Double balanced mixers: various questions & probems with my first attempt - Page 1
Forum section: electronics / rf-microwave

author: Whales
author_rank: Super Contributor
timestamp: April 24, 2020, 02:55:04 am
content: I'm having a lot of issues making my first double-balanced mixer. The goal is to use it to make a better radio for listening to broadcast AM (~0.5-1.2MHz) than my existing amplified crystal set. W2AEW's videos have entranced me.Recommended drive & termination impedancesIf I look at a double-balanced mixer then I can't see anything that easily or obviously defines an intrinsic impedance, other than perhaps the on & off impedances of the diodes:Q1:Does this mean I can just arbitrarily choose an impedance and I will be fine as long as I use it at every port? Eg go for 1Kohm instead of the common 50ohms; because it's much easier to make amplifiers for.Driving mixersFrom what I've read: the RF-in and LO-in need to be terminated properly to minimise reflections (& hence extra mix cycles).I've had a look at some BITX radio schematics and they seem to drive things with (what I see as) variable-impedance output amplifiers. ie plain old common-emitter amps. This seems at odds with the idea of proper termination.As a different example: let's pretend the very last stage of amplification for my RF-in before it gets to the mixer is a simple emitter-follower:The output impedance of this amp is anything from 1Kohm to almost 0 ohms, depending on what the transistor is doing.In my mind this is one way of fixing the problem:Unfortunately this would be very hard to do if I wanted a 50ohm output. I'd need a 5-ohm or less pull-up, or I would have to build a full class AB amp just for driving this mixer (something I have not seen in any radio schems I've glanced over).Q2:Is it really this important to properly terminate all the ports? Am I missing something?(Solved) Really high loss through coresAt the moment I'm running my test mixer with a small DC current as the LO, just to see how much of the input signal makes it to the output.Diodes are KDS187's, small switching diodes with very low reverse capacitance (ie not schottkys). Coils are 10T trifilars. 10x probes are at the end of the red and yellow wires to measure the mixer input and output.Observations:Input: 1MHz 200mV p-p --> Output: 1MHz 500uV p-p (-50dB)Input: 100KHz 500uV p-p (can't go higher without crazy distortion) --> Output maybe 250uV p-p, too low for my scopeEDIT3: I think I had a bad conn somewhere. Now:At 1MHz I can't put in more than 100mV p-p of sinewave without weird distortion appearing at the output (input still looks fine).At 100KHz I can't reliably measure anything coming out before I hit horrible distortion.At higher freqs there seems to be less loss. Could be parasitic coupling?I'm not sure what the cores I'm using are, but wrapping 30 turns around them yields below-measurable inductance on my cheap transistor tester (it seems it can't measure below 0.01mH). I presume this means they're material designed for high-freq coupling, not for energy-storage inductors.My gut is however telling me that these cores are no better than air-coupling.EDIT: actually signal is making it across the first transformer fine, things seem to be getting eaten by the diodes. Hmm.Q3 (solved):Anything obvious I might be doing wrong here? Any suggestions? Wild or sane, I'll take all.Solution:changing the cores. The red ones pictured above (very low mu) don't work in the way I've used them. Switched for some unknown grey ones I had in my drawer (much higher mu), now my DC-as-LO tests are working as expected.

author: bob91343
author_rank: Super Contributor
timestamp: April 24, 2020, 03:41:04 am
content: Perhaps you don't have enough inductance and the signal is getting shorted out before it reaches the diodes. Or perhaps not enough mutual inductance.

author: Whales
author_rank: Super Contributor
timestamp: April 24, 2020, 04:27:21 am
content: You might be right. This could also be why I can't drive it above a tiny voltage.I've been fiddling some more. It seems the first transformer has a small amount of voltage loss (about 10%). Overall loss is now much more sane (only about -6dB), not sure what I did to fix that, possibly a bad connection.If I turn up the gain then I first see distortion on the output, then I start seeing distortion on the input. I wound the two cores slightly differently IIRC (one 12T, the other 10T), so perhaps I'm seeing some core-related effect here too.

author: profdc9
author_rank: Frequent Contributor
timestamp: April 24, 2020, 05:01:48 am
content: You should probably use type 43 or 61 ferrite cores. It looks like you are using red #2 iron powder cores.Are you winding the transformers trifilar? Use one of the windings for the primary and two together for the secondary, using the center tap where they join as the grounding point.

