Forums :: NEW! Web Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Dec Wed 08, 2021 9:55 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 57 posts ]  Go to page 1, 2, 3  Next
Author Message
 Post subject: Popular Electronics Signal-powered signal squarer
PostPosted: Jan Wed 25, 2017 7:49 am 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 5865
Location: Littleton, MA
I need a square-wave generator to test out the step response of the amplifier in my HP 400C AC VTVM.

I've been fighting some oscillation problems in the circuit. But none of my square-wave generators are in service – they are all in the restoration queue.

Image
link to high-res PDF file
I remembered an old article from the October 1967 issue of Popular Electronics. (Many thanks to AmericanRadioHistory.com for making scans of Popular Electronics available.) Leslie Solomon published an add-on Schmidt trigger squaring circuit for an audio sine wave generator. The squarer was powered by a voltage doubler running off the input sine wave - a clever idea.

So, I built one to run off my HP 200CD. The original design used 2N404 PNP germanium transistors. I used 2N2222A NPN silicon transistors in my version, so all the power polarities are reversed from the published circuit. I built it on a piece of proto board and started testing it.

Image
link to high-res image
At low input voltages, the circuit worked fine.

Image
link to high-res image
But as I turned the output higher on the HP 200CD, I started getting some odd bumps on the low side of the waveform.

Image
link to high-res image
At full voltage, you can see that the negative portion of the input sine wave is breaking through.

I puzzled over this for a while, until I realized that the circuit puts the full negative portion of the input sine wave on the base of the first transistor. Those old 2N404's must have had a very high Vbeo reverse breakdown voltage to withstand that rough treatment. The base-emitter junction of modern silicon transistors usually breaks down at around 5 or 6 volts, so when the input reverse-biased the base-emitter junction on the first transistor, the junction broke down and applied a negative voltage to the emitter of the second transistor. Now on the low portion of the output square wave, the second transistor is saturated, so the negative half sine wave on the emitter of the first transistor is coupled directly to the output.

Image
link to high-res PDF file
The fix is to add a 10K series resistor to the base of the first transistor, and add a diode clamp to prevent the base-emitter junction from breaking down.

Image
link to high-res image
With this modification, I get nice square waves up to the maximum output voltage. I also added a zener on the voltage doubler power supply to limit the voltage to less than 40 volts, as the 2N2222A is only rated for 40 volts maximum Vceo.

Image
link to high-res image
I packaged the circuit in a small die-cast aluminum box from Hammond. The banana plugs are spaced at 3/4 inch to plug directly into the output of the HP 200CD. They are insulated by some fiber shoulder washers. The output uses a BNC connector.

Image
link to high-res image
It was a bit of a squeeze getting the banana plugs connected up.

Image
link to high-res image
So, I now have another nice addition to the collection of test fixtures in my lab.

Edit: I fixed the links to the hi-res images

_________________
Steve Byan https://www.byan-roper.org/steve/steve-at-play/


Last edited by stevebyan on Sep Sun 09, 2018 10:02 pm, edited 1 time in total.

Top
 Profile  
 
 Post subject: Re: Popular Electronics Signal-powered signal squarer
PostPosted: Jan Thu 26, 2017 1:21 am 
Member

Joined: Feb Sun 17, 2008 11:36 pm
Posts: 1847
Location: Dayton, Ohio
Nice.

I think it is a good idea to keep documentation of all of your TEST FIXTURES in a three ring binder, to include schematics, specifications and use instructions.

Five-ten years from now, you may not remember what it is for.

Charlie


Top
 Profile  
 
 Post subject: Re: Popular Electronics Signal-powered signal squarer
PostPosted: Jan Thu 26, 2017 2:21 am 
Moderator
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 22530
Location: Somers, CT
Being a young squirt, I would have grabbed a 555 timer chip. LOL.

