10 Questions You Should to Know about small signal schottky diode
Question about simple schottky diode mixers - Physics Forums
I am new to mixer circuit design, but I need to build a simple mixer that will downconvert a 10 GHz signal to a DC to 3 kHz signal. I know I can do this with a schottky diode and lowpass filter to isolate the lower harmonic (f-IF = f-RF - f-LO). The problem is that the RF signal will be in the uV range. Will mixing still work with a weak signal or do I need to amplify it? I read somewhere that the RF signal should be much lower than the LO signal such that the diode transconductance is affected by the LO signal only, but how low is too low?
Thanks for your help! Very difficult.
This would require a very elaborate receiver to convert this signal to a frequency where you could isolate one sideband.
You would have to buy most of it unless you have a lot of experience building equipment.
If you bought a receiver capable of receiving single sideband at 144 MHz, you could look for a converter that could mix your 10 GHz signal with a stable 10 GHz minus 144 MHz sine wave voltage.
You would be looking at at least $ to do that.
Just using a Schottky diode wouldn't work at all. You would need a very stable oscillator on 10 GHz and a filter at the same frequency that just doesn't exist.
And you would need a very high gain amplifier at 10 GHz. Also very difficult.
Then you need an antenna. The question of using an LO and schottky diodes to mix uV signals is something I have no experience with.
Mixing 10GHz to 0 and using a quadrature detector isn't that difficult, conceptually. But there are practical issues that complicate it.
You would need a 10GHz oscillator with very low phase noise in the +-3KHz band, which I think is a problem. Mixing to 144Mhz (as suggested, or some other frequency) means you need low phase noise out at 144Mhz, which is *maybe* more realistic. Then you can gain up and filter the 144Mhz before mixing to zero.
Remember that the mixer output will contain the +-3KHz components from both the LO and the signal.
Either you have to gain it up at 10GHz and then mix, or mix to an intermediate frequency, filter the image, gain it up, and mix to 0. I don't think I was specific enough. The entire setup would be composed of my ~10 GHz signal (which has already been received through a close range transmission from another circuit), the diode, and a 10GHz VCO. The intermediate frequency would be from DC to 3 kHz. The filter would then be applied at the output to get the lower sideband, which should be the easy part since the sidebands would be 20 GHz apart at the output. I'm not too concerned with output power as long as it's in the uV range at least. There are definitely diodes that move fast enough to achieve something like this. Heck, there are mixers that can do it that cost less than $40 or less. In fact, I have a microwave sensor that does the exact same thing I'm trying to do, and it costs $8. I just can't use it because I need to move my frequency wirelessly to another point in my design and DC-3 kHz is impossible to transmit on a small scale.
This is a low power project so I don't need tons of gain either. And the antenna is not the issue; I'm pretty sure I have that part down. Is my proposal still unfeasible? Technically the intermediate frequency is -3Khz to +3Khz.
The mixed down signal you see in DC to 3Hz would contain the information from the signal's 10Ghz +- 3KHz plus the LO's 10GHz +- 3KHz. You have -3KHz to +3Khz summed in the DC to 3KHz signal. Does that make sense?
If you want to receive only 1 sideband of the +-3KHz signal, You need a quadrature mixer and detector (I and Q). If you really don't want 3KHz single-sideband, then your only issue is LO phase noise in the +- 3KHz band and whether you can pull your signal out of that. I see, I wasn't accounting for negative harmonics. Well I'd rather not use a quadrature mixer if I don't have to, but it depends. See the actual frequency I am trying to oscillate will be fed into a frequency counter of some type and information will be extracted from it. Will the -3 kHz image hurt me too much when trying to do that? Sorry for all the questions. I'm actually still a student. Maybe you could clarify further.
What sort of modulation are you using?
How is your 10 GHz signal being generated? Is it a stable signal locked to a crystal oscillator?
Gain and selectivity are very difficult to obtain at microwave frequencies and not easy at 144 MHz.
So it is usual to use multiple conversion to get to a crystal filter at about 10 MHz which can deliver good selectivity.
In the conversion process, gain can be added to bring a 1 uV signal up to a volt or so where you can actually use it.
Most of this can be provided in the receiver (possibly 144 MHz because these are available) and a converter from 10 GHz to 144 MHz is very specialised but probably available.
A microwave detector for microwave ovens is a very different thing.
It operates on 2.4 GHz on millivolts or volts of signal.
It does not need to demodulate anything either. Whether the negative frequency aliasing helps or hurts depends on the nature of the signal you are trying to detect and the noisy environment it exists in.