author: Xnke
author_rank: Regular Contributor
timestamp: April 24, 2020, 05:40:05 am
content: To build a nice diode bridge mixer, a few things that will really help:Use very small type 43 binocular cores, or if you only have toroids that works too. The little 2404 binocular cores have very (VERY) good coupling and bandwidth.Get some BAT-something series diodes-they are super cheap and available, and most of them (BAT40 comes to mind) work very well in this application.Terminate the IF-port into a diplexer. The IF port needs a good broadband 50ohm match over the widest bandwidth you can get.Terminate the RF-port into a well matched 50ohm bandpass filter, or 50ohm source impedance.Don't worry about the LO source impedance, it is far less important. Just make sure you have enough LO to drive the diodes, and a square wave with good sharp, symmetrical edges will give approx. 10 to 20dB of improvement in the intermodulation performance.Make your transformers identical. They aren't just transformers, they're transmission lines. They are doing an impedance transformation, and they need to be as tightly matched as you can possibly make them. Trifiliar windings are a must.

author: Whales
author_rank: Super Contributor
timestamp: April 24, 2020, 06:50:21 am
content: Yes I'm winding trifilar. Xnke and profdc9: thankyou for the tips.I've remade the bridge with two new cores. Notably:- I can now shove in a much higher p-p waveform without distortion (probably higher impedence)- I can run it to much lower freqs (eg 200Khz), where as before the red ones were useless at anything below around 1MHz.Q3 is now resolved, I'll mark it as such.I'll look into diplexers.

author: Yansi
author_rank: Super Contributor
timestamp: April 24, 2020, 06:58:02 am
content: You need a high-mu ferrite cores for these RF transformers. Whyt you have there appears to be a mix No.2 iron powder cores, which are for resonant circuits or chokes, not for transformers!Mix No.2 is especially the one with almost the least mu of them all. Not enough inductance!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 24, 2020, 07:02:22 am
content: Quote from: Whales on April 24, 2020, 02:55:04 amDepends on signal level. At small levels, it's very high; at high levels, it's approx. Vf/If. LO is the only one you should be driving at high level, so that it depends on level means you get your match from setting signal level.It's not very particular (as long as it's stronger than the RF signal level), and it's also nonlinear, so you want a well damped source anyway. Which doesn't mean much at these frequencies, just buffer it and use a nice fat source termination resistor or whatever. Doesn't need to be an emitter follower, can be collector output just as well (in which case the termination is parallel -- Norton vs. Thevenin circuits, basically).The remaining ports then are whatever impedance they are, since the port impedances will dominate over diode impedances.The transformers also need to be much higher impedances than the ports. So, if you choose 1kohm ports, you'll want at least 3kohms reactance at the lowest bandpass frequency, and also the same or more capacitive reactance at the highest frequency.QuoteOr less, under suitable conditions, namely a capacitive impedance at the base gets rotated to a negative impedance at the emitter. So some "base stopper" resistance, or series emitter resistance, can help with that.For the Norton version, use an ordinary common-emitter stage, and assume Miller effect is negligible (which isn't much of a problem at these frequencies). The collector load is the parallel combination of the transformer primary, and the load resistor. (Some series resistance between collector and load might still be used to help isolate parasitics, in case it wants to sing at stupid frequencies, but that's pretty unlikely I think. And again, emitter degeneration can help out.)For better isolation, simply use a common-base or cascode stage. Again, not likely important at these frequencies, but can be handy at VHF+.QuoteYeah, at 1MHz for example you need >= 470uH, and a T37-43 at 0.35 uH/t^2 needs 37 turns already. If that's a powdered iron core, it may be something like 8 nH/t^2, utterly useless here.High-mu ferrite is best here. Anything for pulse transformers and common mode chokes will do.Tim

author: Whales
author_rank: Super Contributor
timestamp: April 25, 2020, 01:47:27 am
content: Hooray, set it up with an antenna on the input and a lowpass filter + audio amp on the output (ie made it into a direct conversion receiver), now I hear stations whistle as I tune around with my siggen as the LOQuoteOoh, I think I have exactly some of those in my drawer.QuoteQuoteWow. I thought transformers were an application were lower mu helped, seems I had it completely backward.I guess it makes sense that I don't want the transformers to appear as shorts, but now I wonder why you wouldn't want the transformer to appear as the same/similar impedance as the source (ie thus getting max power transfer).Q: Is keeping Zload << Ztransformer also important (as well as keeping Zsource << Ztransformer)?Ie if I want a 455KHz IF output then I'll need to consider the impedance of the transformers at that freq? And if I want direct-conversion output (ie audio freq) then I'm basically SOL with this type of mixer (because the Z of the transformers will be way too low)?QuoteI read once that common-mode chokes tend to use an intentionally-lossy material. Does this just mean extra high-mu so saturation is easily hit, or are they intrinsically lossy through some other means? I assumed mains common-mode choke cores should be avoided.