Pete

_________________
Just because it can be done doesn't mean it should be done.


Top
 Profile  
 
 Post subject: Re: Popular Electronics Signal-powered signal squarer
PostPosted: Jan Thu 26, 2017 2:31 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1803
Location: Wayside, NJ Monmouth
Thanks for post it. I just may put one together this weekend. I have all the needed parts. And I can add it to my collection of homebrew test fixtures.


Top
 Profile  
 
 Post subject: Re: Popular Electronics Signal-powered signal squarer
PostPosted: Feb Wed 01, 2017 7:56 pm 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 5865
Location: Littleton, MA
tinwhisker wrote:
Nice.

I think it is a good idea to keep documentation of all of your TEST FIXTURES in a three ring binder, to include schematics, specifications and use instructions.

Five-ten years from now, you may not remember what it is for.

Thanks, Charlie. I keep an electronic notebook, rather than a paper one, but I'd be lost without it.

Peter Bertini wrote:
Being a young squirt, I would have grabbed a 555 timer chip. LOL.

Pete, that's an interesting idea. A CMOS 555 could be powered using the same voltage-doubler approach and should give pretty good performance. I'll have to give that a try.

N2LXM wrote:
Thanks for post it. I just may put one together this weekend. I have all the needed parts. And I can add it to my collection of homebrew test fixtures.

I'm glad you enjoyed it. Let us know how it works out for you.

_________________
Steve Byan https://www.byan-roper.org/steve/steve-at-play/


Top
 Profile  
 
 Post subject: Re: Popular Electronics Signal-powered signal squarer
PostPosted: Feb Fri 03, 2017 2:09 pm 
Member
User avatar

Joined: Dec Sun 22, 2013 5:03 am
Posts: 2382
Location: Santa Cruz mountains
If you add a switch, you can have a dual-purpose box. It can be either a squarer or a banana plug to BNC adaptor.


Top
 Profile  
 
 Post subject: Re: Popular Electronics Signal-powered signal squarer
PostPosted: Sep Mon 10, 2018 5:25 pm 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 5865
Location: Littleton, MA
Someone recently asked for details on the zener, which was a late addition and is not shown on my updated schematic.

I put the 40 volt zener right across the power supply. That is, directly in parallel with the 100 µF capacitor.

I didn't put any series resistance in, because I expect to only power it from audio oscillators that have a relatively high output impedance - 600 ohms or more, and that don't exceed 20 volts output by very much, maybe 30 volts max peak-to-peak.

Trying to protect the zener from a source with a low output impedance like an HP 205AG or an HP 200AB, or a high output voltage, like a GR 1214-A, is problematic. You would need to add a series resistor, and that would likely interfere with correct operation when the signal squarer is used with an oscillator with a high output impedance.

_________________
Steve Byan https://www.byan-roper.org/steve/steve-at-play/


Top
 Profile  
 
 Post subject: Re: Popular Electronics Signal-powered signal squarer
PostPosted: Sep Tue 11, 2018 5:50 am 
Member

Joined: Dec Tue 29, 2015 8:23 am
Posts: 286
Location: Australia
stevebyan wrote:
Trying to protect the zener from a source with a low output impedance like an HP 205AG or an HP 200AB, or a high output voltage, like a GR 1214-A, is problematic. You would need to add a series resistor, and that would likely interfere with correct operation when the signal squarer is used with an oscillator with a high output impedance.

Don't overlook the old lamp trick - a few ohms cold, 10-20x that when hot. Something like a 12v 0.15A or 0.3A lamp should be about right ;).


Top
 Profile  
 
 Post subject: Re: Popular Electronics Signal-powered signal squarer
PostPosted: Sep Tue 11, 2018 3:22 pm 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 5865
Location: Littleton, MA
Rod, thanks very much! That's a great idea!

