Toroid selection for diode ring mixer - Page 1
Forum section: electronics / rf-microwave

author: AlexResch
author_rank: Regular Contributor
timestamp: August 26, 2018, 02:01:32 pm
content: I want to experiment with a diode ring mixer as attached for SWreception, around 3 to 30Mhz. Toroids scare me and I must ask for assistance with following questions :1. How do I select a core suitable for the frequency range ( given that I want to get 455Khz out ).2. Does the core size matter ? For example is the number of turns dependent on core diameter/section like in a normal transformer ? This being reception only, can I use a very small core ?3. I would like to have 50 ohm impedance in and out of the mixer. What needs to be done ?Sorry for the vagueness of the questions. I am only a high school pupil and would want to get some orientation first before asking more informed questions.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: August 26, 2018, 06:38:22 pm
content: you have to take a ferrite material, I would say 2 toroids FT 37-43 from Fair Rite.For diodes commonly are chosen schottky; for optimal symmetry you should match their parameters. They are low impedance devices.Don't go too low with the 1. IF, better choose 9MHz or 10,7MHz; filters are readily available; this offers you a much better image rejection than 455kHz

author: iMo
author_rank: Super Contributor
timestamp: August 26, 2018, 08:37:23 pm
content: You may use the multi-hole cores (aka binocular), the ferrite core material 43 and the smallest binocular size would be ok.The trifilar winding with 0.2mm enameled magnet wire, 6-10 turns (such it fits).For example:https://eu.mouser.com/Search/Refine.aspx?Keyword=multihole-coresFor these kind of transformers you may calculate the minimal number of turns such its inductive reactance XL (in ohms) at the lowest frequency of interest is at least 4x50ohm. Each core type (the toroid or binocular with its core material num.) has a specific AL (in nH/(w*w) where w is the number of turns). One turn with the toroid core is when the wire passes through the toroid, with the binocular one turn goes through both holes.As said above the 455kHz is not the best choice, better go with something like 4..9MHz as the IF (you may have several IFs btw.).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 27, 2018, 03:30:16 am
content: This is a transformer application. All you need is enough permeability so that the winding impedance is much larger (usually >5x) than the system impedance.The figures given above are quite typical for this sort of thing.Offhand, FT37-43 is 0.35uH/t^2, so 6 turns gives 36 times or 12.6uH, which is 237 ohms at 3MHz. This is 4.75 times higher than 50 ohms, not bad.More important is to keep the primary and secondary close together, to minimize leakage. If the turns ratio shall be 1:1+1, cut three strips of wire, equal lengths, twist them together, and wind as one. Connect two strands in series to make the CT secondary, and the remaining one is the primary.This isn't actually ideal, but it's the simplest good method, and will probably still get you better than 20dB balance at 30MHz.Ideal would be a series of transmission line transformers, isolating the primary and secondary and enforcing secondary balance. This is rather more complicated, and you probably don't need 60dB balance (probably can't get diodes that well matched, anyway?), so stick with the easy way until you know you need better.Tim

author: AlexResch
author_rank: Regular Contributor
timestamp: August 27, 2018, 08:16:22 am
content: Thank you, Tim !Tell me please : what does it mean 20dB balance at 30Mhz and what is a "series of transmission line transformers ? What does that look like ?Unfortunately I am stuck with 455kHz I.F as I experiment with superhet stages and I already did some stuff with coils and ceramic filters. By the way, should I couple the sig gens and spectrum analyser to the mixer with capacitors or just straight to the windings ? I am going to use 1N4148 diodes at first and then different diodes to see the ( eventual ) improvement. Am also looking on eBay for a diode ring.

author: KE5FX
author_rank: Super Contributor
timestamp: August 27, 2018, 08:31:53 am
content: Quote from: AlexResch on August 27, 2018, 08:16:22 amIf you don't already have a copy ofExperimental Methods in RF Design, it would probably be a good idea to look for one... hopefully for less than US $120.

