Taming a switching supply?

I've been comparing an external 5V USB battery versus assorted wall warts, to get a sense of the extra noise introduced by assorted 5V switching supplies. There are definite differences, mainly at the low end below1 MHz. I wonder if something as basic as a large capacitor (such as a 1F super capacitor) placed across the 5V supply outlet would suppress the extra RF noise. Or is it still necessary to use a(n) inductor(s) in combination with a capacitor to truly "tame" a switching supply? And yes, I realize that I shouldn't be using a switching supply to power a KiwiSDR.

Comments

  • There is normally a fair amount of mixed mode noise around switchers so a lot depends on the supply and how it is getting in to the radio. I won't pretend to be any kind of expert but in my tests it's important to stop radiated noise and doing that is easier before you tame generated noise from the supply.

    Get some large clamp on ferrite beads (multi turn on the lead) and use Kiwi channels* to do simple comparative measurements, have a couple of RX channels on the go, one real slow and others zoomed in to problem area(s), add ferrite to the lead from the supply and to the kiwi, work on the principle that both are antennas and you are looking to make them really bad antennas more like simple resistors turning the unwanted energy into heat

    Once you have made the leads more immune to pickup by suppression and separation/orientation, look at adding common mode filtering to the input and output. Many cheap PSU's skip fitting the input filter componenets so feed RF back to the mains (which can be a decent antenna). Dealing with the noise conducted through the DC lead is only a part of the issue and perhaps the simpler area to work on, skip the rest and gains may be hard to find as it's rarely just a bit of riple on the DC getting past the Kiwi filtering.


    *One slow channel will show changes that are marginal but can be detected over time (make notes of change against time), more zoomed in channels can show more comb detail lost in the wider sampling.


    If Martin G8JNJ drops by he can give a more learned response.

  • edited April 12

    Sorry to say all this yet again...

    As one who successfully uses SMPS on multiple Kiwi's and has systems with noise floor below propagated noise levels at least through mid-HF, I would suggest that ferrites are almost never the solution. The problem they attempt to solve is worse than the ability of any inductor of any kind to solve. The impedances involved are often several hundred ohms and against the requirement for perhaps 30 dB reduction, no inductor can provide the extremely high Z over the several decade bandwidth. Even tuned systems can't reliably do this is in a narrowband case.

    For a Kiwisdr, at least, 'ground isn't ground' and coaxial cable only shields TEM and differential signals. Common mode signals and noise is injected even when every conductor of a cable is at equipotential with respect to the others. These that have zero differential noise can support current that then passes through the so-called "ground" of a kiwi and develops IZ (it's not just IR) drop that appears between the preamp input and the reference to the ADC and appears as unwanted response.

    The kiwi can detect signals that are only -157 dBm in 1 Hz bw. The amount of current required to achieve this with the Z of these common paths through the Kiwi is so small that many orders of magnitude of reduction can be required in typical situations in order to push it down far enough to not be detectable.

    By injecting common mode noise one can easily verify that only 80-100 dB of isolation is present where maybe 30-60 dB more than this is necessary to avoid impairing system performance. MIcroamperes of unwanted signal and noise flowing through the Kiwi common, the so-called 'ground' is enough to destroy potential performance as can be seen by examining the bulk of publicly available Kiwis. Most systems do not do this well. The few good ones are markedly different.

    Rather than fussing with inductors, C's and L's at the PS I suggest first being sure that the balance/isolation against the common mode path out the SMA connector be addressed. Generally a 1:1 transformer like a MiniCIrcuits T1-1 located right at the SMA is a good starting place. These transformers have no more than about 1 pFof inter-winding capacitance - the path which CM noise must traverse - and may be able to provide 30 dB of additional rejection at 30 MHz, more at lower frequencies. Paths other than those involving the antenna SMA are possible but this is usually the worst.

    As I've written on this forum way too many times before, the biggest issue is common mode paths. The Kiwisdr doesn't have a significant differential noise ingress problem. Capacitors and series L's don't solve the fundamental problem. Please see my Notes at wsprdaemon.org.

  • edited April 12

    I approached the problem more "pragmatically" than "scientifically". 😉 My Kiwi is solar powered, so 12..14 V are supplied (which I also use directly for my homebrew antenna switch). For the Kiwi I use a small 3A step-down converter, which I built completely shielded into a tinplate case, with all lines to the outside going through ferrite chokes and feedthrough capacitors. (It is even placed right next to the kiwi.) Subjectively, I am very satisfied with the result, and Kiwi's own SNR calculation often reports values of more than 40 dB, so it can't be all that bad. However, a comparison test with a linear power supply is still pending.

