RF Noise and voltage regulators. - Page 1
Forum section: electronics / rf-microwave

author: cdev
author_rank: Super Contributor
timestamp: February 11, 2022, 02:12:35 pm
content: I'm building a small low noise amplifier using an MMIC whose noise figure in the UHF range is specified as under 1 db. I'm hoping to use a 3.3 to 6 volt voltage regulator. How do I minimize - and I mean absolutely minimize any noise from the voltage regulation device? I am using small project boxes and I likely will heat sink the voltage regulator using this box (on the outside) Are there any tips for doing this when RF noise is particularly unwanted, even small amounts of it. Obviously use a feed through capacitor optimized to be a low pass filter to be used in this kind of application. I have that kind of part, and plan to use it. I'm also going to attempt to use my nanoVNA to evaluate the DC power path for the device.Most voltage regulators come with suggested values for capacitors and resistors specified in sample circuits. Are they the last word on minimizing noise?This is a very noise sensitive application.

author: mawyatt
author_rank: Super Contributor
timestamp: February 11, 2022, 03:58:03 pm
content: We had a similar problem long ago with a chip design where the MW VCO needed to have extremely low phase noise. Ended designing a somewhat unique on-chip regulator (patent 8692529) for this purpose. The LF noise is the difficult problem to reduce, the HF noise can be effectively reducing by a simple pi filter with quality components. The LF noise is best addressed with some of the Low Noise Linear regulators from Analog Devices, where you don't need a massive decoupling cap.If you are considering a boost type switching converter, then this is a more difficult issue and you'll likely need to follow the boost converter with a good low noise linear for the best overall noise performance. For the boost maybe consider a charge pump rather than inductive type, likely easier to deal with the switching induced noise components.Best,

author: David Hess
author_rank: Super Contributor
timestamp: February 11, 2022, 06:42:07 pm
content: Use a feedthrough capacitor and pi filter in the supply line.

author: cdev
author_rank: Super Contributor
timestamp: February 11, 2022, 07:59:01 pm
content: Thats what I plan to do. Using a special kind of feedthrough that combines a 1000 pf feedthrough and RF choke. I have noticed that the voltage regulators seem to increase the noise figure versus battery A bit. It varies from device to device.Also cleanliness of the PCB results in a noticeably better NF, it seems. I don't know if people believed me but I swear its true. I only notice this with very weak signals.

author: mawyatt
author_rank: Super Contributor
timestamp: February 11, 2022, 08:24:57 pm
content: Make sure you use the feedthru with the choke facing away from the LNA, I've seen folks put these in backwards!! Then decouple the choke side with a good HF chip cap close to the feedthru (effective pi network as mentioned), and a good tantalum large value for the LF PS noise. If you don't need precise regulation at the LNA adding a small series resistor to the PS from the tantalum will help with the LF noise.Believe it or not, LF power supply noise has a means of finding its way onto RF signals by way of slight bias level modulation and the slight non-linearity of the active devices, like varactor effects of PN junctions and so on.For the best results use a good Analog Devices low noise regulator, and large tantalum (maybe with series R), followed by RF decoupling pi network.Best, and good luck with your design.

author: radiolistener
author_rank: Super Contributor
timestamp: February 11, 2022, 09:15:21 pm
content: Quote from: cdev on February 11, 2022, 02:12:35 pmferrite bead then a couple of ceramic and tantalum capacitors (1 nF + 10 nF + 100 nF + 1000 nF + 47 uF) in parallelSomething like that:

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 12, 2022, 06:21:06 am
content: Irrelevant: regulators have essentially no bandwidth up there. Ambient sources are probably the bigger concern. A lossy pi filter (see above) will do.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: February 12, 2022, 06:56:19 am
content: don't forget about pre-made LCLC... filters that have high order and small form factor

author: KE5FX
author_rank: Super Contributor
timestamp: February 12, 2022, 08:46:44 am
content: High-Q LC circuits and LDOs don't mix especially well. They tend to cause noise peaks to appear near their corner frequency at levels well above what would be present with no filter at all. RC or RLC filters are often quieter despite being lossy.Job #1 is to understand if you actually have a problem, of course. VCOs, crystal oscillators, and PLL chips need very clean power, but MMIC amps running class A are usually OK without a lot of special effort. With those parts there is no (obvious) mechanism that can translate power-rail noise from baseband up to the RF frequencies of interest. If they are acting as mixers, you've got bigger problems.

