Noisy  toroids. Saturation? - Page 1
Forum section: electronics / rf-microwave

author: LM21
author_rank: Regular Contributor
timestamp: August 01, 2024, 04:02:27 pm
content: I made a simple amplifier to see the input impedance of a medium power transistor, and to play with NanoVna. PBSS4041NX input resistance seems to be around 14 ohms at 50mA 10V and 3(.5)MHz. Gain seems to be around 20dB.But every time the NanoVna makes its "sweep", the collector coil makes a soft ping. I have had 1-2 turns on two toroids, and both make that sound. How can I calculate the max current of a toroid. I have used two toroids, small yellow and large black. I have to check toroid types too.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 01, 2024, 04:26:56 pm
content: Beats me, but it may be of interest to know that ferrite (particularly when driven near/into saturation) has some magnetostriction, and can resonate at various frequencies. With enough energy, at the right (wrong) frequency, this can even fracture the part -- I've done it before, it was a, I think around 20mm toroid, and around 50kHz I think it was, it all of a sudden snapped into about ten evenly-sized fragments.Ungapped ferrite is a poor choice for several reasons; use a proper inductor here.Tim

author: LM21
author_rank: Regular Contributor
timestamp: August 01, 2024, 05:07:22 pm
content: Yes, Toroids saturate easily. But with toroids it is also easy to get large inductance for low frequencies. Current toroid is about 35mm, and has 44uH with 18turns. I have several types and of course I mixed some of them. So now it is not easy to tell them apart.The NanoSA in generator mode does not make any noises.Edit: does make->does not make

author: LM21
author_rank: Regular Contributor
timestamp: August 01, 2024, 05:29:36 pm
content: and:What did you mean by proper coils. Toroids, mine atleast, have a problem, that they give too much of inductance. To make for instance 50 ohms to look like 5000 ohms, I need 1 to 10 turns ratio. But 10 turns on my toroids give very high inductance. That makes the Q value of the resonance circuit very low, like 1-4, with 2500-5000 ohms.I think I'll try without any capacitor and use the toroid as a transformer. But I need selectivity somewhere.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 01, 2024, 07:35:02 pm
content: Toroids, in general, saturate at whatever. You can even get those that don't saturate at all (mu_r = 1).Ferrite, is a (generally) high-mu material, that saturates easily unless air-gapped. Toroids are sometimes made of ferrite.I have no idea what material you have. You'll have to test or otherwise identify it.Tim

author: LM21
author_rank: Regular Contributor
timestamp: August 01, 2024, 08:04:14 pm
content: Digikey seems to have some suitable toroids, mine have too high AL.

author: 10maurycy10
author_rank: Newbie
timestamp: August 02, 2024, 05:20:00 am
content: If the sweep is going through the audio range (near DC-20 kHz) it could be magnetostriction or coil whine caused by individual loops of wire acting as electromagnets and pushing against the field in the toroid. Both of these effects would be quite small, but it doesn't take much energy at all to make an audible sound.If the sweep is entirely above 20 kHz, it might be something weird like self quenching oscillation.Perhaps you could use the NanoVNA as a signal generator to find the frequency at which it makes noise?

author: Andy Chee
author_rank: Super Contributor
timestamp: August 02, 2024, 06:09:36 am
content: Quote from: T3sl4co1l on August 01, 2024, 04:26:56 pmThat would make for an interesting YouTube video.

author: mtwieg
author_rank: Frequent Contributor
timestamp: August 02, 2024, 11:29:12 am
content: I'm guessing these toroids were on collector and/or emitter of the BJT. The sweep must have been causing a disturbance in the bias current somehow.In my experience, audible noise from magnetic components comes from ferrite cores made of multiple pieces with an extremely small or zero air gap. The separate pieces butting up against each other can greatly increase aucoustic noise. A solid core (i.e. a toroid) usually won't have this issue, unless the core is actually cracked somewhere. You can also get acoustic noise just from the windings due to lorentz forces, but that takes quite a bit of current.Quote from: T3sl4co1l on August 01, 2024, 04:26:56 pm

author: LM21
author_rank: Regular Contributor
timestamp: August 05, 2024, 09:34:07 pm
content: Late answer, but I have made some more tests. I started with VNA range from 20kHz and up.The noise is still there with a VNA range from 1MHz-to 5MHz. I use a large toroid with18 turns. The collector loop is around 4-5 turns, output loop is now 2 turns. The collector current is around 50mA. The transistor is not in pure class A, because current consumption varies a little. The circuit has a broad resonance at about 3MHz.There is some gain, but I have not calibrate that part of the VNA yet, so I can only quess..

