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Old 2nd Jun 2021, 11:29 pm   #1
Cruisin Marine
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Default Replacing Germanium Diodes with Schottky diodes?

Does anyone here have any PRACTICAL experience of replacing old Germanium diodes in either a detector, or in a bum basic field strength meter with low capacitance Shottky diodes like the 1N5711, which are cheap in quantity, as "Geraniums" are so hard to come by now?

I would love to hear of the practical experience of doing so as I have just about exhausted my Germaniums now.
TIA

Last edited by Radio Wrangler; 3rd Jun 2021 at 7:35 am. Reason: typo fix
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Old 2nd Jun 2021, 11:57 pm   #2
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Default Re: Replacing Germanium Diodes with Shottky diodes?

There are many types of small signal schottky diodes and some are specifically made for detector duty. HP's power meters have a range of -70 to +20dBm but that is a cheat by using two detectors and an attenuator at RF, while autoranging switches the metering between the outputs of the two detectors.

But wait one minute, -70dBm in 50 Ohms means a voltage far, far below the turn-on threshold of even a Schottky diode.

The answer is that the turn on-threshold hinges on just what is meant by 'On'? it's usually taken as some current value. But look at the real characteristic and it goes down to far lower voltages, but at far lower currents. A detector can be considered as a rectifier at high levels, giving an output voltage proportional to the input voltage. At lower levels it enters what's called the square law region where the DC output goes proportional to the RF power . With care, shaping of the DC result can accommodate the two modes.

This shifts the difficulty into sub-microvolt DC work. In the HP power meter heads this is done with an FET chopper and an AC-coupled amplifier. In the main meter unit this gets a bit more switched AC gain, then it is unceremonially bunged into a stereo audio ADC and the rest is done in DSP The two channels of the ADC handle the outputs of the two detectors.

-70dBm is far far below where you'd ever think to go with Germanium diodes but the right schottkys do it reliably and hold calibration. The low level diodes have 4 or 8v reverse ratings.

The HP diodes went to Avago (who took over Broadcom and then assumed their name) Just after the corporate shenanigans, they ditched all the small RF prts they made, so data can take a bit of finding, then you'd need to find equivalents. But look for parts specifically for detector applications rather than general purpose, and look for low voltage ratings.

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Old 3rd Jun 2021, 12:26 am   #3
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Default Re: Replacing Germanium Diodes with Shottky diodes?

David, wasn't it basically the same technique used by Racal and Marconi in their RF millvoltmeters with the the head probe "chopper" circuit?
Oh, and at least you can spell Schottky and I can't lol
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Old 3rd Jun 2021, 12:47 am   #4
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Default Re: Replacing Germanium Diodes with Shottky diodes?

I don't think there is a one size fits all answer to this because there are so many types of Ge or Schottky diode and also there are lots of circuits to try them in.

However, the most difference I ever saw in a detector was in an AM CB radio. This used the classic 1N60AM Ge diodes and these were run with a light forward bias to improve the detector performance. When these diodes were replaced for 1N5711 Schottky diodes the efficiency of the AM detector improved significantly at very weak signal levels. This meant than an AM test signal from a sig gen would hold up the volume better as the signal was turned down towards noise floor. This meant (in comparison) that the Ge diodes showed a marked reduction in background noise when there was no signal but this was because the detector efficiency had collapsed relative to the 1N5711 version when detecting near the noise floor. At first this appears to be a victory for the Schottky diode but in reality, it is rather nice for the noise floor to suddenly sink down when there is no station signals present. It's a bit like having an auto soft squelch.

The 1N5711 detector efficiency was better when there were no stations present so it detected the background noise much better meaning the radio was noisier between distant stations transmitting. When listening to regular stations that weren't close to the noise level both detectors were the same in terms of efficiency and volume.

I think I preferred the Ge diode characteristic in this case especially when using the radio at night as the pauses between stations was much quieter. However, from a technical point of view the Schottky diode version was superior.
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Old 3rd Jun 2021, 12:59 am   #5
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Default Re: Replacing Germanium Diodes with Shottky diodes?

