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Old 13th Aug 2019, 1:17 am   #1
Bazz4CQJ
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Default Heathkit GDO using a Tunnel Diode

For some reason, the design of this GDO is something that I come back to at periodic intervals. It was made in the early 60's, used a tunnel diode (TD) with no useful ID as the oscillator and 3 Ge transistors as a meter amp, all running from single AA battery.

Although TD's went out production a long time ago in the West, there are lots of Russian TD's for sale on line. Most of these are described as 'switching" diodes, a few as 'amplifier' diodes and just a small number are described as 'oscillator' diodes. I'm unsure as what significance to put on these designations; what would be the major difference between them?

One parameter that would come in to play in an oscillator circuit would be the capacitance of the diode and quoted values of 10-20pF seem common. That value will obviously limit the upper frequency limit.

Anyone got any insights on this?

B
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Old 13th Aug 2019, 6:41 pm   #2
m0cemdave
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Default Re: Heathkit GDO using a Tunnel Diode

The GE Tunnel Diode Manual was once considered to be the standard text on the subject of TD's, and is well worth a read.

It's now available online:
w140.com/Ge1961TunnelDiodeManual.pdf
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Old 13th Aug 2019, 10:12 pm   #3
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Default Re: Heathkit GDO using a Tunnel Diode

The Wikipaedia page on tunnel diodes has an aura of accuracy about it (not always the case)
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Old 13th Aug 2019, 11:39 pm   #4
Bazz4CQJ
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Default Re: Heathkit GDO using a Tunnel Diode

Thanks for the GE manual Dave, I seem to have missed that.

Re the Wiki page, I see that there is a lot of data on this page http://w140.com/tekwiki/wiki/Russian_tunnel_diodes.

When you compare the specs of the switching, amplifier and oscillator diodes, the latter seem to be running appreciably higher currents, and have somewhat lower capacitance.

All that said, looking at some web pages today, the recurring story seemed to be that Heathkit's Tunnel Diode GDO was not very good (though very neatly designed and assembled). People seemed content with the valve and transistor Heathkit GDO's but the TDO is typically described as not being very sensitive.

Consistent with previous threads on this Forum, there are aways lots of very favourable comments to be found about the valve GDO's made by Millen and Measurements Inc., the latter using an Acorn valve.

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Old 14th Aug 2019, 9:17 am   #5
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Default Re: Heathkit GDO using a Tunnel Diode

I know someone who has a heathkit tunnel dipper. Ill ask him to post about it here. Not sure he will as hes allergic to his PC. AFAIK his dipper doesnt work. This might be in line with your point about them not being very good.

Ive had a large number of tektronix scopes with good tunnel diodes in them go through my hands over the last couple of years. I have played with some surplus tunnel diodes on occasion and found them to be tetchy and difficult to drive. My experiments were mostly limited to attempting to use them as pulse generators however and plotting the curve of one. Ill leave the latter here as it was interesting:

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Old 14th Aug 2019, 4:46 pm   #6
Bazz4CQJ
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Default Re: Heathkit GDO using a Tunnel Diode

There's a Youtube video which discusses the Heathkit TDO (model number is HM-10A). That and other reports suggest that all of the old composite resistors have drifted in value and must be changed. The three transistors in the meter amplifier are mounted in sockets, so can easily be removed and checked. The Youtube vid suggests one of them may be best replaced (very leaky) with something more modern to improve performance.

The need to have three transistors in the meter amp maybe suggests that the oscillator is not really up to the job. Makes you wonder about the particular tunnel diode that was selected; could another component do better? Of course this leads to the thought that perhaps a better tunnel diode (there lots of Russian ones to consider), together with a modern amplifier chip for the meter, could be an interesting project... or may be not .

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Old 14th Aug 2019, 5:28 pm   #7
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Default Re: Heathkit GDO using a Tunnel Diode

If the intent is to have a play with tunnel diodes, then why not? Have some fun!