author: glenenglish
author_rank: Frequent Contributor
timestamp: August 08, 2024, 09:16:15 pm
content: put a spectrum analyser sniff on it.....it might be oscillating at 400 MHz....Are the DC conditions changing ?Have you done the DC magnetizing bias calcs ?Remove some turns if it is out of bounds.Have you done the AC saturation calculations for the low freq ?

author: Bud
author_rank: Super Contributor
timestamp: August 08, 2024, 10:09:16 pm
content: Agree. The amplifier may become unstable and break into oscillation. Connect a spectrum analyzer to the output to check for oscillation, which actually may be well above the amplifiers design range.

author: LM21
author_rank: Regular Contributor
timestamp: August 12, 2024, 07:34:23 pm
content: My toroid seems to be of type 67. Its diameter is 27-29mm. I found the Digikey bag it came in.https://www.digikey.fi/en/products/detail/fair-rite-products-corp/5967001201/8599643More specs of the toroidDiameter 1.142" (29.00mm)Initial Permeability (µi) 40Effective Length (le) mm 73.0Effective Area (Ae) mm² 68.0Effective Magnetic Volume (Ve) mm³ 5000.0There is a slight momentary rise in current consumption with NanoVNA with some settings.This page says the coil is not saturating. But it only knows about AC.http://pigeonsnest.co.uk/stuff/core-saturation.htmlAll formulas have the problem that units are random. Like in "Ae - Cross section area of toroid (sq. mts)" : What is sq. mts.Spectrum analyser is a good idea. My only 3MHz generator is in TinySA, so I have to a get an other spectrum analyser. I don't have buy one, which makes this easier.A 200MHz scope didn't show anything odd, sinewaves with a little distortionQuestions from you:it might be oscillating at 400 MHz....Are the DC conditions changing ?Have you done the DC magnetizing bias calcs ?Remove some turns if it is out of bounds.Have you done the AC saturation calculations for the low freq ?Some answersNo, I did some calculations now. It looks like the toroid is not saturating

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 12, 2024, 11:23:54 pm
content: Again, ungapped ferrite is a poor choice for RFC. #67 properties:https://fair-rite.com/67-material-data-sheet/In particular, notice the wide hysteresis loop (B-H curve), permeability vs. field strength (peaks +25% at ~10 Oe), and amplitude permeability (AC rather than DC bias).Design equations that assume linear materials, do not apply to a material like this.NiZn materials also generally say to avoid magnetization (saturation in particular), or mechanical shock, which can permanently affect the material properties. Magnetic materials areweird, we use linear design equations when we can, and we try to use materials linear enough for that to work out, but realize that both of these are special and contrived cases that do not apply in general across the vast spectrum of possible magnetic materials. These ferrites in particular, are pushing the boundaries of what is reasonable to calculate.An RFC with µ_r ~ mid 10s is easily made out of powdered iron, which does not suffer such aberrant behavior (amplitude permeability change might be only a few percent), and also has fairly high saturation current (100s, 1000s A*t).As for units, stick to SI, simple as that. Don't care what an Oe is, don't care what "lines of force" are, no geometric factors are needed, or rather they're all 1 in the standard parallel-field calculations we use for these reluctance equivalent circuit calculations. Or if you like: use mm, MHz and µH, same thing (carry the multipliers around to prove it). You can enter units explicitly in many calculators as well:https://www.google.com/search?q=25%5E2+*+magnetic+constant+*+100+mm%5E2+%2F+0.24mm+in+microhenrieshere's a typical example, for an EE33 core I have a bunch of (Ae = 100mm^2, le = 104mm, µ_r = 2600), gapped ~0.2mm, 25 turns.Tim

