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Old 24th May 2021, 7:48 pm   #1
Radio Scotland
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Default Signal tracer help needed

Hi folks.

Recently built a signal tracer from a design i found on youtube, I know it's a proven circuit as the guy builds and tests it on camera,

It seems to work well when amplifying audio, But when i hook in to RF Sections of a radio via a detector diode the audio is broken and distorted. But as above it works well when coupled to the radios volume pot with no detector diode in circuit.


The circuit calls for a 1N34A detector diode, I'm using a 1N60. Can't see that being the issue, But worth mentioning.

See attached schematic

Cheers...
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Old 24th May 2021, 7:59 pm   #2
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Default Re: Signal tracer help needed

You should be able to work this one out for yourself.... But here's a clue. This is a surprisingly frequent issue.

That detector diode is another word for rectifier... so it converts a varying amplitude of RF signal to a varying DC voltage/current. That's how a detector works.

Hmmmm

Direct current.... so where does that DC current from the diode flow?

David
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Old 24th May 2021, 8:02 pm   #3
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Default Re: Signal tracer help needed

Quote:
Originally Posted by Radio Wrangler View Post
You should be able to work this one out for yourself.... But here's a clue. This is a surprisingly frequent issue.

That detector diode is another word for rectifier... so it converts a varying amplitude of RF signal to a varying DC voltage/current. That's how a detector works.

Hmmmm

Direct current.... so where does that DC current from the diode flow?

David
Yeah but DC is blocked by the capacitor on the tracer input. I maybe missing your point though.


Cheers
Jay
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Old 24th May 2021, 8:14 pm   #4
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Default Re: Signal tracer help needed

Same as this Heathkit signal tracer, spot the DC path:

http://nvhrbiblio.nl/schema/Heathkit_IT12.pdf

Try an 1N34.

Lawrence.

Last edited by ms660; 24th May 2021 at 8:26 pm.
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Old 24th May 2021, 8:34 pm   #5
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Default Re: Signal tracer help needed

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Originally Posted by ms660 View Post
Same as this Heathkit signal tracer, spot the DC path:

http://nvhrbiblio.nl/schema/Heathkit_IT12.pdf

Try an 1N34.

Lawrence.

Not sure what difference that would make lawrence provided its germanium.


Jay
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Old 24th May 2021, 8:36 pm   #6
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Default Re: Signal tracer help needed

Not absolutely sure but it might have a lower forward voltage and reverse resistance, I'll leave it to you.

Lawrence.
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Old 24th May 2021, 8:38 pm   #7
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Default Re: Signal tracer help needed

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Not absolutely sure but it might have a lower reverse resistance, I'll leave it to you.

Lawrence.
Yeah you might have a point, I'll take a look at the specs.


Jay
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Old 24th May 2021, 9:47 pm   #8
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Default Re: Signal tracer help needed

Without a dc load on the detector in principle its output would charge up to the peak rf voltage and stay there unable to follow successive lower peaks down at audio frequencies.

In practice a Ge diode may well be leaky enough to work OK-ish but it's not a well toleranced design.
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Old 24th May 2021, 11:05 pm   #9
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Default Re: Signal tracer help needed

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Originally Posted by Herald1360 View Post
Without a dc load on the detector in principle its output would charge up to the peak rf voltage and stay there unable to follow successive lower peaks down at audio frequencies.

In practice a Ge diode may well be leaky enough to work OK-ish but it's not a well toleranced design.

It seemed to work perfectly in the guys video. He used it to probe a valve set with no distortion heard on the output.
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Old 24th May 2021, 11:14 pm   #10
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Default Re: Signal tracer help needed

That's it. There is no DC path, so with a good diode all it does is charge the capacitors up to the peak RF voltage and leaves them there.

A detector has to be able to follow the envelope of the modulated RF and that means follow its downslopes as well as its upslopes.

Capacitors can't pass DC (unless they are lousy leaky ones) and diodes make DC from AC, yet in this circuit, there is no path for the manufactured direct current to circulate.

It is literally the same as trying to measure a DC voltage with a blocking capacitor on your voltmeter. You get an initial kick as the capacitor charges, and then that's it, nothing forever more. To work properly there needs to be a DC path between ground and the anode of the diode (in case you are probing somewhere with a blocking capacitor to keep high voltage DC off of your diode) This could be resistor or an RF choke, or even an RF transformer. You also need a resistive path from the cathode to earth. This resistor forms a time-constant with the capacitors and sets the highest frequency your detector can demodulate because it sets the recovery speed at which the output voltage can fall otherwise it fails to track the envelope of the signal and gives reduced output along with bad distortion. Sometimes called slew-rate limiting.

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Old 24th May 2021, 11:26 pm   #11
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Default Re: Signal tracer help needed

OK I think I'm following you now Radio Wrangler, The charge on the input capacitors needs to bleed off in time with the rise and fall of the input signal? Can you suggest a way to modify the circuit to make it more usable?

Still wondering how the guy in the YouTube video had his working perfectly..
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Old 25th May 2021, 12:34 am   #12
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Default Re: Signal tracer help needed

Try adding a pair of 10k resistors as a guessed starting value. One from anode to earth, one from cathode to earth.

There is now a DC current loop.

David
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Old 25th May 2021, 12:40 am   #13
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Default Re: Signal tracer help needed

Cheers I'll try that tomorrow.

Jay
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Old 25th May 2021, 10:05 am   #14
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Default Re: Signal tracer help needed

The current path is via the circuit under test in conjunction with the reverse resistance of the diode. (Post#6)

Lawrence.
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Old 25th May 2021, 10:19 am   #15
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Default Re: Signal tracer help needed

So it fails to work if your diode doesn't leak enough, or the circuit under test doesn't provide a DC path.

David
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Old 25th May 2021, 10:28 am   #16
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Default Re: Signal tracer help needed

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Originally Posted by Radio Wrangler View Post
So it fails to work if your diode doesn't leak enough, or the circuit under test doesn't provide a DC path.

David
Obviously.

I think think this has been discussed in the past.

Most testing points in a receiver or whatever will have a DC path to ground.

Lawrence.
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Old 25th May 2021, 10:49 am   #17
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Default Re: Signal tracer help needed

Swap D1 and C2 in your circuit, and put a resistor in parallel with the diode - about 100k should do the trick.

Better still, put a switch in series with the diode.Off = AF probe, no rectification. On = RF probe, using the diode.
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Old 25th May 2021, 11:05 am   #18
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Default Re: Signal tracer help needed

Quote:
Originally Posted by ms660 View Post
The current path is via the circuit under test in conjunction with the reverse resistance of the diode. (Post#6)

Lawrence.
So because i'm using a different diode the reverse resistance will be different hence the distortion ?


Cheers keith i'll try your suggestion.



Jay
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Old 25th May 2021, 11:16 am   #19
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Default Re: Signal tracer help needed

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So because i'm using a different diode the reverse resistance will be different hence the distortion ?
Probably, the diode you are using is a Schottky diode, it's not the best choice.

As well as in the Heathkit IT-12 (in the link I posted earlier) the same trick was also used in the Heathkit T-1 and T-4 and IT- 5283 at least and Eico also used the same trick in at least one of their signal tracers, the Eico147a, they did that because it worked.

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Old 25th May 2021, 12:27 pm   #20
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Default Re: Signal tracer help needed

Quote:
Originally Posted by Radio Wrangler View Post
Try adding a pair of 10k resistors as a guessed starting value. One from anode to earth, one from cathode to earth.

There is now a DC current loop.

David
Adding a 10k resistor from anode to ground does clear up some of the distortion, Cathode to ground makes little difference,






Jay
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