    I wonder if something as basic as a large capacitor (such as a 1F super capacitor) placed across the 5V supply outlet would suppress the extra RF noise.

    This could be problematic as it will slow down voltage rise while powering on. Digital circuits, especially processors, often don't like that and won't properly start then, so it could lead to strange misbehavior. However, don't know how critical the Kiwi is in that regard.

  • You can generally 'tame' switched mode supplies on frequencies > 1MHz, by the use of suitable screening, capacitors, inductors and ferrites.

    However as Glenn has already indicated at length, the main problem is common mode noise, and the 'ground' path through the KiWi which is difficult to fix, especially on the lower frequencies, due to the size of the componnets required to be effective.

    All sorts of common mode paths can exist, and some experimentation is required to minimise the effects. Although it's usual to find that as soon as you fix a problem on one set of frequencies a new set pop up in another part of the spectrum.

    It's a never ending task, like playing 'whack a mole' or peeling an onion, to reveal yet another layer.

    In my experience, most switched mode supplies benefit from a 0.1 (SMD) and a 1000uF conencted directly across the DC output at the PCB, then several turns (8 to 12) on a FT240-31 ferrite ring fitted to the Mains and DC cables.

    Building a set of E and H field 'sniffer' probes to use in conjunction with your KiWi, other SDR or Spectrum analyser, is the quickest and best way to locate, identify and tame noise sources.


    Images from https://www.emcstandards.co.uk/updates



    Regards,

    Martin

  • I really appreciate the detailed and thoughtful replies to my original query. Setting up a second Kiwi at a new location (EM83 grid) prompted me to explore this question around switching power supplies. The one I am currently using only causes "mischief" below about 0.6 MHz, with a series of noise blobs that raise the floor 10-15 dB - enough to take away the fun of trolling for the weaker NDBs. And given that NDBs are on their way out, this seemed like a good time to put a few in the log. It definitely seems like the common mode pathway is the issue, since I can re-introduce the same noise blobs simply by connecting the SMPS negative connector to the (battery powered) Kiwi ground. Overall not a big deal, but will be fun to work on mitigating it via the assorted suggestions here.

  • This could be problematic as it will slow down voltage rise while powering on. Digital circuits, especially processors, often don't like that and won't properly start then, so it could lead to strange misbehavior. However, don't know how critical the Kiwi is in that regard.

    Absolutely correct. The power management IC (PMIC) on the Beagle enforces a 50 msec rise time on the 5V input. It simply won't power on if it is slower than that.

  • The inexpensive ~960 kHz buck converter mentioned elsewhere on the forum will likely get you low end performance uncompromised. I think they cost about US$1 each...

  • @n6gn can you give a little more detail on your Minicircuits T1-1 setup? Is it mounted to a small circuit board with SMA connectors? Or just free hanging in a small project box? The center tap is just left disconnected?

  • edited May 15

    Just 1:1 on small PCB, SMA male to Kiwi Completely isolated from SMA female to antenna feed line. This leaves only about 1 pF of common mode coupling between Kiwi and antenna line.


    top side of board shown above, Center tap, if present T1-1T, is unused.

    T1-1 from Mini-Circuits,

    SMA edge connectors from eBay,

    unzip & drop .brd file onto OSHPark.com. 3 boards postpaid in US in about 10 days, probably < US$5.


  • edited May 14

    The minicircuit isolation transformers are also available in a metal can with BNC connectors, sometimes there are cheap used ones available on ebay.

    FTB-1-1 with the T1-1 transformer (0.2-500 MHz at the -3 dB insertion loss points)

    I use FTB-1-6 with T1-6 transformer, they go a bit lower in frequency (0.05-150 MHz at -3 dB) at the cost of slightly higher coupling capacity. I measure 2 pF (with the metal can).

  • As Glenn has mentioned "Generally a 1:1 transformer like a Mini-CIrcuits T1-1 located right at the SMA is a good starting place. These transformers have no more than about 1pF of inter-winding capacitance - the path which CM noise must traverse - and may be able to provide 30 dB of additional rejection at 30 MHz, more at lower frequencies"

    It is possible to improve the common mode rejection of the transformer on the HF bands by also including a ferrite common mode choke on the transformer connecting cable.

    The hybrid combination of a flux coupled transformer and transmission line choke can work well, if the high frequency transmission line choke is designed to complement the lower frequency common mode rejection of the transformer.

