Balun and signals not 180 degrees out of phase - Page 1
Forum section: electronics / rf-microwave

author: arildj78
author_rank: Regular Contributor
timestamp: February 25, 2020, 12:10:31 pm
content: Anyone with experience with baluns? I've tried the calculator on this pagehttps://leleivre.com/rf_LCBalun.htmlwith the following input: 0.457MHz - Zunbal:50ohm, Zbalanced:28ohm. When simulating, the output (Va and Vb) is not 180 degrees out of phase as I expected them to be. What is going on? Are they not supposed to be out of phase or is this a suboptimal design?The circuit is to be used on the input of an LT5560 and is shown in the datasheet Figure 19, page 23.https://www.analog.com/media/en/technical-documentation/data-sheets/5560f.pdf

author: richard.cs
author_rank: Super Contributor
timestamp: February 25, 2020, 12:40:37 pm
content: Having just played a little in SPICE myself you get the expected 180 degrees only with the balanced output high impedance.Doing an AC sweep from a decade below the nominal frequency to a decade above is very revealing and shows that on-load each arm is giving you a phase shift of about 55 degrees at the design frequency. Possibly it's just a bad approach, possibly you could improve it by designing with two 45 degree sections in each arm.

author: richard.cs
author_rank: Super Contributor
timestamp: February 25, 2020, 12:56:55 pm
content: This balun design requires coupled inductors? That's not clear from his reference. Checking it quickly in SPICE:uncoupled 90 degrees totalcoupled 55 degreescoupled (other phase) 110 degreesBoth coupled approaches have a flatter amplitude response than the uncoupled one.(I just discovered I can do negative K values in SPICE to parameterise transformer phase - cool)It's still a long way from 180

author: chris_leyson
author_rank: Super Contributor
timestamp: February 25, 2020, 01:13:01 pm
content: Balun does not need coupled inductors, try 1.3uH and 93nF instead of 13uH and 9.3nF

author: chris_leyson
author_rank: Super Contributor
timestamp: February 25, 2020, 01:45:18 pm
content: Found this pagehttps://analog.intgckts.com/impedance-matching/lumped-lc-balun/and it gives 13uH and 9.3nF. Something doesn't add up.

author: arildj78
author_rank: Regular Contributor
timestamp: February 25, 2020, 01:59:07 pm
content: Quote from: chris_leyson on February 25, 2020, 01:45:18 pmI know! Both your link and the one in the original post have the same design as the one I found in the datasheet for the mixer. The values are the same all over the board but the simulation doesn't seem to give a balanced output... What detail am I missing?

author: rfeecs
author_rank: Frequent Contributor
timestamp: February 25, 2020, 10:40:07 pm
content: It appears that this circuit is not really a balun, but more like a single ended to differential splitter. That is, it can input one single ended signal, and output two single ended signals that are 180 degrees out of phase. This is probably what you want to drive your differential input mixer.In that case, if you ground the midpoint of the load resistor, the circuit does what you want.

author: arildj78
author_rank: Regular Contributor
timestamp: February 26, 2020, 07:33:06 am
content: By grounding I get the same result as @rfeecs but I'm still confused.Since this circuit is suggested in the datasheet for LT5560, is it fair to assume that such a ground reference is present on the input of the mixer?Another thing. My signal source is a loopstick antenna fed through an opamp. For the calculations I need to input an source impedance. What factors should I consider when choosing the resistor in series with the opamp output? No series resistor, 28Ω, 50Ω, 600Ω?Does anyone know of a YouTube content creator that might be interested in covering this topic? Maybe @w2aew?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 26, 2020, 09:30:36 am
content: So this is super low frequencies? Why bother with a balun, why not use another opamp? Why the expensive LT5560?Do you have the parameters of your loopstick and tuning network?Tim