author: iMo
author_rank: Super Contributor
timestamp: August 27, 2018, 09:47:17 am
content: The 1n4148 diodes are ok, you have to use a little bit bigger Local Oscillator (VFO in your schematics) amplitude (or "power") than with Schottky or Ge diodes.Do match the diodes such all 4 have the same Vf (forward voltage) with say 5-10mA If (forward current) current.Also mind with 455kHz the minimal 250ohm impedance (5x50ohm) is with 88uH.

author: AlexResch
author_rank: Regular Contributor
timestamp: August 27, 2018, 11:32:24 am
content: Found "Experimental methods...." for download in the usual place.

author: AlexResch
author_rank: Regular Contributor
timestamp: August 27, 2018, 11:35:37 am
content: Quote from: imo on August 27, 2018, 09:47:17 amNow I am getting very confused ! I need 50 ohms in/out on all ports. How do I get THAT ?

author: iMo
author_rank: Super Contributor
timestamp: August 27, 2018, 12:07:23 pm
content: You have got:your Local Oscillator with Zout=50ohmyour RF with Zout=50ohmyour IF with Zin=50ohmThe mixer has got 3 ports: LO input, RF input, IF output. All inputs/outputs of all modules involved have to see "50ohms" or as closer to 50ohms as possible.The impedance of a transformer winding and the Zin/Zout are inparallel, for example 50ohm||250ohm = 42ohm or better with bigger winding impedance..The broadband transformers are usually designed such their WINDING impedance at the lowest frequency of interest is at least 4-5x 50ohm.The higher the winding's impedance the better match (you get closer to 50ohms when both in parallel).BUT, higher winding impedance means a lot of parasitic capacitance (because of a lot of turns), more core saturation, higher ohmic resistance, etc.So for broadband transformers people are happy with 4-5x 50ohm = 200-250ohm.PS: Mind we talk "50ohms" here. That means "50.0 + j0.0" complex impedance..

author: HB9EVI
author_rank: Frequent Contributor
timestamp: August 27, 2018, 12:11:46 pm
content: like imo calculated; it needs about 88uH inductance, so you need more turns on the toroid.And since you are so eager for the 50 ohms system impedance -> don't forget proper termination on all of the ports.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 27, 2018, 12:20:16 pm
content: Quote from: imo on August 27, 2018, 12:07:23 pmLess core saturation -- flux goes as (applied volts) / (turns).The rest is correct.Ohmic losses of course aren't a big deal here, unless maybe you're using hair-fine wire and very small transformers. Well, even then, the transformer will be so small that it won't take much wire... hard to go wrong in this band, in this respect, really.Capacitance is indeed a concern, in fact the more fundamental property at work here is the electrical length and impedance of the windings themselves. Capacitance and leakage inductance is directly proportional to wire length, when the wires are bundled together (as I suggested for construction above). C and LL also set the upper frequency limit (where resonance occurs, and transformer action breaks down).Impedance isn't too hard to manage here: twisted magnet wire has a fairly low impedance, around 50 ohms, thanks to the thin dielectric.Keep in mind the secondaries are 200 ohms end to end (1:2 turns ratio <--> (1:2)^2 = 1:4 impedance ratio), so you get a big hit there, and it would actually be advantageous to use a more heavily insulated wire (more distance between wires = higher impedance). I don't think the 4x mismatch is going to be a problem, as the wire length used is probably much less than 4x shorter than the maximum length set by the upper frequency range.That is, 30MHz is 10m, and 1/20th wavelength or 50cm is the point where you start to worry about bandwidth limitations. 1/4th of that is 12cm, which, hmm, you might need about that much wire, actually; or, half as much, but because the secondaries are in series, they count double, so, still.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 27, 2018, 12:21:49 pm
content: Quote from: HB9EVI on August 27, 2018, 12:11:46 pmThe IF port is not being transformed, only RF and LO. So Fmin is 3MHz (or 2.55MHz if the LO is below RF, but that would be weird).Tim