author: Xnke
author_rank: Regular Contributor
timestamp: April 25, 2020, 03:15:39 am
content: The impedance of the transformer is immaterial, as the device is symmetrical. Load the RF and IF ports with 50, and they'll be happy-as long as they are BOTH 50.The LO port would "in theory" want to see 50but in reality, the performance is better when you just drive it with enough power to fully switch the diodes as fast as possible-a square wave LO is best.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 25, 2020, 05:50:19 am
content: Well no, transformer impedance matters for raw mixing action. It can't be less than the diode dynamic impedance, or you'll short out even that.More importantly, transformer impedance needs to be high enough to allow the signals to pass, that is, you want to avoid insertion loss.This is easy to illustrate with an extreme case: suppose you try getting audio or DC out of the 2nd transformer. Say you apply LO between CTs, and RF to the other transformer. The impedance of the 2nd transformer is approximately zero at DC, shorting out all signal, and not allowing any voltage to develop across the diode ring.Both transformers act in parallel, more or less.If the transformer coupled ports are intended to be bandpass, they can be tuned, assuming the transformer losses are alright, but this just fits into filtered ports, see below.Impedance matching does not apply to reactive components (which absorb zero average power), and you certainly wouldn't want to couple maximum power to intermediate loss elements for no reason. Hence, the transformer impedance is made high so that overall gain can be as close to ideal mixer loss (6.something dB) as possible. You're matching the antenna (or RF preamp) and IF amp stages, anything else is detrimental.Lossy materials are fine, as long as they retain impedance over the applicable bandwidth. Typically a hi-mu ferrite has mu ~ 10k at DC up to ~100kHz, with losses rising up to that point, where resistance then exceeds reactance. This is plotted as mu = mu' + j mu'' in datasheets. It's basically a lowpass cutoff point for the inductive core.Imaginary mu is simply giving the resistive (loss) component in a somewhat backwards way. Imaginary mu means imaginary inductance, which gives an impedance:Z = j w L = j w (j R') = -w R'A real impedance. mu'' is negative by convention so it comes out to real loss, not negative resistance (which would be gain, and would require an external power source). Note that mu'' is always either rising with frequency (in which case mu' is large and ~flat), transitioning between the two (technically speaking: it can be locally flat, d(mu'')/dw = 0, but d^2(mu'')/dw^2 will not be zero), or falling as fast as inversely proportional with frequency (in which case mu' is dropping even more quickly).mu'' falling with frequency, when mu'' is dominant, is another way of saying it's a dumb resistor. Z = w (R / w) = R, a constant, real, resistance.So that's how we measure that. If that sounds like a mess to you, yeah, you can just refer to the impedance curve of the core. Typically the impedance |Z| continues to rise with frequency until some higher cutoff point (which is to say, mu' and mu'' aren't dropping quite as rapidly as inversely proportional to frequency, the inductance is still going down but the impedance at frequency is still going up), which is all that really matters. As long as |Z| is large, you don't care that it's likely quite lossy up there. |Z| > 5x or 10x Zin or Zout is a good idea.#43 ferrite, for example, is an intentionally lossy material, with a modest mu (~800). Its impedance peaks at a few MHz, then drops off surprisingly slowly above there. This makes it lossy over a wide range, very useful for damping resonances on cables (in the cable's equivalent circuit, the ferrite bead looks like a series resistance, which serves to terminate standing waves, flattening dips in the cable's noise rejection response). The lower mu means we'll need relatively more turns (a little toroid or binoc core can fit plenty of turns if you have very fine wire to wind it with), or we should use shapes with high inductivity to begin with (thick walled beads, and binoc cores, are typically quite good).And, again, it's a lossy material, but that doesn't matter because we're making |Z| large enough not to care either way.As for Zin and Zout, they more or less act in parallel, as far as each other is concerned. You are alternately