author: arildj78
author_rank: Regular Contributor
timestamp: February 26, 2020, 10:04:54 am
content: Reason 1: I'm trying to figure out how and when to do balanced/unbalanced conversion as well as impedance matching.Reason 2: I'm trying to downconvert 457kHz to 30kHz.My loopstick is resonant at 457kHz with an inductance of 70µH. It is then amplified by a low noise instrumentation amplifier (AD8421).I am aware that 457kHz is so low that I don't have to be concerned about transmission lines, but when searching for a mixer on digikey, this is one I found, and all calculations in the datasheet assumes higher frequencies than what I'm concerned with. The header states 0.01MHz to 4GHz so the part should be able to do the trick.My main problem is that I'm finding quite a bit of info about RF online, quite a bit of DC as well. But in this segment, where I'm doing work in low RF and mixing down to audio, I'm not that lucky.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 26, 2020, 12:55:18 pm
content: How about SA602 or 612?What ESR does the loopstick have? What is your desired bandwidth?So here are some tips on network theory:- Q Factor, is a matter of definition. As usually given, it's the Q factor of a component: the ratio of its reactance to ESR, or EPR to reactance. Well, when we put that component into a circuit, it doesn't stop having ESR or EPR (whichever one we're preferring to use), so it still has the same component Q. But there are other resistances connected to it, and so we can also ask what its Q factor is in the system.So in general, we're concerned about two kinds of Q. System Q can never be higher than component Q, but it can be much lower (and we generally prefer this, to keep efficiency up, or insertion loss down).- Impedances all lie in the complex plane. Disregard the name, there is nothing imaginary or complicated about them; it's the same number system you've always used, expanded to include direction as well as magnitude (length). You only need high school math to evaluate things here, but some higher level insights will greatly facilitate things.In general, we write an impedance as its series equivalent R + jX (j == sqrt(-1)). Addition and subtraction are trivial. Multiplication is carried out in the usual way, distributing terms, collecting and simplifying (j^2 = -1, so all terms can always be collected into real (no j) and imaginary (one j) parts). Division is done in the usual way as well, though to avoid complex denominators, we can multiply top and bottom by the complex conjugate (conjugation just means negate the sign of the j terms) and simplify. Something of a PITA to write everything out in this way, but it serves a purpose.We also find it useful to express impedances in polar format Z ∠ φ = Z cos φ + j Z sin φ. This is just trig, plotting R and X on a 2D plane. Geometric intuition proves useful: addition is the same as stacking vectors in series (simply adding up the x and y components), and multiplication multiplies the lengths while adding the angles.So we can find it helpful to work in rectangular or polar coordinates, depending on what we're doing.In circuit, addition is simply connecting things in series. Connecting things in parallel is a bit of all three (addition, multiplication and division), with the division being the most important part I think I'd say.- We also find ratios supremely useful. Note that sqrt(L/C) has units of ohms, and if we divide this by the system impedance Zo (or vice versa), we get the (system) Q of that LC tank in the system. sqrt(L/C) might be called the characteristic or resonant impedance of the tank.Likewise, the characteristic frequency, ω = 1/sqrt(LC), has units of (radial) frequency. (Anywhere we have ω, just replace it with ω = 2 pi F when you want cycles frequency.)We can keep our equations clean, abstracted from component values, in this way. We can always fill in RLC values later.- Filters are generally made by stacking LC tanks together, where each LC has the same resonant frequency, and there is some coupling factor between each stage. The coupling factors define the filter profile, for which there are named prototypes for specific purposes (Butterworth, Bessel and Chebyshev most prominently, but there are also filters approximating the Gaussian function or its relatives, or filters optimizing impulse, phase or step responses.)When you see the traditional ladder network circuit (for a lowpass: series L, parallel C, ...), you're seeing a particular realization of this. In fact, when the coupling factors are small, the impedance ratios to Zo (the system Q) are very large and small, respectively, and this can give ponderous values compared to impedances available from real components.The point isn't actually all that clear (or even relevant!) with lowpass filters, but it's a concern for wideband highpass filters, and very apparent with band-pass/stop filters. The bandpass prototype uses parallel parallel-resonant tanks (L || C) and series series-resonant tanks (L+C), and the impedances of these tanks differs from Zo by ratios near system Q.Matching:- Bandwidth, Q or any roughly equivalent parameters constrain the kind of matching possible.At high Qs, you can use resonant tanks to filter and convert impedance at the same time. Consider for example connecting a resistance in series with your loopstick, versus in parallel: the former will be relatively small to affect the tank (indeed, comparable to ESR). The latter will be significantly higher however. (How much, may not be obvious at a glance, but the proof lies in simply writing out the parallel equivalent impedance and simplifying to get it in series form R + jX.)We can also absorb a transformation ratio in a capacitor or inductor divider, if we have some excess L or C to balance it out.Impedance ratios are roughly limited to Q or less. At low Q, we might get away with multiple stages (actually hm, I don't think that can work), more likely we'll need a tapped inductor or transformer.- Note thatsomeresistance is required, somewhere. It is meaningless to speak of an LC circuit with no resistance whatsoever. There is always a source or load resistance, losses in the components, or anything equivalent to these. Resistance is important because it sets the system impedance, the basis for the entire development of filters, matching and so on.- If you aren't up to solving whole networks en masse, stick to a standard, typically 50 ohms. We have coaxial cable in this impedance. We can match to any other impedance, within some limits (the limits are tighter as frequency goes up, of course), and we can always terminate with a load resistor if we need to, insert buffer amps, etc.Filters can be designed for single or double termination, or anything inbetween. An example where this might be applicable: a DDS chip with constant current (open collector/drain) output. The output has a high Thevenin impedance, capacitive dominant. We can absorb that capacitance into the leading C of a CLC(LC..) filter -- of course, the filter can't be designed to use less than the chip's capacitance, which puts limits on impedance and bandwidth -- saving, heh, well, all of one component (a termination resistor) and about 3dB of SNR. Sometimes that's worth the effort, often it's not.Note that a lowpass filter between Zo and say 10*Zo can't match the pass-through impedance by itself -- it's not a transformer, its DC voltage gain is still 1 -- the filter design just means its response will be correct. It's still up to you to get power transfer, or voltage gain, or whatever it is you're optimizing!- So, for all this work -- you'll most likely want to stick to simple double-terminated filters, buffers, and transformer or 2nd order (L-match) matching networks.I... certainly wouldn't expect you to become proficient in networks overnight. It's been many years before I gathered an intuitive understanding of them myself. Do try some "homework", solving simple circuits. 2, 3, maybe 4 or 5 elements are tractable enough by hand, but expressions quickly get unwieldy from there. Write it out fully, R, L and C. Absorb R, L and C into ratios of ω/ω_0, Zo, Q and so on. Run simulations, say in Octave/MATLAB/NumPy/etc. Investigate interesting points like integral phase shifts (90°, ...), or peaks or valleys or cutoff thresholds (-3dB, -20dB, ...). Get a visual feel for things. Plot the phasors (complex vectors). Plug it into SPICE, start manipulating RLC values and optimizing for a given application. Set up transmission and reflectance instrumentation.Tim