If the intent is to create a GDO, then don't bother. The heathkit ones proved that though it could be done, the end result's performance was seriously disappointing, and not good enough for most day to day uses. On the other hand, there are much superior dippers using valves, bipolars, FETs and even the triode part of a magic eye has been pressed into use. Take your pick.

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Old 14th Aug 2019, 6:07 pm   #8
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Default Re: Heathkit GDO using a Tunnel Diode

If you recall the thread we had ~2 years ago asking what is the best GDO, the consensus was that the Megacycle Meter made by Measurements Inc was very hard to beat.

Subsequently, I started to make a copy of that unit and got as far as making up a pcb ready to accept an Acorn valve and with a variable capacitor mounted right up against it.... but then I found another variable that I though was a better choice, so some re-work needs doing, but finishing that is high on the 'To-do' list for this winter. I also have a small solid-state vibrator board which I'm hoping will allow it to be run from a 12V SLA if it needs to be used outside.

That would compliment/supersede my existing FET GDO, which I built at least 40 years ago. I also recently bought an old Japanese GDO with a bipolar transistor; not too good >50MHz, but really quite fair below 30MHz.

So, that's my route to a 'super' GDO. The TDO is more of a curiosity which breaks out like a rash now and then .

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Old 19th Aug 2019, 9:28 pm   #9
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Default Re: Heathkit GDO using a Tunnel Diode

Someone has just offered a Trio DM801 dip meter at our local radio society, he's asking 50
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Old 19th Aug 2019, 11:18 pm   #10
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Default Re: Heathkit GDO using a Tunnel Diode

Presumably, the original design appeared when tunnel diodes were new on the scene and a little mystical to most, beyond the fact that they had application at microwave frequency and must therefore be Special and Wonderful. Perhaps the marketing folk at Heathkit won over wiser counsel and felt that the use of a TD would be a useful USP, regardless of whether there was genuine merit in their use. It makes me think of that later episode in consumer technology where several Japanese hi-fi manufacturers felt that "Ga-As FET!" was a must-have in the blurb for their latest FM tuner offering, irrespective of whether they were necessary or even suitable for the application.
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Old 20th Aug 2019, 3:10 am   #11
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Default Re: Heathkit GDO using a Tunnel Diode

Quote:
Originally Posted by Bazz4CQJ View Post
For some reason, the design of this GDO is something that I come back to at periodic intervals. It was made in the early 60's, used a tunnel diode (TD)
I can see the fascination with the TD. Tek did use them quite a lot and they also made a TD "Pulser" as a small laboratory tool to generate perfectly rectangular waveforms for oscilloscope alignment. I copied out the schematic of one of their units , scroll to page 25 of this article:

http://worldphaco.com/uploads/TEKTRO...THE_DS1225.pdf

While one could argue that all components and everything in the universe is quantum mechanical, I still regard the TD as the only "quantum mechanical part" in my workshop as the electron tunneling is a quantum mechanical effect. Also the negative resistance region makes for a great 2 terminal oscillator.
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Old 20th Aug 2019, 4:20 am   #12
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Default Re: Heathkit GDO using a Tunnel Diode

Quote:
Originally Posted by Argus25 View Post
Tek did use them quite a lot and they also made a TD "Pulser" as a small laboratory tool to generate perfectly rectangular waveforms for oscilloscope alignment.
Back in the early nineties, on a foray south of the border, I called in to see George Dobbs, G3RJV. There, sitting on the mantelpiece in the G3RJV shack was a little Tektronix unit.... the high speed pulse generator used for testing sampling scope rise times.

"Yes, that's a rather fine looking thing" he said, "I bought it off Chris-the-sheds because I could use the knobs and case for building something in. If you have a use for it, you can have it."

George was very generous man.