author: mawyatt
author_rank: Super Contributor
timestamp: February 12, 2022, 05:00:05 pm
content: Quote from: KE5FX on February 12, 2022, 08:46:44 amThink all the discussions from folks are about a pi type filter where the L is very lossy by design, thus bypassing (pun intended) the noise peaking issue!!Guess you haven't experienced the LF noise modulation on various MMIC devices, we often have. This even includes ECL/CML pre-scale dividers, as well as amplifiers, and the usual VCO, Mixers, Phase Comparators and such. The mechanism is that if the active device voltage or current "sees" the LF noise, so will the RF signal since all active devices are non-linear to some extent and thus mix the LF noise with the RF signal. This is especially prevalent in VCOs, but also in MMIC amplifiers where the LF enters the bias network and other circuits like phase detectors and obviously mixers which are supposed to mix signals!!The levels can be small but observable by injection techniques on the PS and looking at the result on a Phase Noise Instrument or setup. This is often why batteries are utilized to make close in phase noise measurements, any PS LF noise or mains feedthru will show up in the close in phase noise.Best,

author: TimFox
author_rank: Super Contributor
timestamp: February 12, 2022, 05:25:05 pm
content: A class-A amplifier can certainly modulate the desired signal with unwanted low-frequency components on the power supply.The main point here is that voltage regulator circuits that achieve good low-frequency noise (hum, ripple, etc.) do not accomplish anything at frequencies well above 1 MHz, and that RF filter techniques are requiredin addition tothe DC stabilization.

author: mawyatt
author_rank: Super Contributor
timestamp: February 12, 2022, 05:35:48 pm
content: ExactlyBest,

author: rfclown
author_rank: Frequent Contributor
timestamp: February 24, 2022, 11:06:11 pm
content: A little over a year ago I did a SATCOM LNA to replace one that used 42V (provided on the RF feed). I needed 50 dB of gain and NF<=0.5. My LNAs (3) all used 5V. A linear regulator would have taken too much power. For a buck converter I used an LT1777 and put a huge LC after it (150uH, 100uF) followed by a 5V LDO (garden variety, not specialty low noise). The unexpected problem was noise from the switcher getting back to the radio... and it wasn't the Rx that was the issue... is was a 4 MHz modem that was used to control the other LNA/Antenna that I was replacing (which was fed on the coax along with the 42V bias and the 1.5GHz RF). So I put a second huge LC on the input of the switcher. In my case I didn't have any issue will noise on the DC affecting my noise figure. I think it was about 0.35I'm just remembering the funniest part of the story. When I learned about the switching noise getting to the radio, I thought, "I'll just put the same LC on the input", so I did... Not thinking about the voltage. The 100uF cap wasn't rated for the 42V on the input. I wired it up, and without trying it out gave it to the software engineer who was getting the system working. I didn't put the cover on the chassis so she could see an on board LED come on to know that she had connected things right. When she turned on the system... BANG!. She had her face right over the unit and fortunately was wearing glasses. She happens to be the wife of the VP of Engineering (my boss', boss', boss). I still keep the remains of the cap (aluminum electrolytic) in a container at my desk.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 24, 2022, 11:13:24 pm
content: And you've been using that username ever since?..Hah, yeah, I can see the interest in holding onto that cap. The "somehow this didn't cost me my job" collection.Tim

author: e.sotillet
author_rank: Contributor
timestamp: February 25, 2022, 03:50:29 pm
content: In my daily work, I face this problem: a big DC input (48V) and low noise amplifiers that needs 3V. I notice that, using only a pi filter, the signal was AM modulated and 2 sides signals apperar just at the same frequency of the converter.I ended using a buck converter at 3.6V and later a low noise LDO at 3V and problem solved.Finally, any one can recomend part numbers for those integrated LC ceramic filters? I saw a very peculiar one on the output section of a 5kusd siglent generator.Best regards

