RF interference in charge amplifier - Page 1
Forum section: electronics / rf-microwave

author: DaveHardy
author_rank: Regular Contributor
timestamp: September 01, 2018, 01:56:29 am
content: Hi, I'm in the process of designing a piezoelectric charge amplifier with 500M ohm input impedance. It is being powered by a single rail. The op amp GBW is 11MHz. Unfortunately, I'm picking up a fricking 100mHZ radio station that is very close to me. It's so bad, I can hear the program and identify the call letters when listening to the output! The station is roughly 2 miles away and I can see the antenna because it is on top of a very tall building.Right now, I'm powering it from batteries that are fed into a common mode choke and then through a LC(T) filter and a 100nf cap. I can filter out the radio station by winding the power cable around a ferrite core thing 20 or so times!What is a good approach to solving this problem? I have a 100nf cap right on the power input pin of the op-amp. Should I put a 100pf cap on there paired up with a 10nf cap? Can you guys recommend any good tutorials?

author: coppercone2
author_rank: Super Contributor
timestamp: September 01, 2018, 02:04:41 am
content: What kind of power cable is it?Whats a charge amplifier? Is it a broadband device? Block diagram?Maybe shielding is your best bet? Did you check the response of your filter at 100MHz? You can increase filter order or add another series filter that has a better HF response.

author: DaveHardy
author_rank: Regular Contributor
timestamp: September 01, 2018, 02:24:51 am
content: Here's what a basic charge amplifier looks like. I don't think I'm picking up RF through the input because it is dead quiet if I wrap the power cord around ferrite. I'm curious how to remove RF here. I have decent shielding, but the main problem is RF coupled through the power line.

author: coppercone2
author_rank: Super Contributor
timestamp: September 01, 2018, 02:27:20 am
content: I mean your system diagram with wires and stuff. Did you try probing the input?I don't get what you mean by power line, if you are using a battery. I am kinda imagining a power/amplifier box connected to a sensor by some length of wire?Or do you have a power supply/post processing box thats connected to a remote amplifier/probe head, so you have power and signal going through one cable?

author: DaveHardy
author_rank: Regular Contributor
timestamp: September 01, 2018, 02:34:53 am
content: Sorry, I meant DC line. The DC line is coming off of my high quality PSU and the cable to the enclosed box is about 5 feet. If I wrap the power cable 20 or so times around a big ferrite core the problem goes away.I can't probe the input. The input is at virtual ground and the circuit is a very high impedance. The input impedance is determined largely by the 1G ohm feedback resistor. The only thing holding the circuit in place is a 10pf feedback cap. Input capacitance of the op amp is around 1pf.

author: coppercone2
author_rank: Super Contributor
timestamp: September 01, 2018, 02:39:23 am
content: Ok, so you have a power supply, that runs a (shielded? unshielded? twisted pair?) wire to a probe box.Is the sensor attached to the probe box by a cable?What is the probe box attached to?I am guessing triaxial cable (guarded) to the probe, or coaxial, shielded cable from the PSU to the box, and BNC to the test equipment after the box?

author: DaveHardy
author_rank: Regular Contributor
timestamp: September 01, 2018, 02:45:24 am
content: The cable I am using is an untwisted pair with a foil shield. Ground wire and shield are tied together and grounded to project box. Everything is enclosed in project box except for Ferrite, choke, and LC(T) filter. I also have a 100 ohm resistor on output of op-amp to prevent interference. I don't have any fancy tiraxial cable or BNC hookups. The sensor is in the box.

author: coppercone2
author_rank: Super Contributor
timestamp: September 01, 2018, 02:46:47 am
content: I am having a hard time imagining this honestly.Why are those components outside of the project box? What is connected to the box??

author: DaveHardy
author_rank: Regular Contributor
timestamp: September 01, 2018, 02:50:26 am
content: The components are outside of the box because I didn't think I would get RF interference and I have hastily assembled them to solve the problem.

author: coppercone2
author_rank: Super Contributor
timestamp: September 01, 2018, 02:51:13 am
content: So you got the sensor, batteries, op-amp and voltage regulator all inside of a box?? What is the box connected to?It sounds like the box has no openings.If thats the case then you need better shielding. Maybe gaskets, layered boxes, etc. A good sealed box will give you 100dB attenuation.I am guessing it has a LCD readout? Or pannel meter?You said you listened to it, so BNC output on a fully sealed box?

author: DaveHardy
author_rank: Regular Contributor
timestamp: September 01, 2018, 03:00:32 am
content: I've got a shielded cable coming out of it connected to my oscilloscope and a DC power cable going into it. The majority of the DC cable is unshielded. The cable is shielded after the common mode choke and LC(T) filter. The shielded part of the cable is about a foot long. I have a ferrite core right before the common mode choke on the unshielded DC cable and another clamp on right before entry into the box. If I remove the ferrite, I can hear the radio when I hook it up to a small amplifier.