author: iMo
author_rank: Super Contributor
timestamp: August 13, 2024, 10:09:05 am
content: AL=40 is too low for the 1..5MHz band.You would need a lot of turns to get the inductive reactance of its primary or secondary (XL= 6.28*f*L) at least 5x higher than your load resistances..Ie. for example with 50ohm load your XL should be at least 250ohm..With AL=40 [nH/t^2] that would mean aprox 17 turns in order to get 12uH (250ohm at 3.5MHz)..For such low HF frequencies you would need material 43..PS: BTW. - the more turns the less chance to saturate the ferrite toroid..Anyhow, the saturation of your core with AL=40 is highly unlikely even with 1 turn (in your setup).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 13, 2024, 03:50:17 pm
content: Quote from: iMo on August 13, 2024, 10:09:05 amIt's a choke not a transformer (or, first then second respectively), same DC on more turns saturates faster.Keep in mind how much current the transistor in question is capable of drawing.But perhaps it's time for OP to reveal their schematic, biasing, and VNA settings (signal level) so we aren't just guessing anymore.Also, mu = 40 is pretty much just right for energy storage purposes (SMPS, 10s to low 100s kHz), I don't see how it's "too low" for RF a decade higher?Tim

author: iMo
author_rank: Super Contributor
timestamp: August 13, 2024, 04:19:54 pm
content: Quote from: T3sl4co1l on August 13, 2024, 03:50:17 pmIt is too low for the 3.5MHz band (OP is experimenting at) because when creating a transformer (or a choke) you would need a lot of turns, that increases the parasitic capacitancies (creating issues like lower Q, self resonance somewhere within you freqs of interest etc).As I wrote above the rule of thumb in this business is to have the inductive reactance of primary/secondary windings (the source or the load see) at least 5x higher than the impedance/reactance of the source/load at the frequency of interest (or at the lowest freq of interest when we talking the broadband transformers)..OP writes he is using material 67 which is better for higher bands like 14-50MHz, afaik.For the lower frequencies like the 3.5MHz the material 43 (mu'=850 in his toroid size) is better from my experience.Btw. we are not talking SMPS with 10s to 100s kHz here but HF bands..PS: FYI - the HAM bands like 1.8MHz-7MHz are considered "lower HF bands", the 14MHz-28MHz "higher HF bands", and for example the 50MHz band is already a start of the VHF band (50MHz/70MHz/144MHz).. Based on the bands (and the actual usage) HAM people decide on the best ferrite or powder materials (Amidon/FairRite/FeroxCube/Neosid/etc)..PPS: except the mu' (function of frequency) there is the mu'' which represents the losses (function of frequency) - that is a parameter amateur radio operators/enthusiast are interested in too..

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 13, 2024, 06:16:42 pm
content: Explanations not needed, I've done all this before. Or, not sure how familiar you are with my posting history and topics of interest, but, suffice it to say I know how to design a network?Some concerns:Quote from: iMo on August 13, 2024, 04:19:54 pmCapacitance is irrelevantat a given center frequencyFc until such point as the parallel-resonant (impedance peak) frequency drops below Fc. The impedance just keeps going up and up until then.Whether one is interested in harmonics, is another matter, but generally in radio, it's desired to filter those out, for which the capacitance would even seem desirable.(My experience is primarily SMPS and wideband, and SMPS is generally quite wide. Last year I characterized a PFC choke up into the 200MHz range; modern SJ MOSFETs happily excite ringing this far up, and following such ringing through the circuit is critical for EMC emissions. Don't let the "10s of kHz" fool you)I would say wideband network design is far more comprehensive than narrowband, but NB design can still be done as a primary path; just be sure to check for spurs once in a while.QuoteYou're defining a Q less than 0.2 already; I don't see what the point is. That is, for XL > 5 RL, the system-embedded inductor Q is less than 1/5by definition. That's pretty damn low to start with. The Q merely reduces to 0 if the core is 100% lossy, in which case efficiency drops by merely a few percentage points / insertion loss goes up a dB or so.Unless you're doing a high-power transmitter on battery power and have extremely high pressure to optimize battery life... it seems a difference unlikely to matter? Or compact parts where power density is high, so that temperature rise would be an issue.Indeed the rule is most often applied to pulse and isolation transformers, baluns, etc., where thesignal loading due to core lossshould be kept acceptably low, i.e. insertion loss of a dB or so. Reactance isn't important, just that the transformer can be treated as a more-or-less ideal transformer over the frequency range of interest. It's a rule of expediency -- or perhaps laziness, but one must not be completely lazy and forget why such a rule exists, under what assumptions it was created, and should be applied.Ferrites are often predominantly resistive in the VHF range, so the raw signal loss is important. Inductance is irrelevant or inapplicable in that case. Note that larger and higher-mu ferrites can be resistive even down into the MF range, i.e. 1MHz or below.Only the low-frequency cutoff case is given by XL and RL, and whatever dB attenuation/reflection is acceptable there.Powdered iron cores are typically inductive up to some MHz, except for the worst (#52 and #26) types of course. They would seem a suitable basis for an RFC here, and Micrometals' publications concur.You wouldn't normally choose a powdered iron core for a transformerper se, because of the low inductivity, but you would when the DC bias is considerable, and making it a transformer isn't abjectly worse than doing a parafeed connection with a ferrite transformer. Or putting it another way: a single-ended tube amplifier output transformer, is still a transformer even if its effective core permeability is ballpark 60 or so. It just means you don't get the same LF cutoff you would for the same number of turns on an ungapped push-pull core. But that push-pull core saturates at some 10s mA if you try using it single-ended, it ain't gonna work, you have to gap it.QuoteAssuming of course, it doesn't saturate; typical toroids used in radio saturate in the couple to low tens of At, fine for signal currents, but again, the transistor in question can draw several amperes with very little base bias, so it's worth asking the OP what their bias actually is.QuoteStrange you aren't aware of the applicability of complex permeability to SMPS inductor design. CMCs too!In fact, I've used an alternative expression of it, the generalized Steinmetz core loss formula (which normally fits ~okay~ (+/-20%?) to common power ferrites), to estimate (via a single closed expression, amazingly enough) the number of turns and air gap required for maximum Q on a given core, for given value and ratings. The aforementioned PFC choke was calculated to have a Q around 350, and was measured a bit over 300 at 100kHz; but still got too hot for comfort and a larger design had to be chosen! Alas!Tim

