In this video, Nick M0NTV takes on a hot topic in ham radio homebrewing: The matching of diodes in diode ring mixers. How should the matching be done and -- more controversially -- is this matching necessary?
I won't spoil it for you by giving the answer. Watch Nick's video to find out if it matters. (But a hint appears below.)
I think it is great that Nick has taken the trouble to look carefully at this issue, and has found info that will be of great use to homebrewers. And I really liked Nick's response to the fellow who suggested just going out and buying a commercial diode ring: Nick replied that he homebrews because he likes to, and because he wants to know how these circuits work. FB Nick.
I was also pleased that Nick gave some much warranted recognition to Pete Juliano for his idea regarding the placement of a trim pot on a diode ring. This idea made it into the Experimental Methods in RF Design book (under Pete's old call: W6JFR). Page 6.56.
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ReplyDeleteNice video presentation The topic appears to be balanced modulators?
Most often we match diodes to get the best port isolation and thusly examine our diode ring matching with this in mind. This whole business is complex and involves diode parasitic series resistance, diode junction resistance and diode junction capacitance amongst other things.
The use of adding resistor for impedance matching in networks and structures like diode mixers dates back to the mid 1960s.
Petr
Here are a few observations:
ReplyDelete1) The random (unmatched) bag of Schottky's have usually yielded me better Vf matches than a similar bag of 1N914's. But your results may vary. So Nick's unmatched Schottky's may have been close enough to start with.
2) Maybe the transformers need to be characterized for match? A bridge circuit of some kind? Reason I mention that is Pete's pot effectively nulls both transformer and diodes in the single-balanced modulator.
3) Another possible reason for Nick's disappointing results is poor return loss at the ports. The three ports need good termination at harmonics and IF freqs as well as in-band. This is why Minicircuits, etc use ~10dB pads at the ports when testing mixers,etc. They show that in their app notes. So one can have a great, well matched mixer, but get poor (or confusing) results with poor wideband termination.
Old habits die hard-- think I'll still match 'em.
I usually try for 2 millivolts Vf match on all 4.
73!
WN2A
Two questions:
ReplyDelete1) shouldn't M0NTV have tested those same diodes at 1 mA ? Judging from those overlaid I-V curves the matching would be the same as at 10 mA ?
2) why use the1n5711 Schottky diode ? The forward voltage at 10 mA is about the same as the cheaper conventional 1n4148.
1) Depends upon what you want to use it for. A low-level mixer ~4-7dbm LO, might use the 1 mA level. For a high-level ~17-23dBm LO, the higher test current is probably suitable.
ReplyDelete2) I find matched 1N4148's very good at low-HF, but going up to ~30 MHz and above, I see better performance with the Schottkys. Part of this is Cj , but also better reverse recovery time (trr). The 1N5711 is listed at 1ns max trr (Diodes,inc); IN4148's 4ns.
With a DBM, we should switch rapidly through the transition zone. Both Cj and trr are factors, especially at higher frequencies.
Nick didn't get what I expected for carrier suppression in either case (matched or unmatched diodes). Something amiss otherwise?
Thanks WN2A, so the forward voltage isn't really the important spec, it's the capacitance and recovery time, for the BAT42 has a lower forward voltage at 10 mA but larger capacitance and trr.
DeleteWell, absolute Vf is important for the drive level. Lower Vf for ~4-7dBm, higher Vf for ~17-23dBm LO drive on a DBM.
DeleteA good transformer null, a good wideband match at the ports (Nick did touch on that subject), and the proper drive levels and diode Vf matching all contribute. The LO power and LO spectrum also are factors.
Nick did mention that by increasing the LO drive, he could get greater carrier suppression! Maybe a Eureka Moment?? Said another way, his IF tone levels may have been too much for his original LO drive level- hence the disappointing carrier suppression-even with matched diodes. Generally with DBM's, the LO power should be >>10dBm the IF and RF ports.
Yet another "rule of thumb", sorry.
By "checking all the boxes", the LO-RF isolation can be >>50dB at HF for your HB DBM.