connecting them together, in parallel, or in anti-parallel. Follow the diodes, that's what they do.The act of swapping polarities doesn't change the impedance, it just changes the frequencies where that impedance is presented at (which is to say, it's a mixer).You don't really need to carefully match small signal impedances here, again, just in general because of the low frequency. It's not like you have wavelengths of cable you need to terminate. You just want to keep signal losses down. And, keeping a stable impedance for the harmonics and out-of-band mixing products can be useful.I forget if presenting a short or open load for harmonics and mixing products is supposed to have useful or detrimental effects. That is to say, putting a conventional bandpass filter right up to the mixer port. Filters reflect out-of-band signals, by effectively shorting or opening the circuit at the rejected frequencies.A transformer with significant reactance, can be tuned by putting a capacitor in parallel with it; this simply makes it a bandpass filter.If both filtering and termination are desirable, the reflection from a conventional bandpass filter can be dampened with a modest size attenuator (say 3 to 6 dB). Obviously, this costs exactly as much dynamic range (raising the noise floor).More sophisticated, a diplexing filter can be used, which has a flat port impedance (the rejected bands can be absorbed in terminating resistances, or even directed to additional ports!).Or an amplification stage can be added between filter and termination, which could introduce less noise or lost dynamic range than a brute-force attenuator does.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 25, 2020, 06:02:55 am
content: Regarding the diodes, you want higher impedance diodes for a high system impedance. More specifically, you want diodes whose off-state impedance is much higher than Zo, and whose on-state impedance is much lower than Zo. The closer the impedances are to Zo, the more critically they must be matched.Schottky diodes are good for lower impedances, like 50 ohms. You may find better performance with junction diodes, mainly just because the voltage drop is higher (~0.7V vs. ~0.3V).Note that, because the voltage drops across opposing diode pairs are constrained to be ~equal by the circuit, matching is not so simple as choosing the same type for all of them. That is, random 1N4148s might drop 0.645V, 0.638V, 0.650V, etc. at 10mA. At the same (e.g. 0.640V) voltage, their currents won't match, and neither will their dynamic impedances. The diodes might all be very close to 2 ohms dynamic resistance at 10mA forward current, but their currents won't match unless the diodes themselves are matched.Or for a JFET mixer, the JFETs need to be matched by Vpo, and Rds(on) or Idss; or for a MOSFET mixer, same thing; etc.Likewise, in the off state, the leakage current is negligible for both smallish schottky (BAT40, BAT54, BAT85, whatever, all in the uA ballpark) and junction diodes (~nA for 1N4148), capacitive reactance dominates. The BATs are ballpark low to mid 10s of pF, while 1N4148 is a few pF (1-4 depending on who you ask, and at what voltage). Diodes always have a variable capacitance, with bias voltage; you'll be in the low voltage range obviously, so the capacitance (and its change) in the 0-1V range or so is what matters most.Anyway, ideally the capacitances match, both in terms of absolute C, and C(Vr) (the curve).Imbalance allows signals, even harmonics, and even mixing products, to pass between ports -- the ideal double balanced mixer isolates these, but a non-ideal one lets some through. If you design your band plan to avoid these products anyway, it doesn't matter; you can also use a single balanced mixer, or even an unbalanced mixer (e.g., couple RF + LO into a single diode or transistor, and filter IF off the output). The DBM is then only for its marginally lower mixer loss, and... style points, I guess.You need a properly balanced mixer when those bands overlap. For example, the conventional AM radio has a 530-1700kHz RF range; with an IF of 455kHz, its LO range must overlap the RF range. (Although historically, tracking filters were cheaper than very carefully balanced mixers -- part of the reason for the multi-section variable capacitors in old sets. In effect, the RF and LO bands never overlap because they're varied simultaneously. Making it a hybrid partial TRF (tuned-RF) and superhetrodyne receiver.)Tim

