Importance of forward voltage drop of the diodes of a DBM  - Page 1
Forum section: electronics / rf-microwave

author: Dom13c
author_rank: Regular Contributor
timestamp: December 20, 2018, 02:20:15 pm
content: Is it critical to match the forward voltage drop of the 1N4148 diodes(Im using them) for a double balanced mixer like the one in the Beach 40 transceiver? If so or not then why?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 20, 2018, 06:33:23 pm
content: Specific to the Beach, I don't know. Usually a DBM is chosen because the RF and LO rejection is required (which in turn is driven by the particular RF BW, LO range and IF center and BW chosen for that mixer). Assuming that drove the choice here, then yes, matched diodes are required: consider the effect of the RF being switched backwards and forwards by the LO. When each pair of diodes is conducting, its voltage drop is in series with the RF-LO path. Therefore if the voltage drops are unequal, some of the LO (pairs of diodes offset) or RF (pairs proportional) will leak through.Normally, better than 1mV matching is needed (which gives ~60dB balance compared to a ~1V signal), which is very difficult to ensure with discrete diodes, and a monolithic array is needed.Unmatched diodes probably give around 34dB balance, which isn't terrible, considering. If RF and LO rejection is not actually required, it would be fine.Tim

author: cncjerry
author_rank: Unknown
timestamp: December 25, 2018, 02:18:01 am
content: Hey, if you are building your own DBM, find that NIST paper on using 2n2222 transistors in place of the diodes with the base/collector shorted together. I did some preliminary testing but was more interested in it as a phase detector. Someone on time-nuts felt that since the transistors are being used as switches matching wasnt necessary. Two papers were written, both felt the performance was better than commercial alternative.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 25, 2018, 08:48:34 am
content: With some effort, you could use BJTs as switches in a similar circuit to the JFET mixer; I think you'd have to use isolated drive (B-E) windings though, to free up dynamic range -- otherwise you're stuck with a 4-diode DBM and that's no particular advantage.I don't think that would necessarily grant you full Vce(sat), including inverted operation, though. After all, with B strapped to E with a winding, you can't exactly invert it any further than the DBM again. So, I think the ultimate realization of that circuit would require bias voltage as well as B-E isolated drive, and with some rearrangements, and perhaps opting for current instead of voltage transmission -- you very neatly end up with a Gilbert Cell or something close to that.To put some historical context on that, it would very much be the sort of circuit Tektronix used back in the day, from their 500 series scope plugins (e.g., pentodes + diode current steering, gating channel A/B to the output), to the 400 series (transistors gated by emitter current, or collector output + diode current steering), and further in integrated circuits (until everything went CMOS, probably relatively recently).BJTs have certainly been used for low-sat applications before, early solid state DACs for example; but I think to use them this way, you must have precision matched current sources and sinks, to turn on the BJT bidirectionally without introducing an error current in the result. I mean, you could still implement a mixer this way, but holey moley what a headache; and it's still not passive, because the current source/sinks need a supply rated for the full compliance voltage range (or the bias I spoke of earlier, needs to be added in series, to the same end).Well...Practical matters aside -- yes, I would expect "Rce(on)" to be much lower than for JFETs, and consequently, there being less need to match them. Capacitance you might still want to match, but not much beyond that.Tim

author: SeanB
author_rank: Super Contributor
timestamp: December 25, 2018, 11:16:29 am
content: If you want matched 2N2222 transistors look for 2N2223 transistors, they are a matched pair in the same TO100 case.

author: cncjerry
author_rank: Unknown
timestamp: December 25, 2018, 11:17:00 pm
content: I found the paper referenced below so you don't have to be an IEEE member to download it, just look around a little:Residual PM noise evaluation of radio frequency mixersChad A. Barnes, Archita Hati, +1 author D. A. HowePublished 2011 in 2011 Joint Conference of the IEEE International…Direct observation of phase-modulation (PM) noise is often difficult due to the high dynamic range that exists between the carrier and the modulated sidebands. A common tool used to reduce the dynamic range is the phase detector, which removes the carrier and down-converts its noise sidebands to baseband. The double balanced mixer (DBM) is the most widely used phase detector for high-resolution PM noise detection at most carrier frequencies. For Fourier offset frequencies close to the carrier… CONTINUE READINGTo summarize, their mixer, and I use "their" loosely as I've seen others at NIST referencing the same design and I didn't go back to review the authors, performed as good or better than a number of commercial mixers. The authors present a chart listing 10 or so. I've built it up to use as a phase detector and it compares favorably to a TUF-3 as well as another Minicircuits (number unknown) and since I am using it as a PD, it doesn't present the non-linearity at 180 degrees as found in the AD8302, at least at the frequency I am using of 10Mhz. I happened to have two MCL transformers on-hand that fit well with their design, threw it together and I'm pleased. By the way, someone on time-nuts mentioned that the transformers aren't that critical as long as they are the same. Once I am done with my PN testing, I plan to run some high level signals through it to see what the IMD looks like, but again, since I was using it as a PD to measure PN, I was more interested in not adding PN. Also, they mention the terminating diplexor to be important and I've seen noise on the positive voltage swing without it.btw, I'm not offering this as a general solution as there are a ton of new mixers out there, h-mode, etc, AD has some interesting units. Just that the OP was wondering about 1n4148s forward voltate and I would think this mixer should outperform a homebrew based on those diodes but have no data to backup this up.Jerry

author: Dom13c
author_rank: Regular Contributor
timestamp: December 27, 2018, 10:09:26 am
content: Well, I've built the mixer and it's working good enough for me. Matched the forward voltage within 2 mv ( tried to match them within a millivolt but the reading was fluctuating enough(swinging within two milivolts each second) to get a stable measurement . For not having propet measurement equipment I can't specify the carrier rejection or drive level. Built two seperate crystal oscillators with same frequency crystals ( with one of them having a series variable capacitor to alter the frequency) as signal generators to check whether it was working or not. From the IF port connected to an audio amplifier, I could hear the difference frequency which swinged whthin 500-1000 Hz as I tweaked the capacitor. That's how I knew that it worked. I'll try the 2N2222 mixer mentioned by you guys in the next transceiver that I'll built from scratch.