What you are dealing with is not a "negative harmonic", but just an aliasing negative frequency. Think of a spectrum analyser view of the 10GHz signal. Replace the 10GHz with 0Hz. That is what you are creating with the mixer. Add LO feedthrough at 0 Hz, and LO phase noise. Then basically fold it over at zero. I expect LO noise and LO feedthrough will be the major issues with respect to the required signal level. You will have twice the noise bandwidth.
I have no idea how you will tune the LO to be right on to make this work. To the extent it drifts, the carrier drifts.
How fast are you trying to count? Is it just pulses (a signal level detector, basically)? I appreciate your replies.
I'm trying to build a speedometer for skis and longboards and the like that charts a user's speed by sending off off a signal and determining the velocity by reading the doppler-shifted return signal, like a radar gun only on a much smaller scale. I have a sensor that can do this; it transmits a ~10.525 GHz signal, receives the return signal, and mixes it with the same oscillator used for transmission (which is an integrated Dielectric resonator oscillator) to produce an IF signal that is at the doppler-shifted frequency, which should be DC to 6kHz.
The problem is, I need to output what the speed actually is on a wrist display, while the sensor will be mounted close to the ground, so I need to get the information wirelessly (hopefully) from the sensor to the display unit. To keep antennas small, I would like to transmit in the X-band, but the frequencies that contain the information will be DC-6kHz depending on how fast the user is going, hence, my downconversion predicament.
I'm also pretty inexperienced so forgive my naivety.
I think the sensor oscillators may be stable though and I may be able to access the onboard mixer in the device, which I could use to downconvert the signal for me. Otherwise, not sure how I would do it on my limited budget.
Thanks for your help! Very difficult.
This would require a very elaborate receiver to convert this signal to a frequency where you could isolate one sideband.
You would have to buy most of it unless you have a lot of experience building equipment.
If you bought a receiver capable of receiving single sideband at 144 MHz, you could look for a converter that could mix your 10 GHz signal with a stable 10 GHz minus 144 MHz sine wave voltage.
You would be looking at at least $ to do that.
Just using a Schottky diode wouldn't work at all. You would need a very stable oscillator on 10 GHz and a filter at the same frequency that just doesn't exist.
And you would need a very high gain amplifier at 10 GHz. Also very difficult.
Then you need an antenna. The question of using an LO and schottky diodes to mix uV signals is something I have no experience with.
Mixing 10GHz to 0 and using a quadrature detector isn't that difficult, conceptually. But there are practical issues that complicate it.
You would need a 10GHz oscillator with very low phase noise in the +-3KHz band, which I think is a problem. Mixing to 144Mhz (as suggested, or some other frequency) means you need low phase noise out at 144Mhz, which is *maybe* more realistic. Then you can gain up and filter the 144Mhz before mixing to zero.
Remember that the mixer output will contain the +-3KHz components from both the LO and the signal.
Either you have to gain it up at 10GHz and then mix, or mix to an intermediate frequency, filter the image, gain it up, and mix to 0. I don't think I was specific enough. The entire setup would be composed of my ~10 GHz signal (which has already been received through a close range transmission from another circuit), the diode, and a 10GHz VCO. The intermediate frequency would be from DC to 3 kHz. The filter would then be applied at the output to get the lower sideband, which should be the easy part since the sidebands would be 20 GHz apart at the output. I'm not too concerned with output power as long as it's in the uV range at least. There are definitely diodes that move fast enough to achieve something like this. Heck, there are mixers that can do it that cost less than $40 or less. In fact, I have a microwave sensor that does the exact same thing I'm trying to do, and it costs $8. I just can't use it because I need to move my frequency wirelessly to another point in my design and DC-3 kHz is impossible to transmit on a small scale.
This is a low power project so I don't need tons of gain either. And the antenna is not the issue; I'm pretty sure I have that part down. Is my proposal still unfeasible? Technically the intermediate frequency is -3Khz to +3Khz.
The mixed down signal you see in DC to 3Hz would contain the information from the signal's 10Ghz +- 3KHz plus the LO's 10GHz +- 3KHz. You have -3KHz to +3Khz summed in the DC to 3KHz signal. Does that make sense?
If you want to receive only 1 sideband of the +-3KHz signal, You need a quadrature mixer and detector (I and Q). If you really don't want 3KHz single-sideband, then your only issue is LO phase noise in the +- 3KHz band and whether you can pull your signal out of that. I see, I wasn't accounting for negative harmonics. Well I'd rather not use a quadrature mixer if I don't have to, but it depends. See the actual frequency I am trying to oscillate will be fed into a frequency counter of some type and information will be extracted from it. Will the -3 kHz image hurt me too much when trying to do that? Sorry for all the questions. I'm actually still a student. Maybe you could clarify further.