author: arildj78
author_rank: Regular Contributor
timestamp: February 26, 2020, 02:03:57 pm
content: Thanks for a thorough walkthrough of these concepts. It did however go a bit over my head. The math part is not a problem and I'm more than happy playing around with vectors and complex numbers. However, the intuitive understanding of impedance matching and single ended vs differential signals is not there yet. Nor how to treat the output of an opamp with regards to output impedance, and when and why to change the output impedance with a series resistor.As mentioned, pure DC and high freq starts sinking in, but it is the area in between that's giving me grief now.I'll read through your answer several times, and try to come up with relevant homework, but I still think there's material here for a video if someone out there is up for the task.

author: David Hess
author_rank: Super Contributor
timestamp: February 27, 2020, 04:27:21 pm
content: Quote from: rfeecs on February 25, 2020, 10:40:07 pmExactly, one leg advances the phase and one leg retards the phase to produce two single ended outputs which are 180 degrees out of phase; it is a phase splitter which is just fine for this application. A balun could do the same job but with either a single ended or differential input as it removes or attenuates the common mode signal and would be used in a wideband application.

author: arildj78
author_rank: Regular Contributor
timestamp: February 27, 2020, 08:27:20 pm
content: Why do I not see 180° phase shift without grounding the center of the load and can we assume that the input of the mixer does ground the center?

author: rfeecs
author_rank: Frequent Contributor
timestamp: February 27, 2020, 09:46:06 pm
content: Quote from: arildj78 on February 27, 2020, 08:27:20 pmYou have a series L, shunt C on one leg; and a series C, shunt L on the other. For this to give you the 180 degree phase difference, you need a real load on each arm. You might be able to look at a Bode plot for each arm and see this works if the poles and zeros are in the right place.If you just connect the two arms together, then each arm sees a complex load and this adds poles or zeros and screws things up.I think if everything were extremely precise, then at your exact resonant frequency the two outputs would be exactly 180 degrees out of phase and exactly equal magnitude. So there would be no current going through the center ground point. In that case it would actually work without ground at that one frequency. But even with simulation it is hard to get everything set to precisely the right values for this to work.QuoteIt looks safe to assume. Figure 4 of the LT5560 shows a simplified input port schematic. Each port is a common base input, and assuming the Vbias connection to the bases is a low impedance to ground at the RF frequency, then yes.