_________________
Steve Byan https://www.byan-roper.org/steve/steve-at-play/


Top
 Profile  
 
 Post subject: Re: Popular Electronics Signal-powered signal squarer
PostPosted: Sep Tue 11, 2018 8:04 pm 
Member
User avatar

Joined: Nov Wed 30, 2016 7:35 pm
Posts: 11326
Location: Sunbury, Ohio 43074
Quote:
I think it is a good idea to keep documentation of all of your TEST FIXTURES in a three ring binder, to include schematics, specifications and use instructions. Five-ten years from now, you may not remember what it is for.

That is a GREAT Idea... I've actually done that. Now, where did I put that three ring binder........................ 8)

_________________
Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


Top
 Profile  
 
 Post subject: Re: Popular Electronics Signal-powered signal squarer
PostPosted: Sep Wed 26, 2018 7:39 pm 
Member

Joined: Mar Sun 25, 2018 10:50 pm
Posts: 356
Location: 07960
I decided to go ahead and build this. (Thanks Steve!) Unfortunately, my square waves aren't so square. This is at the output.

Image

Image

And this is what my HP 204C is presenting to the input.

Image

I'm going to go back and review my work, but if anyone has any suggestions or sees anything amiss in the photos below (aside from the sloppy layout), let me know.

Image

Image

Image


Top
 Profile  
 
 Post subject: Re: Popular Electronics Signal-powered signal squarer
PostPosted: Sep Wed 26, 2018 9:33 pm 
Member
User avatar

Joined: Jan Mon 05, 2009 4:09 am
Posts: 1439
Location: Hudson, MA
Does the waveform look squarer when you operate at a lower frequency? Try a 1kHz input, and have the scope timebase on 500uS /div

_________________
Eddy


Top
 Profile  
 
 Post subject: Re: Popular Electronics Signal-powered signal squarer
PostPosted: Sep Wed 26, 2018 9:34 pm 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 5865
Location: Littleton, MA
Is the scope input set to AC for both photos? Where is the trace 0 volt reference --- is it the center line in the graticule? If so, the 204C seems to have a bit of DC offset, but not enough to account for the pronounced asymmetry in the square wave.

What is the peak-to-peak voltage of the sine wave output by the 204C?

Is the output of the squarer open-circuited, or is there a load on it?

It's a bit hard to read the back side of the board, but I'm not seeing how the negative end of the 100 µF capacitor is connected to the black wires, which appear to be part of the "ground lead"; that is, the negative end of the power supply and circuit common for the signal path.

Try this: remove the ground strap from the back of the 204C so that the output is floating with respect to power line ground. Then set your scope to DC and connect it across the 100 µF capacitor. What voltage do you measure, and is there significant ripple in the voltage?

What is the waveform at the base of the first transistor?

_________________
Steve Byan https://www.byan-roper.org/steve/steve-at-play/


Top
 Profile  
 
 Post subject: Re: Popular Electronics Signal-powered signal squarer
PostPosted: Sep Wed 26, 2018 10:56 pm 
Member

Joined: Sep Tue 30, 2014 6:08 am
Posts: 5955
Location: Norfolk, VA
Peter Bertini wrote:
Being a young squirt, I would have grabbed a 555 timer chip. LOL.

Pete


We always used CMOS Schmitt Triggers to square up things - outputs of tape data units, noisy/non-linear sig gen output to Frequency counters and the like. Not signal powered, but handy as can be.

40106 in the good ol' days, but now a 74C/LVC14 or variant....

Nice job, Steve!

_________________
Brian
"Capacitor Cosmetologist since 1979"
USN Retired 1984-2006 (Avionics/Cal)


Top
 Profile  
 
 Post subject: Re: Popular Electronics Signal-powered signal squarer
PostPosted: Sep Wed 26, 2018 11:40 pm 
Member
User avatar

Joined: Nov Wed 30, 2016 7:35 pm
Posts: 11326
Location: Sunbury, Ohio 43074
the odd thing is the non-symmetry of the square wave. Are you possibly overdriving the input? Otherwise doublecheck the wiring.

