43 - Matching Diodes for Mixers

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Nick M0NTV discusses the challenges of matching diodes for homebrew diode ring mixers. He builds and demonstrates an adjustable constant current source which is simple to construct and easy to use. Best of all - once set to the current level you want to test at - it requires no more tweaking. Just swap in the diodes and read their forward voltage drop on a DMM.
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IMPORTANT!
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Please note that there is an error in the left hand schematic at 22:10:19. The cathode of the diode
(D-) should NOT be connected directly to ground as shown here. It should only be connected to the switch. Apologies for this and many thanks to my friend Alan M0VLT for pointing it out and for building it himself! I have corrected this in the downloadable version.
Schematics and project layout diagrams can be downloaded here:
drive.google.com/file/d/1e-OV...
LM317 Datasheet can be downloaded here:
www.ti.com/lit/ds/symlink/lm3...
Grateful thanks particularly to Alan Wolke W2AEW for his excellent videos which really help amateurs (like me) to get a grip of RF electronics. Check out Alan's video on matching diodes here:
• #302: Back to Basics: ...
To find out more about the GQRP Club and SPRAT magazine please visit:
www.gqrp.com
Thanks for watching!
73
Nick
M0NTV

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Опубликовано:

26 июл 2024

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Комментарии : 42

@alanmacdonald65 10 месяцев назад

Great little device Nick, and a very useful addition to the workshop. I have a feeling that this afternoon's entertainment has just been decided!

@M0NTVHomebrewing 10 месяцев назад

Thanks Al!

@InteraliaTony 10 месяцев назад

Well I'm convinced. The two meter method that you described (without current limiting) produced readings that "crept" a little as the diodes warmed up. I had to clip a small heatsink on the diodes to stop that. The method still worked, but this looks way easier! (Tony G4WIF)

@M0NTVHomebrewing 10 месяцев назад

Thanks very much Tony. Well I'm hoping it might make life a bit easier for us all! Time will tell. 73, Nick

@acestudioscouk-Ace-G0ACE 10 месяцев назад

A very neat little project 😊 Thanks for sharing. 👍

@M0NTVHomebrewing 10 месяцев назад

Cheers Ace! Thanks for watching. 73, Nick

@DustinWatts 7 месяцев назад

I stared at the schematic for a while.... and that switch is not needed as I see it. Either you put a diode in, or you put your DMM in and measure the current. Anyway, did not know the LM317 could be used as constant current source, and that is something I appreciate knowing and definitely something I am going to use . So thank you for that!

@M0NTVHomebrewing 7 месяцев назад

Thanks Dustin. You're probably right about the switch! Thanks for watching and commenting. 73, Nick

@BobMellor1954 10 месяцев назад

Such a great idea Nick, this is the next project; thank you and 73 Bob

@M0NTVHomebrewing 10 месяцев назад

Thanks a million Bob. 73, Nick

@Daklon17 10 месяцев назад

Thanks for this video, I had read about constant current sources but haven't seen a practical use for them. Will build it for sure :)

@M0NTVHomebrewing 10 месяцев назад

Thanks very much. Yes, I think it could be quite a useful little circuit. Happy building! 73, Nick

@technishn 10 месяцев назад

Very useful. I remember replacing 4 diodes in 2 identical discrete mixers . One 1N5711 set gave much better balance, no doubt due to being better matched at the higher current. I have seen diode quads in a single package degrade, again normally due to 1 or more diodes changing. I have used 1SS99 diodes which have a far lower drop at 10mA, translating to less mixer loss. About 5 dB vs 7.5 dB. I haven't tried driving the 1N5711 diodes with higher oscillator power yet.

@M0NTVHomebrewing 10 месяцев назад

Thank you very much. I've not come across the 1SS99 before but it looks as if they are getting harder to find these days. Bit of a shame as they look ideal for mixers. Thanks again. 73, Nick

@technishn 10 месяцев назад

The BAT42/43 looks interesting with the lower voltage drop at 10 mA, however the capacitance of around 7-10pF may degrade the mixer at frequencies near 28MHz/50 MHz. They are glass diodes, at a few pence (cents) each.

@williamglew2851 10 месяцев назад

just got sprat well done more useful gear

@M0NTVHomebrewing 10 месяцев назад

Thanks very much William. 73, Nick

@lmoline63601 10 месяцев назад

Hello Nick! Great video! One other test to refine matching might be to check capacitance of the diodes also and use that to whittle them down farther. This was suggested by W2AEW. What a great test device you made!

@M0NTVHomebrewing 10 месяцев назад

Hi Loren, thanks very much. That's an interesting idea about the capacitance. There has been quite a bit of discussion about this whole topic on the GQRP Club reflector in the last couple of days. Someone suggested that we shouldn't be DC testing diodes for mixers at all - we should test them with RF at the frequency we want to use them at. I haven't got a problem with that - just that it complicates the process a bit. This is where the capacitive reactance will kick in as it is frequency dependent. It is interesting! Thanks very much. 73, Nick

@lmoline63601 10 месяцев назад

I built a couple of mixers in which I used a trimmer capacitor to help balance the mixer which affected the carrier suppression. This was in addition to balancing the amount of carrier to the 2 sides of the mixer windings. This was just another adventure in the mixer world. This all depends on how critical one wants to be in design and construction. If the carrier suppression is great as originally built that is all one could hope for. Thanks again!