    In practice I have found that a further 20dB of common mode rejection (S21 measured in a 50 Ohm system) can be achieved at 30MHz.

    The eight KiWi sdr's at Weston, use a dipole and special balun I built, consisting of a 12:1 flux coupled transformer and transmission line choke, the combination of which works very well as a broadband receive antenna. There is a diagram of the receive system on the KiWi header.

    http://websdr.uk:8078/?wifi=0

    Coil-Craft also have a number of miniature wide band transformers, that may be easier to obtain than Mini-Circuits parts in some areas of the world.

    https://www.coilcraft.com/en-us/products/transformers/wideband-rf-transformers/smt/wb_smt/

    I have used both of the following 1:1 types with good success, but other ratios are available.

    WB1-6TSL_

    WBT1-6SL_

    Regards,

    Martin

  • see edited file previously posted for .brd file to have fab'd.

  • Thanks @n6gn for the OshPark files, I have ordered some. And thanks @HB9TMC and @G8JNJ for your insight.

    Does anybody know if the Minicircuits ADT1-1+ will also work? Looking at the specs, it seems to be comparable, but I'm not sure. 1 pF inter-winding capacitance is not on the spec sheet, is more capacitance better?

    The reason I'm asking is that shipping costs from Minicircuits for the T1-1-KK81+ is more than 5x the cost of the transformer. Mouser has the ADT1-1+, 1.6mm end SMA connectors, and also the BBAI in stock, so maybe I'll do a bigger upgrade 😀

    I realize that the ADT1-1 is much narrower than the pads on Glenn's board, but I'm confident I can wire mod it up. Pin spacing is still the same at 0.100" as far as I can tell.

  • I don't know about the ADT1-1+ but I do know that the T1-6T+, spec'd to go lower in frequency is NOT a good idea. Rather than being a simple flux coupled transformer with low inter-turn capacitance it appears to be a transmission line type, even though they don't say.

    Without verifying low capacitance I would stay away from alternatives, though there may be some.

    It's handy to order the CT versions since these can be useful for CAT5 to 50 ohm use as well.

    Mini-Circuits will sample a small number of parts if you have a commercial/academic connection. Otherwise order 10-20 of them, pay the shipping and share them with your friends😀

  • I have some T1-1s I can share

  • I've just measured some Coilcraft transformers on the bench. All the primaries and then all the secondaries were connected together with short conductors, and the capacitance measured between the primaries and secondaries.

    WB1-6TSL_ 1:1 with no centre tap measures 3pF

    WBT1-6SL_ 1:1 with centre tap measures 7pF

    I couldn't find any Mini-Circuits equivalents to compare these values with.

    Note that the PCB and connectors will increase the primary to secondary capacitance.

    Regards,

    Martin

  • The Coilcraft SWB1010-1-PCL 1:1 40KHz to 175MHz with no centre tap may be promising.

    I just measured a SWB3010-1-PCL with centre taps and that came to a value of 6pF.

    There is also the SWB1010-PCL which has a very impressive bandwidth of 5KHz to 100MHz, but that is likely to have a higher value of inter-winding capacitance.

    If you are not too worried about frequencies below 400KHz then the WBC1-1L_ surface mount 1:1 transformer may be suitable.

    Regards,

    Martin

  • How can I measure such a low capacitance of the primary / secondary transformer coupling?

    Mouser offers a bunch of Minicircuits transformers. Which one is recommended? This one seems to be priced reasonably:

    There is no mention of the coupling capacitance in the datasheet.


    I wonder how would the Minicircuits transformer compare with a hoembrew bifilar transformer on a tiny binocular Amidon core made of 43 material. For example, the softrock ensamble RX receiver has one at the input, with 4 windings bifilar. There is a jumper to connect / disconnects the receiver ground to the antenna ground.

    I also wonder whether a 1:1 transformer is optimal for a wideband receiving antenna such as a dipole or a long wire. For example Nooelec sells a 1:9 transformer, while Minicircuits offers transformers for a range of impedance ratios.

    73, Vojtech OK1IAK

  • edited May 22

    You can measure 1 pf with a nanoVNA or similar VNA in 2-port mode. Connect the grounds at the end of the test cables and hook the primary/secondary as a two port device on the centers. This is just like measuring a small capacitor. If you see something on the order of -25 dB at 10m you are probably in good shape. Recognize that the common mode soure/load impedance you are working against may e a good bit higher than 50 ohms, at least at some frequencies but since most of the noise one typically sees is at lower frequencies a little decoupling goes a long way.