When used passively in a detector (i.e. no deliberate forward bias from a PSU) the 1N5711 generally doesn't detect as well as an old school Ge diode for weak signals.
However, it does load the circuit less when used as an RF probe, especially at lower signal levels. Also there can be marked differences in the AM demodulation bandwidth between a Schottky diode and an old school Ge diode when detecting very weak signals. The dynamic resistance can be very different between the two diode types when detecting weak signals.

It's a bit sad but I have built up a library of IV (forward and reverse leakage) curves for all kinds of Ge, Si and Schottky diodes here and these are taken with great precision down to the uA level using 6.5 digit bench meters and a programmable PSU and a computer program to control and log the results.

I have measured quite a few classic old Ge diodes like this and this provides data that is much more detailed than what can be read from the basic IV curves given in the datasheet. It's also possible to reverse engineer the Is and n factors that go into the Shockley diode equation using the carefully logged IV data. It's quite rewarding to see the results agree with Is and n data sometimes provided by HP/Avago in their datasheets.
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Old 3rd Jun 2021, 7:49 am   #6
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Default Re: Replacing Germanium Diodes with Shottky diodes?

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Originally Posted by Cruisin Marine View Post
Oh, and at least you can spell Schottky and I can't lol
Go on, then. I've just popped in and edited the title of the first post, so this should fix it on the top level and all posts from now on. It helps people using search engines, so it's worth fixing. I hadn't noticed it!

Choppers go back a long way. There were ones with motors whirling interruptor discs blocking light onto photoconductive cells, then bistable oscillators with neon bulbs. The slow recovery of a darkened CdS cell is an issue. With FETs, the gate capacitance injects charge at the switching frequency. A trick is to use a second FET on the opposite phase to cancel it.

You'll find choppers in some bolometric heads as well as diode heads. All instrument firms have used them.

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Old 3rd Jun 2021, 8:21 am   #7
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Default Re: Replacing Germanium Diodes with Schottky diodes?

The main purveyors of fast Schottkys for detectors and mixers is MACOM. Now a lot of their devices are so-called flip chip, which is not something you want to mess with unless you have microwave strip line experience.

But they do regular surface mount packages too, like the zero-bias detector Schottky https://cdn.macom.com/datasheets/MA4E2200%20Series.pdf

They are somewhat fragile, as are many parts that go to 4GHz

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Old 3rd Jun 2021, 9:28 am   #8
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Default Re: Replacing Germanium Diodes with Schottky diodes?

I'm surprised you're having trouble finding germanium diodes though. There are always sellers offering them on eBay, and while prices do seem to have risen, they're not particularly expensive in lots of 5 or 10.
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Old 3rd Jun 2021, 9:32 am   #9
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Default Re: Replacing Germanium Diodes with Schottky diodes?

Good question.

My "Practical Experience" is that I use OA95 Germaniums for everyday detector replacements and for new small signal projects
For a more rugged diode use, I use the 1N5711 family.

Despite much advice to the contrary, I find the Germaniums slightly more sensitive, even at a few hundred MHz.

But for a recent project of a power limiter for a MilliWattmeter, I used 1N5711s.

Both have similar capacitance values.

I bought another 100 OA95s a few years ago.
I found that before soldering them into circuit, I had to test each of these diodes for at least 10/1 resistance forward/reverse, as some were dodgy.
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Old 3rd Jun 2021, 7:24 pm   #10
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Default Re: Replacing Germanium Diodes with Schottky diodes?

For replacing OAxx germanium diodes I just keep a stock of 1N34As. I've used them for some time now to replace dud detectors in transistor radios such as Bush and Roberts and as meter rectifier diodes and they've worked fine in all circumstances. Typically 20 pence or so each with free post from UK suppliers on eBay if you buy 20. (IE £4.00 for 20).

Google this word string will find them:

20Pcs 1N34A DO-35 germanium detector diode AM/FM crystal radio TV;UK

Though it isn't a very scientific test, I've checked them against OA90 and OA79s on my Hameg 203-7 scope component tester and the display looks identical. Quite a useful facility on the scope.