@oldblokeh 10 месяцев назад

Hi Nick. It might be worth mentioning that, like their PN junction cousins the forward voltage of silicon Schottky diodes has a negative temperature coefficient. It's lower than that for PN diodes, but still worth considering at about 1.45 mV/degree. So a difference of, say, 10 degrees due to holding one in a hot clammy hand versus one left to sit for a little while after mounting in a jig could give a discrepancy of some 14.5 mV. Power dissipation will only be of the order of 5-10 mW, but it's probably worth making measurements reasonably quickly once the current is on to avoid self-heating.

@M0NTVHomebrewing 10 месяцев назад

Thanks for this. That’s helpful advice. 73, Nick

@hflorin07 10 месяцев назад

Hi Nick. The best method for matching diodes: Use a Peak Atlas DCA75 Pro with PC software for diode characterization. Choose those diodes that have the closest curve. Zoom in on the chart for a closer look. Regardless of the method used, try to use a plastic tweezer to change the diodes at the measuring point, in order not to change their temperature. Silicon diodes have a voltage variation with temperature of 2.5 mV/°C. At a variation of 10 °C, you will have a difference of 25 mV! Good luck! 🙂

@M0NTVHomebrewing 10 месяцев назад

Hi there, I have one of those devices. That’s how I produced the IV chart in the video. It works a treat but I wanted to try and build something myself which wouldn’t cost much but would give good consistent measurements and was not too fiddly. It does also depend upon your DMM. The DCA75 Pro is brilliant though. 73, Nick

@G0FUW 10 месяцев назад

Good work Nick and thanks for the G-QRP plug. In your previous video you tested a number of mixers for carrier suppression, etc. Have you done any tests with mixers made from matched/unmatched diodes? I get the theory of why it is good to match, but does it make any difference in the real world?

@M0NTVHomebrewing 10 месяцев назад

Thanks Steve! I haven't made a diode ring mixer without any attempt to match the diodes. However, for some of the early ones I just used the diode function on my multimeter. I'll have to try it at some point. It has certainly provoked some discussion on the reflector! Thanks again. 73, Nick

@amirb715 10 месяцев назад

matching the diodes just based on their forward voltage drop is nice but that is not the main source of problems in ring mixer (unless the discrepancy is really huge). Things like transition time and recovery time are much more important in operation of the ring mixer and they are difficult to measure and match . The best way is to use diode array packages.

@M0NTVHomebrewing 10 месяцев назад

Thanks for your feedback. From my own experiments it would seem that the forward voltage matching is a bit more forgiving than I imagined. I found that even more important than this is how carefully you build the circuit. But, hey, on balance I'll try and build a mixer the best I can - including Vf matching even if it's not the most important thing in the world. It is interesting stuff! 73, Nick

@alanmacdonald65 10 месяцев назад

On the schematic you've got the DUT cathode connected to ground, I think this is incorrect, should be as per the wiring layout.

@M0NTVHomebrewing 10 месяцев назад

Thanks Al. I think I know where you mean - on the V3 pencil schematic. The D- terminal should only be grounded through the switch. I'll correct it. Cheers.

@jamesmoffat9754 10 месяцев назад

Why is a mixer bridge arranged differently from from a rectifying bridge?

@M0NTVHomebrewing 10 месяцев назад

Very good question! Essentially although the two circuits are similar they perform different functions. I explained what the diodes do in a mixer but check out the link below (under the ‘Rectifier’ section) for a similar walk through of the current flow in a diode ring rectifier: en.m.wikipedia.org/wiki/Diode_bridge Hope this helps. 73, Nick

@andye2005 10 месяцев назад

A mixer is a diode RING not a diode bridge - two different configurations. Draw it out and you will soon see the difference Andy

@DAVIDGREGORYKERR 3 месяца назад

What about a Double Balanced Mixer by implementing the SO42P using 12V Vacuum tubes made for a B+ of 12V?

@M0NTVHomebrewing 3 месяца назад

I've never dabbled with valves/tubes/thematrons myself David. I'm a solid state guy really. Never say never though! 73, Nick

@h7qvi 10 месяцев назад

The mismatch in voltage amounts to 10% or less. Efficiency/gain/loss change is minimal. Only big effect is the LO leakage to the output might be 10-20dB down instead of 40dB down. With proper designs that filter that output, it's hardly worth worrying about. You can also get good LO suppression just by matching diode pairs instead of matching all 4

@M0NTVHomebrewing 10 месяцев назад

Thanks for watching and for your comments. I agree that loss is not a huge issue because that can always be made up for but the LO leakage is certainly, as you say a more serious problem. As I've said in previous videos, one of the main issues I've had in building SSB transceivers has been poor carrier suppression in the balanced modulator. Once that signal is there then it can be a pig to try and notch it out. Far better not to have it there in the first place. So anything I can do to improve the performance of the mixers/modulators that I build is, for me at least, time well spent. Thanks again for taking the time to comment. 73, Nick.

@h7qvi 10 месяцев назад

@@M0NTVHomebrewing You can also inject a small amount of DC from a trimpot to get near perfect suppression

@wendelljackson1488 10 месяцев назад

I appreciate your postings, but I have a suggestion. When you refer to your drawings or charts, etc., you use directives such as here, this, there, etc., and use your mouse cursor to focus attention. However, the cursor is so tiny it is almost impossible for me to quickly locate it on the screen. It would be quite helpful if your cursor were larger so it could be followed around the screen more easily. Thanks.

@M0NTVHomebrewing 10 месяцев назад

That’s a very fair comment and thank you for the feedback. I’ll see what I can do about that for the next video. I appreciate you letting me know. 73, Nick