    Yes, you can use a small core but I'd suggest not using bifilar windings, rather, just make separate primary/secondary windings in a simple flux coupled arrangement with low inter-winding C. This should have good performance if the core material is fairly high permeability. Even 300:75 ohm FM/TV cores, often binocular, can work fine.

    I don't think you need do any matching at this transformer, rather move that out nearer the antenna, though if you want to use CAT 5 for (balanced) feedline you could perhaps do 3:2 turns ratio and have success. Don't get too carried away in trying to perform a lot of broadband matching. After all, a pretty bad mismatch, like 3:1 SWR ,only results in about 1 dB of mismatch loss which you will be very hard put to measure much less hear.

  • As Glenn has commented, don't try to bother with impedance transformations at the KiWi, do it else where.

    If you want to build a simple passive broadband antenna, try something like this experimental design I originally posted elsewhere.

    WRT the dipole, this is terminated with a 600 Ohm impedance. By doing this the variation in signal levels should be minimised across the 1-30MHz frequency range.

    Assuming:-

    • A rural location (as defined by the ITU noise curves).
    • The dipole termination impedance is 600 Ohms.
    • The centre point is about 15ft (5m) above ground and is configured as an inverted V with the ends at about 3ft (1m) above ground.

    If you are only interested in reception from 1 to 30MHz, then anything longer than approximately 30ft (10m) per leg should be fine.

    In order to get coverage down to the 300 - 500 KHz NDB band you would probably need something like 75ft per leg (25m).

    If you are in a particularly quiet rural location (as defined by the ITU).

    Then you may need to use a minimum length of around 100ft (30m) per leg to obtain sufficient sensitivity at 1MHz, or >120ft (40m) per leg to cover the NDB band.

    As you increase the sensitivity at the low frequencies, you are more likely to have problems with strong LW & MW broadcast stations overloading the receiver.

    This will vary between day and night, due to changes in propagation.

    As a result you may have to compromise on sensitivity, and reduce the length of the antenna legs until the signal levels become acceptable.

    In most locations I think a length of somewhere around 30 to 45ft (10 - 15m) per leg would be a good starting point, maybe more if you can manage it.

    The choke transformers are simply 5 turns of the twisted CAT 5 passed through the BN73-202 core with both wires providing electrical continuity between the input and output sides.

    This is to help improve common mode isolation between the input and output of the transformers and chokes when connected together as shown.

    The use of a balanced CAT5 twisted pair should also help minimise unwanted common mode noise and interference on the feedline.

    We are using something very similar to this antenna to feed our 8 KiWi's at Weston in the UK.

    http://websdr.uk:8060

    Regards,

    Martin

  • | you can use a small core but I'd suggest not using bifilar windings, rather, just make separate primary/secondary windings in a simple flux coupled arrangement with low inter-winding C.

    The Mini-circuits transformer is wound on a binacular core. I understand the concept of what you wrote, however I don't know how to wind such a transformer on a binocular core. Would you please suggest how to wind such a transformer?

    > Even 300:75 ohm FM/TV cores, often binocular, can work fine.

    Binocular FM/TV cores dont have sufficient permeability for low HAM bands.

    | I don't think you need do any matching at this transformer, rather move that out nearer the antenna,

    I saw LZ1AQ placed a balun into the loop amplifier, while I though one would want to get rid of common mode currents close to the radio. Why is the balun beneficial close to the antenna and not to the radio?

  • edited May 22

    Actually eBay FM/TV cores with only 2 or 3 turns on each winding DO have enough permeability to get down to the low end of HF, though as you say not MF and LF as well. Asking any inductor for three to four decades of BW is a very tall order since they are trapped by permeability (even of free space) on one end and self resonance on the other at typical impedances.

    Here are two modified with a couple of turns on each winding (one turn is counted as through both holes and back out the same end) and a BNC for the previous F connector:

    Not sure you can see it but if you use fine wire both windings can fit, exiting opposite ends.

    Two of them measured back-back show -3 dB each at about 3 MHz and up past 6m.

    Balance is beneficial everywhere: the antenna wants to be balanced as does the feedline, ideally. With total symmetry, shorting the (balanced) antenna gets rid of the desired differential signal and leaves the undesirable CM to verify that it is at least several dB lower.

  • Hi Glen and Martin.

    Interesting info indeed.

    I am mostly interested in HF bands. I live at the edge of Prague with a lot of radio noise. I don't have space for a long dipole, however I received LZ1AQ loop amplifier, which I plan to set up as far as possible from all the houses around. I wonder whether anybody uses the LZ1AQ loop amplifier with the Kiwi. For the noisy location as mine, being able to switch between the crossed loops may be beneficial.