What sort of modulation are you using?
How is your 10 GHz signal being generated? Is it a stable signal locked to a crystal oscillator?
Gain and selectivity are very difficult to obtain at microwave frequencies and not easy at 144 MHz.
So it is usual to use multiple conversion to get to a crystal filter at about 10 MHz which can deliver good selectivity.
In the conversion process, gain can be added to bring a 1 uV signal up to a volt or so where you can actually use it.
Most of this can be provided in the receiver (possibly 144 MHz because these are available) and a converter from 10 GHz to 144 MHz is very specialised but probably available.
A microwave detector for microwave ovens is a very different thing.
It operates on 2.4 GHz on millivolts or volts of signal.
It does not need to demodulate anything either. Whether the negative frequency aliasing helps or hurts depends on the nature of the signal you are trying to detect and the noisy environment it exists in.
What you are dealing with is not a "negative harmonic", but just an aliasing negative frequency. Think of a spectrum analyser view of the 10GHz signal. Replace the 10GHz with 0Hz. That is what you are creating with the mixer. Add LO feedthrough at 0 Hz, and LO phase noise. Then basically fold it over at zero. I expect LO noise and LO feedthrough will be the major issues with respect to the required signal level. You will have twice the noise bandwidth.
I have no idea how you will tune the LO to be right on to make this work. To the extent it drifts, the carrier drifts.
How fast are you trying to count? Is it just pulses (a signal level detector, basically)? I appreciate your replies.
I'm trying to build a speedometer for skis and longboards and the like that charts a user's speed by sending off off a signal and determining the velocity by reading the doppler-shifted return signal, like a radar gun only on a much smaller scale. I have a sensor that can do this; it transmits a ~10.525 GHz signal, receives the return signal, and mixes it with the same oscillator used for transmission (which is an integrated Dielectric resonator oscillator) to produce an IF signal that is at the doppler-shifted frequency, which should be DC to 6kHz.
The problem is, I need to output what the speed actually is on a wrist display, while the sensor will be mounted close to the ground, so I need to get the information wirelessly (hopefully) from the sensor to the display unit. To keep antennas small, I would like to transmit in the X-band, but the frequencies that contain the information will be DC-6kHz depending on how fast the user is going, hence, my downconversion predicament.
I'm also pretty inexperienced so forgive my naivety.
I think the sensor oscillators may be stable though and I may be able to access the onboard mixer in the device, which I could use to downconvert the signal for me. Otherwise, not sure how I would do it on my limited budget.
How to tell germanium from schottky diodes - EEVblog
0 Members and 1 Guest are viewing this topic.
For more information, please visit Hornby Electronic.
The company is the world’s best small signal schottky diode supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.
Contact us to discuss your requirements of high voltage diode. Our experienced sales team can help you identify the options that best suit your needs.
vpittman
- Newbie
- Posts: 6
- Country:
How to tell germanium from schottky diodes
« on: February 16, , 06:19:30 pm » I've heard that a lot of so called germanium diodes are actually schottky's. Is there and easy way to tell them apart ? The Vf of each of these is around 0.2 V so that's not a good indicator. Is there anything else that could be used to tell them apart ?There is a youtube video ( ) and at 2:13 he compares a schottky to a silicon diode using an oscilloscope. Latter he increases the frequency from 60 Hz to 300 kHz and shows the the silicon diode has a reverse current leakage while the schottky does not (@ 3:20). Could this procedure be used with germanium as well ?
Thanks !!
Victor
Terry Bites
- Super Contributor
- Posts:
- Country:
- Recovering Electrical Engineer
Re: How to tell germanium from schottky diodes
« Reply #1 on: February 16, , 07:51:26 pm » Part number?vpittman
- Newbie
- Posts: 6
- Country:
Re: How to tell germanium from schottky diodes
« Reply #2 on: February 16, , 07:57:47 pm » D9eKleinstein
- Super Contributor
- Posts:
- Country:
Re: How to tell germanium from schottky diodes
« Reply #3 on: February 16, , 08:00:35 pm » One may see a difference in the temperature dependence of the forward voltage or leakage current in reverse. Germanium has a significant lower bandgap energy - even with a shottky diode it is generally still silicon. I would expect more temperature dependence for the silicone based diode, not just normal PN diodes, but also Schottky diodes.iMo
- Super Contributor
- Posts:
- Country:
Re: How to tell germanium from schottky diodes
« Reply #4 on: February 16, , 08:25:36 pm » Germanium diodes are always packaged in glass (or metal or ceramic).. I have never ever seen a germanium diode in plastic The TC of Ge diodes (and transistors) is higher than the TC of the Si stuff. Also the forward drop with small signal Ge is usually a bit lower than with small signal schottky.. « Last Edit: February 16, , 08:51:05 pm by imo » Readers discretion is advised..Vovk_Z
- Super Contributor
- Posts:
- Country:
Re: How to tell germanium from schottky diodes
« Reply #5 on: February 16, , 08:50:22 pm »D9eVery unusual partnumber for an American or Europian diode. But it is suspiciously very similar to old Soviet '9' ('9 series) - small signal germanium diode (widely used in old AM radios etc.).