_________________
Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


Top
 Profile  
 
 Post subject: Re: Popular Electronics Signal-powered signal squarer
PostPosted: Sep Thu 27, 2018 12:14 am 
Member

Joined: Mar Sun 25, 2018 10:50 pm
Posts: 356
Location: 07960
stevebyan wrote:
Is the scope input set to AC for both photos? Where is the trace 0 volt reference --- is it the center line in the graticule? If so, the 204C seems to have a bit of DC offset, but not enough to account for the pronounced asymmetry in the square wave.

What is the peak-to-peak voltage of the sine wave output by the 204C?

Is the output of the squarer open-circuited, or is there a load on it?

It's a bit hard to read the back side of the board, but I'm not seeing how the negative end of the 100 µF capacitor is connected to the black wires, which appear to be part of the "ground lead"; that is, the negative end of the power supply and circuit common for the signal path.

Try this: remove the ground strap from the back of the 204C so that the output is floating with respect to power line ground. Then set your scope to DC and connect it across the 100 µF capacitor. What voltage do you measure, and is there significant ripple in the voltage?

What is the waveform at the base of the first transistor?


So I'm not sure what happened, but here's what I have now, inputting a 1 kHz sine at 6V peak to peak.

Image

Perhaps Barry's right, and I was overdriving the input, though the max output from the 204C is 5V.


Top
 Profile  
 
 Post subject: Re: Popular Electronics Signal-powered signal squarer
PostPosted: Sep Thu 27, 2018 12:27 am 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 5865
Location: Littleton, MA
It's still not as symmetrical as it should be.

My guess (and it's only a guess, pending further information) is that your 204C is out of whack and has a considerable voltage offset in it's push-pull output stage, and that the electrolytic output coupling capacitor (C11) is leaky from non-use but slowly reforming. That would explain a voltage offset on the output terminals that decreases over time.

With the signal squarer disconnected, what DC voltage do you read between the two output terminals of the HP 204C oscillator?

What DC voltage do you read at the junction of R41 and R42 in the push-pull output stage of the 204C? It should be -0.35 volts. You'll want a DMM to make that measurement.

_________________
Steve Byan https://www.byan-roper.org/steve/steve-at-play/


Top
 Profile  
 
 Post subject: Re: Popular Electronics Signal-powered signal squarer
PostPosted: Sep Thu 27, 2018 12:38 am 
Member
User avatar

Joined: Nov Wed 30, 2016 7:35 pm
Posts: 11326
Location: Sunbury, Ohio 43074
Just for grins try a .1 cap between the generator and the squarer. That should eliminate any dc offset from the generator

_________________
Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


Top
 Profile  
 
 Post subject: Re: Popular Electronics Signal-powered signal squarer
PostPosted: Sep Thu 27, 2018 12:41 am 
Member
User avatar

Joined: Nov Wed 30, 2016 7:35 pm
Posts: 11326
Location: Sunbury, Ohio 43074
Also.... and I have not studied either the signal gen or the squarer.... but verify that your sine wave is symmetrical and no modulation

_________________
Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


Top
 Profile  
 
 Post subject: Re: Popular Electronics Signal-powered signal squarer
PostPosted: Sep Thu 27, 2018 1:10 am 
Member

Joined: Mar Sun 25, 2018 10:50 pm
Posts: 356
Location: 07960
The DC voltage at the 204C outputs is zero. When I adjust the amplitude it will go up a little, as high as 16mV. But as soon as I stop at an elevated value like that it goes back down to 0mV. I won't be able to open up the oscillator to check the voltage at the junction of R41 and R42 now, but I will later tonight or tomorrow and report back.


Top
 Profile  
 
Post New Topic Post Reply  [ 57 posts ]  Go to page 1, 2, 3  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 7 guests



Search for:
Jump to:  




































Privacy Policy :: Powered by phpBB