author: AlexResch
author_rank: Regular Contributor
timestamp: August 27, 2018, 12:35:44 pm
content: Quote from: HB9EVI on August 27, 2018, 12:11:46 pmIf I may ask, what does "proper termination" mean ? My sig gens and SA are already 50 ohms. Must I add anything ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 27, 2018, 01:10:47 pm
content: Quote from: AlexResch on August 27, 2018, 08:16:22 am20dB balance, meaning, the common mode (error, imbalance) is 20dB below the differential mode (intended signal).Common mode is the average of two lines, (V1 + V2) / 2. Differential mode is the difference, V1 - V2.Error arises due to how the transformer is wound. Ideally, it would be geometrically symmetrical, but the primary is itself unbalanced, so this isn't possible. The next best construction would be a flat (cylindrical, not toroidal) winding, primary in a single layer, then a metal shield (a cylinder, tied to ground, slitted so as not to form a shorted turn), then the secondary (in one single layer, CT in the middle). Transformers of this construction have been used for bridge instruments.For a construction like I suggested (three wires twisted, wound together), you have the transmission line mode coupling the primary directly into the same ends of the secondaries. But one secondary is turned completely around. So what you will actually get is, above some crossover frequency, the negative secondary side will droop down, phase shift, and flip around to ~0° phase. Then the transformer output will be ~100% common mode! Down below this resonant frequency, in the passband, differential mode will be dominant, but not perfect, and because we know a resonance is two poles, it will have a -40dB/dec asymptotic response. That is, if it's ~0dB balance (e.g., one side of secondary zero, other side nominal) at Fmax, then at Fmax/10 it will be closer to 40dB balance.Whereas this kind of transformer design kind of grudgingly accepts transmission lines despite their problems, there is a better way.The ideal construction method harnesses transmission lines in the best way possible. An ideal transmission line has no common mode current: it has two ports (input and output, say; but mind they're both bidirectional, so implying an input and output is kind of disingenuous), only the differential voltage of which matters. The relative voltage between the two ports can be whatever, it doesn't matter. That's an ideal port.A real transmission line doesn't have ideal ports, but we can make one look arbitrarily good by wrapping it in ferrite and other core materials.So, suppose we take the input signal, delay it by some time T (a transmission line is an ideal delay element), and hook up the far port backwards. Now its signal comes out inverted, with respect to ground. An inverting transformer! If we look at the voltage from input to inverted-output, we see it's pretty good (as a centertapped transformer), but one pin is delayed and the other is not. So, let's add another transmission line, that doesn't do anything (it's wired positive-to-positive on both ends), just adds delay. (This line doesn't need any ferrite cores on it; the other one does.) Great, now we get a perfect (delay matched) positive-and-negative (centertapped winding) version of the input!Note that, because the transmission lines are straight through, there is only delay added -- this type of transformer does not depend on frequency. It has infinite bandwidth, no Fmax!In practice, of course, Fmax is limited by the properties of the transmission line. It might be too leaky, or the transmission line itself breaks down into higher (waveguide) modes. (It's rather hard to perfectly invert the geometry, along an inverting transmission line winding, from one end to the other. Usually, twisted pair is used, or two pieces of coax with a crossover joint in the middle. These will inevitably radiate some imbalance, particularly at very high frequencies.)That's the first step, a Guanella TLT. But it's not so much a transformer as an autoformer. We don't have isolation.To obtain isolation, follow it with a 1:1 isolation transformer. An isolation transformer cannot have unlimited bandwidth, sadly, but we can do better with a single (two winding, 4 terminal; or 6 terminal if CT:CT) transformer, than we can with doing everything at once*.This gives good balance, isolation and CMRR. Note that it requires two cores, so each transformer needs to be designed for double the inductance, to get the same Fmin (LF cutoff frequency).At LF to HF, it's probably not a big deal where the isolation transformer is placed, on the primary or secondary side. Primary might be better, because of convenience and bandwidth (easier to get a 4 terminal transformer to behave nicely?). A VHF+ IF output might want it the other way, though (less stuff floating above ground!).*At this point, we're really not much, if any, better off, compared to the shielded transformer case. That may get better with other tweaks, though.What tweaks would those be?We can further improve balance and CMRR by adding chokes. Again, at some expense to loss, and Fmin or balance. An isolation transformer has isolation capacitance; this can be improved by introducing a common mode choke. A CMC is just a length of ordinary transmission line -- a pair (or triple, or whatever) of wires wound up on ferrite as needed. Downside, it's more (electrical) distance from the CT winding, which means balance can be worse (not from the primary side -- but because unequal loading will result in unequal voltages).Putting the CMC on the single ended (primary) side would seem to be the best option. Again, if it's a priority to have fewer components floating at RF/LO/IF frequencies, it would be better placed on the secondary, and then you have to deal with balance.Finally, balance can be improved (at expense of Fmin) by adding a CT choke. Just another TL, this time with a twist back to itself so it's a 1:1 inverting autoformer. With no other windings on the core, these can be quite compact, giving good balance. Again, downside is more stuff loading down the signal -- with fully three cores loading the signal now, you need triple the inductance for each, to get the same Fmin.You might not use all these options, but some combination is often very beneficial when shooting for a particular impedance ratio, isolation requirement and so on. Here are two examples:https://www.seventransistorlabs.com/Images/Wideband_Amp_4W_50MHz_Sch.pnghttps://www.seventransistorlabs.com/Images/Wideband_Amp_4W_50MHz.jpgL1 is on the phenolic former, just a plain transmission line to match delay. L4 is an inverting transformer, wound on the blue and gray cores. Thus, L1 and L4 form a 1:1+1 Guanella TLT.L3 is the central pot core, wound with red-white-green wires (enough strands wired in parallel to give the specified characteristic impedance). Note that opposite ends of L3 are bypassed to ground, and supplied with +20V: this is a 1:1 inverting Guanella TLT, which enforces balance on the two output phases.L2 is the sideways pot core, wound with, uh, whatever other colors, six strands total (connected in parallel and series as shown, to give the characteristic impedance shown). This is a 1:2 transformer, connected in such a way that one phase's voltage gets doubled and the other's goes to ground (the input terminals are +V/2 and -V/2 at 12.5 ohms, the output is +V and 0 at 50 ohms).This was rather overkill for what bandwidth the amplifier ended up with (about 50MHz); I made these transformers for it, more as an exercise.Tim