If it is so then there is no need to make an experiment, and it will be hard to find a difference with a Shottky because those diodes are very fast too. « Last Edit: February 16, , 08:55:12 pm by Vovk_Z »
Vovk_Z
- Super Contributor
- Posts:
- Country:
Re: How to tell germanium from schottky diodes
« Reply #6 on: February 16, , 09:00:01 pm »I would expect more temperature dependence for the silicone based diode, not just normal PN diodes, but also Schottky diodes.Yes, but there is not a significant or noticeable difference between germanium and Shottky types - both have large reverse DC current. And both types have similar (and large, I mean very significant) reverse current temperature dependence. « Last Edit: February 16, , 09:09:07 pm by Vovk_Z »
iMo
- Super Contributor
- Posts:
- Country:
Re: How to tell germanium from schottky diodes
« Reply #7 on: February 16, , 09:01:44 pm » The D9E has got typically Ir<250uA at Ur=10V. That is much higher Ir than with a schottky at ambient temp.Small signal schottky is typically 100nA-1uA Ir. « Last Edit: February 16, , 09:08:42 pm by imo » Readers discretion is advised..
Vovk_Z
- Super Contributor
- Posts:
- Country:
Re: How to tell germanium from schottky diodes
« Reply #8 on: February 16, , 09:08:13 pm »Germanium diodes are always packaged in glass (or metal or ceramic).. I have never ever seen a germanium diode in plastic+1, by the way: does it looks like as in an attachment? « Last Edit: February 16, , 09:11:49 pm by Vovk_Z »
iMo
- Super Contributor
- Posts:
- Country:
Re: How to tell germanium from schottky diodes
« Reply #9 on: February 16, , 09:10:32 pm » Small signal schottky are also packaged in glass - here helps following: Ge diodes are always bulkier Readers discretion is advised..vpittman
- Newbie
- Posts: 6
- Country:
Re: How to tell germanium from schottky diodes
« Reply #10 on: February 16, , 09:52:25 pm » Very similar, but mine have a red and blue stripe« Last Edit: February 16, , 09:57:33 pm by vpittman »
David Hess
- Super Contributor
- Posts:
- Country:
- DavidH
Re: How to tell germanium from schottky diodes
« Reply #11 on: February 16, , 09:53:34 pm » I have never seen a germanium diode in a plastic package so that helps a little. Small signal germanium diodes like the 1N34A and 1N270 also use the older DO-7 glass package which is larger and easy to distinguish from the smaller glass DO-35 package used by schottky diodes like the 1N, 1N, SD series, and BAT series and silicon switching diodes like the ubiquitous 1N.Vovk_Z
- Super Contributor
- Posts:
- Country:
Re: How to tell germanium from schottky diodes
« Reply #12 on: February 16, , 09:57:11 pm » Red and blue mean 'E', so it is D9E (again). It can be only blue or red+blue.D9E ('9') is an analog to 1N34, but with a lower rated forward current (20 mA, +-). So possibly the case type is the same. « Last Edit: February 16, , 10:03:10 pm by Vovk_Z » The following users thanked this post: edavid
Circlotron
- Super Contributor
- Posts:
- Country:
Re: How to tell germanium from schottky diodes
« Reply #13 on: February 16, , 10:06:26 pm » Has there ever been a commercially available Schottky diode made from germanium?Vovk_Z
- Super Contributor
- Posts:
- Country:
Re: How to tell germanium from schottky diodes
« Reply #14 on: February 16, , 10:09:59 pm » Here is a pic:Kleinstein
- Super Contributor
- Posts:
- Country:
Re: How to tell germanium from schottky diodes
« Reply #15 on: February 16, , 10:11:02 pm » There are quite a few old parts, though I don't see much need for a Schottky diode made from germanium. The Germanium diode are usually fast and low voltage just like the modern silicon schottky diodes. So the problem is not to guess it a germanium diode is a actually schottky type, but to see if a diode is Si-Schottky or old style germanium.If they have the same - why care ?