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 14, 2024, 12:44:45 am
content: Hi LM21, I think a lot of magnetic hot air is here because of insufficient information. Can you put up a sketch of your circuit using the PBSS4041NX?I copy here some of your text:"the collector coil makes a soft pingThe collector loop is around 4-5 turns, output loop is now 2 turns. The collector current is around 50mA. The transistor is not in pure class A, because current consumption varies a little."Are you using a separate RFC ( Radio Frequency Choke) between the V+ and the collector?If so does the RFC -OR- the transformer make the "soft ping"Are you using a DC blocking capacitor between the single transistor class A collector and the toroids you are attempting to use to match to 50 Ohm?

author: Bud
author_rank: Super Contributor
timestamp: August 14, 2024, 12:52:45 am
content: In my case I was measuring a 200MHz amplifier with a VNA and every time I swept the results were different. This indicated to me the amplifier was unstable. I connected a spectrum analyzer and sure enough spotted oscillation at 4GHz. I put some strategically placed bypass capacitors in the collector circuit, killed the oscillation and VNA readings became stable.

author: mtwieg
author_rank: Frequent Contributor
timestamp: August 14, 2024, 11:26:33 am
content: Quote from: LM21 on August 12, 2024, 07:34:23 pmEvery new post from you seems to change the description of these chokes... could you just post a schematic for clarity?

author: LM21
author_rank: Regular Contributor
timestamp: August 15, 2024, 09:57:24 pm
content: I used the toroid I had from testing baluns. (There are better components for baluns)I measured coil reactance to be over 400 ohms with 18turns. That is good for a 50ohms transformer, but not very good for matching 50 to say 5kilo ohms, to get a Q of 10. To get a good a transformation ratio max secondary is 2turns. I would like to use even more turns. At collector there are 5 turns.I put schema here too. Please, notice that tuned circuit is not there, mostly because I connected the turned circuit differently than in my schema and PCB. Capacitance for the tuned circuit is (470+47pF) 500pFForum SW does not support images from clipboard?

author: LM21
author_rank: Regular Contributor
timestamp: August 15, 2024, 10:24:42 pm
content: And I looked at it with TinySa, there wasn't any high frequency signals.Edit: Funny, I checked what 500pF is at 3MHz. It is 100 ohms. The coil is over 400 ohms. Time to do something.

author: boB
author_rank: Frequent Contributor
timestamp: August 15, 2024, 11:38:32 pm
content: Try starting your sweep out at a higher and higher frequency and see if the ping stops when the scanner resets back to the beginning.