"For what one of those is worth, you could buy rather a lot of knobs and cases" was my response. At the time, HP was redoing a 4-channel 500MHz digitising/sampling scope, fitting it into a small plug-in for their modular spectrum analyser, needed for a big contract job for the US navy. I was doing the power supply and re-laying the trigger circuits (sampling scope triggers need to be fast compared to the fastest input signal). I knew they were just about to order a nice, new Tek tunnel diode pulser for the test setup for the new scope. Now at HP or Tek, having to buy something from the other always rankled a bit Bill, Dave and Howard Vollum were quite friendly, but there was a degree of competition at lower levels.

A phone call to the R&D manager involved created a bit of surprise.

I took the little TEK risetime calibrator north with me. It was working and easily calibrated.

So there was the matter of fair payment. George didn't want anything for it. He'd only paid a few quid, he said. It was just a small piece of old, surplus, test equipment. If he didn't want money for it, would he accept some different surplus test equipment for it? asked HP. They'd offered whatever the second hand market value was - half Tek's catalogue price. George wasn't having that! he said he'd only paid a few quid for it.

So I wound up making another trip to Rochdale. In the back of the Land Rover was an older model HP spectrum analyser from the surplus equipment collection, all checked and calibrated, complete with 110MHz and 1250MHz plug-ins, tracking generator, and storage mainframe. Quite a nice setup for an amateur radio shack, and the older style units with one knob per function are a lot easier to learn on. George was flabbergasted!

So that's how a Tektonix tunnel diode unit, found in the study of an English vicarage wound up calibrating new oscilloscopes destined for the (then) latest automated test gear racks of the US navy and other HP customers. That 4-channel scope was an HP catalogue item.

And if you look at the picture of George on the inside cover of SPRAT, in editions from the mid 90's onwards, you'll see behind him the HP spectrum analyser etc in the huge array of radio gear that hid one wall of the G3RJV shack.

Real life can outdo fiction! My jaw dropped when I saw that little box on the mantelpiece. Jaws dropped at HP when I told them. George's jaw dropped when he saw what 'a bit of useful test gear' meant.

It was a pleasure to have known George, and I miss him greatly.

David
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Old 20th Aug 2019, 7:13 am   #13
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Default Re: Heathkit GDO using a Tunnel Diode

What a great recollection David!

Tunnel diodes were used in several Tektronix fast pulse generators, and in the trigger circuits of 'scopes and plugins. Anyone who has tried to get reliable triggering on a 7T11 sampling timebase will know how twitchy these can be. Before the fastest TD in a 7T11 went obsolete, Tek was charging USD800 for that TD.

Leo Esaki won the 1973 Nobel prize (with Ivar Giaever and Brian Josephson) for electron tunnelling. For a while it was known as the Esaki Diode, before Tunnel Diode stuck.

In a sense it is rather sad that they went obsolete not long after he won the Nobel.

If fast rise pulse generators are your bag, Leo Bodnar sells 30-40ps rise time pulsers for 50 to 100 (depending on connector). You set the pulse amplitude via the USB connection from which they derive power. They even come with a calibration certificate, http://www.leobodnar.com/shop/index....ndex&cPath=124 .

I have one of his BNC ones, since most gear that I use uses a BNC.

Craig
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Old 20th Aug 2019, 9:36 am   #14
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Default Re: Heathkit GDO using a Tunnel Diode

I have one of Leo Bodnar’s GPSDO modules. Excellent company and products.

800 USD for a single TD? Wish they were worth that now! I have 20 of the things I got in a lot off eBay. Unfortunately not exotic ones but 4xx trigger ones.
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Old 21st Aug 2019, 8:58 am   #15
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Default Re: Heathkit GDO using a Tunnel Diode

David, interesting story about the pulser.

Also of note a tunnel diode can be deployed in reverse as a detector for low level RF and in this application they act as a zero forward voltage drop detector. I have tried this in my workshop when I was trying to make zero forward voltage drop detectors. In this application they are referred to as a "Back Diode". This, along with the current source driven detector, as a method of voltage drop free detection, are covered well in Horowitz & Hill.
Voltage drop free RF detection has applications in making a good RF probe.
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