author: Yansi
author_rank: Super Contributor
timestamp: April 25, 2020, 08:57:14 am
content: Quote from: Whales on April 25, 2020, 01:47:27 amThat just means you rearrange ports, so the IF port can pass DC, just like this. (Commercially made DB mixers are also made this way.)

author: Whales
author_rank: Super Contributor
timestamp: April 25, 2020, 12:58:35 pm
content: Woah. Thankyou for the fire TimI never knew that the world of magnetics was this bananas. I now understand why people say that it's easier to get closer to ideal caps than coils for practical human use.QuoteAh, that makes sense if I assume losses are proportional to current (B-field).QuoteSo far I have been able to find mu' in datasheets, but not mu''. It seems everyone likes providing core loss in varying other ways. Is quoting/graphing mu'' unique to a certain type of product (eg cores dedicated for radio signal coupling)?Also, I demand positive mu''. I need something that can both warm my pockets and provide a broadband noise beacon for locating me when I'm lost.QuoteI'll presume you're describing three common material types. (3) is only useful for coupling lower freqs and (1) sounds like common 10,000 mu' fe Mn-Zn or sim?QuoteOh god. *runs*. Indeed I plan to put an amp between the mixer and my filters, that bit of separation (with predictable Z either side) makes things much easier for me to think about.QuoteThe ones I'm using are already very well matched, at least to within +-1 LSD on my multimeter's diode test range. Made me happy when I saw that. ~2pF at low V.QuoteHmm. The DBM is the first I've come across in detail, I'll have a look at the other options too. Thanks.QuoteYep. My first mistake was thinking I should start playing with radios by modifying an existing "simple" few transistor AM radio.That's both mixing and oscillating with the single transistor? How many varicaps? What the hell?!?Quote from: YansiOoh, that makes sense. Thankyou.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 25, 2020, 02:15:14 pm
content: Quote from: Whales on April 25, 2020, 12:58:35 pmOh yes, quite. They're also many times larger than capacitors, for the same energy storage.Heh, which, not that it's hard to find nonideal capacitors either -- type 2 ceramic dielectrics are ferroelectric, i.e., the same ferroic phenomenon but connected to electric dipoles rather than magnetic spins. So you get all the same ugliness: anisotropy, saturation at high field, weak response for small signals, hysteresis losses, massive tempco (peaking just below the Curie temperature, above which the ferroic behavior collapses entirely).Probably won't have to worry about that here, but it's good to know for those occasions when you need a nice accurate, relatively large, capacitor, say for timing or signal filtering purposes. For supply bypassing, who cares, for the most part; the main downside being, they're also piezoelectric so acoustic vibration is coupled to supply noise, annoying when you need very quiet supplies (microvolts of noise?).The good kind of both being, gapped inductors for energy storage (the air gap swamps the core's nonlinearity, for the most part), and C0G ceramics or film capacitors.QuoteEh, power materials are usually rated by losses under conditions, and may not have small-signal data like that. Signal and EMI ferrites usually have it. Example, the famous #43:https://www.fair-rite.com/43-material-data-sheet/QuoteJust describing the areas of the plot: note that on the left, mu'' is low and mu' is flat or falling slowly; note also that |mu| is down by 3dB (~70%) by about 2MHz, but it's still dominant inductive. Then mu'' is flattening out, and eventually takes over by 13MHz or so. Above 30MHz or so, mu'' is quite dominant, but it's still not dropping quite as fast as inversely proportional to frequency; this means the impedance continues to rise, even past 100MHz, even though |mu| isn't very much up there.The exact form of the plot doesn't really matter so much; more just spotting ranges of interest, and noting the symmetry between change in mu' and magnitude of mu'' (which is actually quite deep, representing a law of time symmetry, energy conservation).All materials share this general form, as far as I know. This type is more lumpy than others; again, an optimization for EMI purposes. Others might be a bit sharper, and the cutoff may be higher or lower.Another comparison, Ferroxcube 3F45:https://elnamagnetics.com/wp-content/uploads/library/Ferroxcube-Materials/3F45_Material_Specification.pdfthey give both small and large signal curves, which is nice. (That is, mu'/mu'', and Pv vs. B for different frequencies.)Another interesting symmetry is a roughly constant "GBW". That is, high mu types have a lower cutoff (e.g., ~100kHz for ~10k mu) than low (e.g. 1MHz for 1k mu, or 10MHz for 100 mu). If we plot these characteristics for many materials, we find they cluster along a line of constant GBW. Which means those that fall well above the line are doing quite well; we can use this as a figure of merit for the material. The 3F45 being around 6MHz cutoff and 1k mu means it's doing quite well indeed. (Good old transformer iron -- silicon steel -- is pretty poor by this measure, but not to say metallic strip is useless: glass and nanocrystalline metals actually outperform ferrite, having |mu| over 100k into the 10s of kHz. These materials make fantastic pulse transformers; downside is they're a bit more pricey.)Though again, since we aren't terribly concerned about material losses in this application, we're quite content to save turns with a higher mu material instead.Tim