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 25, 2022, 08:17:47 pm
content: Murata has a line of those:https://www.digikey.com/short/t94tfh1qBeware that they're, well, just what they look like -- the ceramic is X7R or worse in larger values, so saturates with DC voltage, while the ferrite beads saturate with DC current. They're rated plenty of voltage and a couple amperes, but won't be fantastic (in AC response) over, Idunno, an ampere or so? And voltage, who knows. (Actual-bead FBs tend to saturate more gradually than chips, I think? They're also generally larger, which helps the most.) For design purposes, I think I'd prefer the discrete components -- that I can design around their characteristics, for which I have enough selection to find parts with chars listed -- but they're perfectly fine when you just need a bit and want the convenience of a single component.Tim

author: rfclown
author_rank: Frequent Contributor
timestamp: February 25, 2022, 08:22:50 pm
content: Quote from: e.sotillet on February 25, 2022, 03:50:29 pmMurata makes them. Example: NFE61HT330U2A9L, Digikey 490-6950-1-ND

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: February 25, 2022, 11:37:53 pm
content: Quote from: T3sl4co1l on February 25, 2022, 08:17:47 pmThey are specc'ed at 50 or 100 V, 2A. Do you build hi end audio where the utmost "headroom"is absolutely necessary? We are talking about a MMIC and 6V. And corner frequencies of10 or 20 MHz.I just designed 3 of these filters in the latest revision of my ADF5356 microwave synthesizer.They simply work.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 26, 2022, 01:19:05 am
content: No, it's not like an audiophool thing. It's a design assurance thing.As with all ferrite beads and ceramic capacitors: the ratings are a safety factor, not a characteristic. The characteristics (C, attenuation) are measured at zero bias, and you only have guaranteed performance at that condition.That's quite adequate for signal purposes, analog and digital say -- audio is an example sure, or logic-level inputs, even logic-level outputs aren't bad with ferrite beads as long as they aren't too small (a FB of say 1kohm and 0402 size will saturate at those kinds of currents (~20mA), but larger sizes are fine).It's a concern, because the attenuation factor might shrink by10 times or worse, when operated at limiting ratings. It's not like it's a subtle, picky thing, where the curve warps a little, 10 or 20%, or even 50% -- it can be out by decades. So I will never miss an opportunity to provide this caution!And that's all that it is, a caution, another design condition to meet, or at least a consideration. I'm not at all discouraging their use in general; I like them well enough, I've used them before. Like I said, I rather use the individual parts, whose characteristics are known, so I can fully understand the conditions they're in -- but this kind is still perfectly usable, as long as these concerns are understood.Actually, scratch the technical explanation -- let me throw this back at you, actually; let us illustrate by example! You say they "simply work" -- but in what way did you measure that "work"? How many dB attenuation do they provide in your application? How much did you need? And what application(s) were they -- power supply, pass-thru filters, bias tees, wired inputs, etc.?Tim

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: February 26, 2022, 06:58:36 am
content: Quote from: T3sl4co1l on February 26, 2022, 01:19:05 amSome 40 dB as promised. Measured with Rohde & Schwarz ZVB vector network analyzer.And yes, there was a reason for them. The standard decouplers alone were not enough toisolate 3V3 / 5V for an ADF5356 fractional PLL at up to 13 GHz.Have we met on s.e.d?Gerhard

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 26, 2022, 08:36:16 am
content: Quote from: Gerhard_dk4xp on February 26, 2022, 06:58:36 amExcellent! This is all thatreallymatters.QuoteLikely. There are a few other SED refugees here too.Tim

author: RJSV
author_rank: Super Contributor
timestamp: March 05, 2022, 06:50:09 am
content: Yeah, I'd start with ferrite beads, (and thanks Radiolistener).A friend had several ham radio setups, using 12 V battery each (!)...He claimed, every once in a while, you can get a really obnoxious -ly noisey dc. Lead acid type.But, apparently he liked the set-up, ...but that was a volunteer disaster prep fellow, in rural community, so ..