author: AlexResch
author_rank: Regular Contributor
timestamp: August 27, 2018, 03:02:03 pm
content: Tim, really don't know how to thank you for taking the time - great answer and right what I was looking for. Now I get a glimpse what the problems are and I will work trough them slowly. I'm shopping for parts now to implement the initial design and once it works I will ask you for help to keep improving it. Many thanks again for taking the time and sharing your hard earned expertise with a nobody.

author: AlexResch
author_rank: Regular Contributor
timestamp: August 31, 2018, 04:32:43 pm
content: Quote from: HB9EVI on August 27, 2018, 12:11:46 pmCould you please explain what "proper termination on all of the ports" means ? I plan to connect two sig gens directly to the transformers and an SA to the "output". Must I have anything between the sig gens/SA and the transformers ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 31, 2018, 05:26:13 pm
content: The sig gens should be 50 ohms, which means any power reflected from the mixer will be terminated into 50 ohms on those ports. That's all that matters.In a real radio application, you'll most likely have a filter facing several ports, which, for ordinary filters, will be an open or short circuit in the stop band. It can be worthwhile to add a few dB pad (attenuator), both to better terminate the filter (the mixer's input impedance at the driven frequency isn't necessarily well-defined), and to help absorb reflection and feed-through. Or if loss is undesirable, a buffer or preamp can be used.The best case is a diplexing filter, with the extra bands being terminated into resistance: this way, the mixer sees a constant resistance at all frequencies, just more or less of each depending on frequency: one or another termination resistor, or the intended source/load. A full diplexing filter is rather complex and overkill; there are simplifications possible to obtain constant resistance with fewer components. (I'm not sure how well it works out with a bandpass filter, though.)Tim

author: AlexResch
author_rank: Regular Contributor
timestamp: August 31, 2018, 05:43:04 pm
content: Thank you, Tim !Once I have it working in a crude form I'll pester you about the filters and the att pads.