There were also a few JFETs based on Germanium, but I have never seen one in real life.
Circlotron
- Super Contributor
- Posts:
- Country:
Re: How to tell germanium from schottky diodes
« Reply #16 on: February 16, , 10:15:28 pm » I was meaning if there was a Schottky made from germanium would it have an even lower forward voltage drop than either silicon Schottky or normal germanium?Vovk_Z
- Super Contributor
- Posts:
- Country:
Re: How to tell germanium from schottky diodes
« Reply #17 on: February 16, , 10:17:15 pm »Has there ever been a commercially available Schottky diode made from germanium?There were galena Shottky diodes, but I can't find any info about germanium ones existing:
« Last Edit: February 16, , 10:22:17 pm by Vovk_Z »
vpittman
- Newbie
- Posts: 6
- Country:
Re: How to tell germanium from schottky diodes
« Reply #18 on: February 16, , 10:22:45 pm » The DO-7 package measures correctly, and if it's also as fast switching as a schottky then my idea of testing with high frequency won't work. I'll see if I can measure the leakage current in reverse.Why care? It's covid, I'm new and I'm learning all this stuff... I find it very interesting and it gives me something to do.
And in this post https://www.eevblog.com/forum/beginners/is-it-really-germanium/msg/#msg there appears to be some difference, not really sure how much affect this will have
vpittman
- Newbie
- Posts: 6
- Country:
Re: How to tell germanium from schottky diodes
« Reply #19 on: February 16, , 10:26:22 pm »D9eVery unusual partnumber for an American or Europian diode. But it is suspiciously very similar to old Soviet '9' ('9 series) - small signal germanium diode (widely used in old AM radios etc.).
If it is so then there is no need to make an experiment, and it will be hard to find a difference with a Shottky because those diodes are very fast too.
What do you look at in the datasheet to tell that these are 'fast' ?
Kleinstein
- Super Contributor
- Posts:
- Country:
Re: How to tell germanium from schottky diodes
« Reply #20 on: February 16, , 10:35:01 pm » For the speed of a diode there are 2 parameters to look at: the main part is the reverse recovery time. The other parameter is the diode capacitance relative to the current rating.With a scope and function generator (square wave with reasonable fast edges , like from a 74HC gate) one can test the diode recovery quite easy. Look at the diode voltage and apply something like a +-2 V square wave with a series resistor of some 1 K ... 50 Ohms depending on the diode size.
The curve of "junction" capacitance versus reverse voltage (especially the range of low votlages) could also tell apart some diodes. Chances are Germanium is different here too, as the build in voltage is naturally relatively small.
vpittman
- Newbie
- Posts: 6
- Country:
Re: How to tell germanium from schottky diodes
« Reply #21 on: February 16, , 10:56:46 pm » Thanks everyone for all the insight !!!Vovk_Z
- Super Contributor
- Posts:
- Country:
Re: How to tell germanium from schottky diodes
« Reply #22 on: February 16, , 11:00:25 pm »What do you look at in the datasheet to tell that these are 'fast' ?For soviet diodes, it was a parameter called "Max working frequency" (it was hard to understand what it really means), and for '9' it is 100kHz. As I know it works up to tens of megahertz (so thats why I say it is hard to understand some Soviet characteristics). « Last Edit: February 16, , 11:03:21 pm by Vovk_Z »
David Hess
- Super Contributor
- Posts:
- Country:
- DavidH
Re: How to tell germanium from schottky diodes
« Reply #23 on: February 16, , 11:03:27 pm » The 1N270s that I have used in power detectors were usable to 100s of MHz.Has there ever been a commercially available Schottky diode made from germanium?
They were advertised at one point for high efficiency power supplies but obviously were not commercially successful.
bdunham7
- Super Contributor
- Posts:
- Country:
Re: How to tell germanium from schottky diodes
« Reply #24 on: February 17, , 12:09:21 am » So I'm following along hoping for a definitive answer, but I'm not sure of one yet.I also have some diodes I have questions about. They look like a DO-7 case, definitely larger than my 1Ns. They have a Vf of 0.16V @ 100uA and 0.22V @ 1mA, IR at 10V is 1.1uA and really doesn't change much if I warm it up with my fingers. They appear to be marked 'DJ 025 T' and they were sold by a surplus dealer as 1N34A germanium. What do I have?
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.