    As of now, I will just connect my long wave using a homebrew balun with one winding wound over the other. I wonder whether the Mini-circuits TC1-1 is wound any smarter than that. The photographs of Mini-circuits products are informative only and often of different products. Would you guys please make close-up pictures of the TC1-1 for us?

    Regarding Martin's dipole setup, I have a handful of questions:

    Do you use just one pair of an UTP cable? Do you leave the rest of the pairs unused? Do you ground them? Would it be better to connect let's say two pairs in parallel to achieve lower ohmic losses?

    With the virtual ground between the centers of the transformers, aren't you actually worsening the common mode suppression? I suppose if everything is perfectly symmetric, then the virtual ground links would have no effect, but if the system is not perfectly ballanced?

    I suppose the 68pF capacitor at the radio end is to improve the transformer at higher frequencies?

    I suppose the 600 Ohm to 100 Ohm transformer at the dipole end is designed mainly for the low bands where the dipole is very short and mostly capacitive?

    73, Vojtech OK1IAK

  • Regarding Martin's dipole setup, I have a handful of questions:

    Do you use just one pair of an UTP cable? Do you leave the rest of the pairs unused? Do you ground them? Would it be better to connect let's say two pairs in parallel to achieve lower ohmic losses?

    I have used just one pair. They have a characteristic impedance of around 100 Ohms. You could use some other twisted pair or parallel the pairs, but you would need to change the transformer ratios to suit.

    With the virtual ground between the centers of the transformers, aren't you actually worsening the common mode suppression? I suppose if everything is perfectly symmetric, then the virtual ground links would have no effect, but if the system is not perfectly balanced?

    The virtual ground was intended as an experiment, which is why it is shown in red. It was a request by someone who wished to provide and electrostatic discharge path. Personally, I'd avoid it but include a resistive static bleed and maybe some 90v or 75v gas discharge tubes. This is what I have done at Weston.

    I suppose the 68pF capacitor at the radio end is to improve the transformer at higher frequencies?

    Yes it's to improve the match on the higher frequencies. It's not essential, I just like to see flat impedance curves if at all possible.

    I suppose the 600 Ohm to 100 Ohm transformer at the dipole end is designed mainly for the low bands where the dipole is very short and mostly capacitive?

    It's to ride out the excursions between high and low impedance resonances. By using a 600 Ohm termination impedance the overall mismatch losses are reduced, and you get a much flatter amplitude characteristic.

    Generally speaking the receive performance on the LF bands will be constrained by the natural noise floor. So even a relatively short antenna can still produce good results on these bands. However on the HF bands minimising losses and having a bit more gain is beneficial, as the natural noise floor is typically 20dB lower at 30MHz in comparison to 1MHz.

    Here's an example based on the antenna in use at Weston. Note that the scales of the two graphs are different. The mismatch loss is shown in bold figures on the left.


    Incidentally this type of design can also work well with an Unbalanced vertical antenna. You just need to change the transformer ratios. The best known version of this type of antenna is probably the Lankford Low Noise Vertical.

    https://tinyurl.com/DallasLNV

    Incidentally if you want to compare various antennas in an urban location try my home KiWi.

    http://g8jnj.zapto.org:8073/?ext=ant_switch

    Antennas 5 & 6 are Wellbrook clones and Antenna 7 is an LZ1AQ.

    The varitaion in noise floor is mainly due to the antenna orientation relative to local noise sources.

    If you have any specific questions send me a PM so that we don't clog up the list.

    Regards,

    Martin

  • Thanks everyone for the info here. I built up several of these isolation boards, Minicircuits was out of the KK81 package so I used the regular T1-1+ and bent the leads down. It seems to help with the noise a bit. My antenna is just a fan/parallel dipole array on the roof, for 40/20/10 meters.

    I'm about to add a Bodnar Mini GPSDO to my Kiwi to help with these new FST4W modes. Should I add an isolation transformer between the Bodnar output and J5 U.FL pads?

    Looking at the schematics, the J5 U.FL ground is the same as the "HF" J2 input. The Bodnar will be powered from the wsprdaemon computer, which is probably a pretty noisy supply.

  • edited September 20

    My method: Add a MIX31 or MIX77 toroid to the wire goes to the Kiwi from the PSU with many turns as you can, and ground the chasis of the PSU.

    I bought LED power supply, similar like this: https://www.amazon.com/Switching-Universal-Regulated-Transformer-Electronic/dp/B0781GBHL3/

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