Pic 1: Germanium diode.
Pic 2: Silicon diode.
Pic 3: Zener diode.
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Old 3rd Jun 2021, 8:41 pm   #11
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Default Re: Replacing Germanium Diodes with Schottky diodes?

Thank you to everyone who has replied with some informative information regarding the change to Schottky from Germanium.
I just checked the Germanium diode component draw, and am currently down to zero.

I asked this question as I am most doubtful over Chinese knock offs on the auction sites nowadays, having been burnt about three times with extremely bad components that quite frankly were only fit for the bin after checking them. I also like to benefit from others on hand experience, as I have not tried using these Schottky substites.

Kindly, a forum member has offered to help by supplying me with some Germanium's to keep me going on ongoing projects and repairs.

As ever, this place is wonderful when a fellow experienced designer, repair/service person or a or learner starting out wants some good advice.

Again, thanks to everyone, I now get the real message as to what this fine forum is all about- and I like it very much.
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Old 3rd Jun 2021, 10:11 pm   #12
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Default Re: Replacing Germanium Diodes with Schottky diodes?

Tailpiece- if i can find the name of the seller who sent me 10 a couple of Germaniums a couple of years ago i will pass it on for info. It was a UK seller.
They were fit for (the) purpose (i put them to) but i don't know their country of origin.

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Old 3rd Jun 2021, 11:56 pm   #13
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Default Re: Replacing Germanium Diodes with Schottky diodes?

Quote:
Originally Posted by paulsherwin View Post
I'm surprised you're having trouble finding germanium diodes though. There are always sellers offering them on eBay, and while prices do seem to have risen, they're not particularly expensive in lots of 5 or 10.

You do have to be very careful though, many have been known to try flogging silicon types as germanium ones.
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Old 4th Jun 2021, 9:38 pm   #14
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Default Re: Replacing Germanium Diodes with Schottky diodes?

Marvellous isn't it? Fortunate that (healthy) Ge/Si are distinguishable with a diode tester.
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Old 5th Jun 2021, 6:17 am   #15
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Default Re: Replacing Germanium Diodes with Schottky diodes?

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Originally Posted by The Philpott View Post
Marvellous isn't it? Fortunate that (healthy) Ge/Si are distinguishable with a diode tester.
Dave
Ge versus Schottky isn't so certain, though.

Schottky diodes can be made in both Si and Ge. And the semiconductor half can be either N-type or P-type (P is more common for lower voltage drop reasons)

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Old 5th Jun 2021, 2:14 pm   #16
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Default Re: Replacing Germanium Diodes with Schottky diodes?

Quote:
Originally Posted by Cruisin Marine View Post
Does anyone here have any PRACTICAL experience of replacing old Germanium diodes in either a detector, or in a bum basic field strength meter with low capacitance Shottky diodes like the 1N5711, which are cheap in quantity, as "Geraniums" are so hard to come by now?

I would love to hear of the practical experience of doing so as I have just about exhausted my Germaniums now.
TIA
I have an Eddystone 770U uhf receiver which had a GEC GEX66 germanium diode as 1st mixer/frequency changer.

I replaced the GEX66 with a Schottky diode from the tuner of an old TV set that I found dumped in the road. The sensitivity of the 770U went up by about 6dB and the signal/noise ratio was significantly improved.
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Old 5th Jun 2021, 3:07 pm   #17
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Default Re: Replacing Germanium Diodes with Schottky diodes?

Quote:
Originally Posted by Radio Wrangler View Post
Quote:
Originally Posted by The Philpott View Post
Marvellous isn't it? Fortunate that (healthy) Ge/Si are distinguishable with a diode tester.
Dave
Ge versus Schottky isn't so certain, though.

Schottky diodes can be made in both Si and Ge. And the semiconductor half can be either N-type or P-type (P is more common for lower voltage drop reasons)

David
Would the silicon versions be the Power diode type, and the germanium versions be the mixer/signal types?
I have none of the power types on hand to measure.
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