serious load to the generator used as a signal source.

It's "traditional" to use a pretty large load resistance for service bench demodulator probes. 10k ohms is much lower. A high value resistor will reduce how much the demodulator loads the signal.

Note that most sweep measurements were usually done at 60Hz, or even lower sweep rep rates, so the low load resistance would not be necessary. To allow an even higher load resistance, the capacitor values might be reduced to retain the response down to 60Hz.

The test equipment designers also attempted to make their demodulators resistant to destruction by connection to B+. Lower capacitor values, first diode cathode to ground, series resistance, can all help keep the diode from popping when the probe hits the wrong pin.

Ted


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 Post subject: Re: Demod probe results
PostPosted: Apr Thu 18, 2019 9:16 pm 
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Looks like I might have fried my last 1N34A removing the 10K resistor and subbing in a 1M resistor. Yes, I did use a heat sink. :oops:

Tom


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 Post subject: Re: Demod probe results
PostPosted: Apr Thu 18, 2019 10:24 pm 
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-The demodulator is not loading the signal source. Your signal source is a mess. You can bump the 10k up to about 33k or so.
-The capacitor parallel with the 10k resistor is probably not necessary. I would drop the input capacitor down to 100 pF, but this is not critical at this point and you can experiment with that later.
-If a heatsink is necessary for any component, something is seriously wrong.
-You can use a Schottky diode instead of a germanium one.
-I don't know what the output impedance of the signal generator is, but you should use a terminating resistor on the output signal.


MOST IMPORTANT:
You need to fix your signal generator so it will put out a proper modulated sinewave, then you can experiment with the response and workings of the probe.


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 Post subject: Re: Demod probe results
PostPosted: Apr Fri 19, 2019 3:08 am 
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Location: Lexington, KY USA
The ugly RF waveform from the generator will still work for alignment. The tuned circuits in the radio will not respond significantly to the harmonics.

Most early service shops had no scope, so they didn't even know.

I'd be more worried that a generator with such an output waveform might not maintain a constant level of the fundamental frequency over the range being swept.

Some frequency counters may have a cow when fed such a signal. Getting a steady display of the fundamental frequency may require some fiddly adjustment of the counter's trigger control.

It's really not clear what the scope screen shots show us. Is this the generator output in every case? Are the scope settings the same in all cases? Where is zero volts on the Y-axis?

You can experiment using silicon small signal diodes such as the 1N4148. Just don't expect good results on mV level RF. For alignment, the signal to be demodulated can be several volts, so fancy diodes may not be needed.

Ted


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 Post subject: Re: Demod probe results
PostPosted: Apr Fri 19, 2019 12:48 pm 
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You want to see something like this, or close. This is a B&K probe which seems better than most. The bottom of the demodulated signal is a bit distorted, but it really doesn't matter. A lot of commercial probes don't do nearly as good of a job at demodulating as this one. Some probes end up putting out nearly a sawtooth due to the time constants in them.

The bottom trace in the picture is the 300Khz modulated signal, and the top trace being the output of the demodulator probe.

It will be difficult to experiment with values in your probe (time constants, etc) if you are not getting a better signal out of it. But as stated by Ted, the fundamental is what will be primarily passed by the radio.

Image


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 Post subject: Re: Demod probe results
PostPosted: Apr Fri 19, 2019 3:35 pm 
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I use service grade scopes and sweep generators, ie: old crap...

The generator sweep rate is 60hz., center frequency is whatever I want to look at, the sync is a saw-tooth (not TTL) that is input to the scope horizontal from the generator, so scope is sweeping 60hz. There is no re-trace blanking so there is a double hump that can be rotated to look like one, the de-modulator probe has the same circuit shown but FWIR it has video diodes in it., works the same. I use a second generator or a grid-dip meter as a marker oscillator to see if the attenuation of the side-bands is somewhat similar. Marker injection is either by loop injection or link into a marker input, what ever gets the signal into the front end without overload.

The traces I get are identical to those often shown in Riders for certain high end radios and in Sams for TV alignment.

Sweep alignment is fun once understood, but the resultant fidelity for AM band is a waste, if there are a dozen Hi-Fi am stations left in the US that is 11 more than I am aware of. For BC and short wave DX and boatanchors, some receivers have to have the sweep alignment.

I will admit, I was first thwarted by the hookup, I used rubber shielded microphone cable and got a lot of losses I could not account for. I changed to 50 ohm coax and the crazy curves went away...

If one is using modern equipment there probably will not be a 60hz sweep rate nor a locked saw-tooth for the 'scope. That means a much faster sweep rate needing a scope much higher in bandwidth. Locking the scope to the sweep is usually done with a TTL trigger output.

Really, all depends on the gear you have to get the job done.

YMMV

Chas

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