author: coppercone2
author_rank: Super Contributor
timestamp: September 01, 2018, 03:01:10 am
content: Ok, did you measure the power cable after the filter? Did you measure the supply by itself? You should be able to measure the supply since its very low Z.Can you try hooking it up to batteries inside of the box?Is the supply floating (ground loop)?

author: DaveHardy
author_rank: Regular Contributor
timestamp: September 01, 2018, 03:06:01 am
content: Batteries won't fit inside the box and I'm not sure how to measure the power cable. I'm assuming that the ripple or RF interference is too minute to measure. I have a 300MHz scope and a very nice multimeter.How would you approach this if it was your project? Do I need a spectrum analyzer?

author: coppercone2
author_rank: Super Contributor
timestamp: September 01, 2018, 03:06:43 am
content: Shove everything into a larger box with batteries, a box inside of a box should not matter too much. A cookie tin might be a good impromptu thing to shove everything into.Or.I mean, you can try to just probe the cable, or put a tap inside of the box with a BNC connector connected directly to the PSU (perhaps through a pass capacitor) so you can connect the scope to it all shielded and nice. Like drill another hole, add a BNC connected as a tap on the power rail.Then the next step would be to get a preamplifier between your scope and your DUT to see if you can measure noise.

author: DaveHardy
author_rank: Regular Contributor
timestamp: September 01, 2018, 03:10:20 am
content: Ok. So what should I look for? I already know the interference is about 100MHz but I don't know how it rectifies. Do you know of any good tutorials for employing ferrite beads and RF decoupling? Should I stuff a nice regulator in there too?

author: coppercone2
author_rank: Super Contributor
timestamp: September 01, 2018, 03:10:42 am
content: RF signals .Sorry, I mean connected to the power rail (like close to the op-amp), not the PSU.Put a high pass filter on the scope too if you can (digital or small coupling capacitor). You might be able to see something at the best settings.But I would recommend just shoving everything into a larger box that seals well.

author: coppercone2
author_rank: Super Contributor
timestamp: September 01, 2018, 03:17:10 am
content: you can obviously put the power tap on the 2ndary shield too if you want to try to measure something.A spectrum analyzer would come in handy sure, but some kind of wideband preamp thats low noise could make the oscilloscope work, just not as nice. I don't know what kind of scope you have, some are very sensitive.Just renember the input impedance issues with measurement, since Oscope is high_z and SA is usually 50Ohms, but you just might be able to use general presence/magnitude of the signal to make design choices without worrying what the true value is

author: DaveHardy
author_rank: Regular Contributor
timestamp: September 01, 2018, 03:19:38 am
content: Thanks for drawing that up!

author: coppercone2
author_rank: Super Contributor
timestamp: September 01, 2018, 03:22:21 am
content: also if you use some bootleg box like a cookie tin be sure to sand the paint off so it seals well, and you can always solder them shut with a high power iron or torch if you are careful and have the right kind of flux (welding store sells good low temperature solder that works well for sealing enclosures but it has a real nasty acid flux you need to wash and use really carefully, I find this stuff works alot better then electronic solder.. you can easily make steel project boxes using this kind of solder.Like the 'stay brite' stuff with the clear flux.I honestly don't know how much harmful vapor diffuses into the container while you seal it up though, it might be bad for permament use, but I have some stuff that seems to work fine.I use this brand but I forgot which variant I used, I ran out of it sealing up some steel boxes;http://www.messerwelding.com/MG%20Welding%20Products/Brazing.htm

author: coppercone2
author_rank: Super Contributor
timestamp: September 01, 2018, 03:29:17 am
content: If you are clever though, what you can do is make a few threaded openings on your box (say by soldering on a pipe thread with a end cap, or some little thing you tapped yourself and seal with a bolt), then wash the insides out after you are done sealing it around the perimeter with distilled water a few times to get rid of any corrosive vapor that diffused into the box. A bolt will seal very well. Or just make some holes that you will seal up with non-corrosive more of a pain in the ass to close with smaller or thinner material.Then you can just dry at low heat for a while, the electronics wont mind. You can probably even throw the whole thing into a ultrasonic cleaner too, that would be the fastest and best so you don't need to soak it in water for long periods of time. Maybe give it a few flushes first though, so if you do get some bead of corrosive flux that drips in there, you quickly remove it before it has chance to diffuse into anything that might be permeable to it.I never bothered though tbh. Pretty sure I used the 120 version.The flux should be extremely sour.I do think though that its some kind of organic acid so it should decompose to something non-reactive when you actually use it, the main corrosion problems I noticed were when it dripped on my holder (machinists vise) and I forgot to clean it off.If you heat it fast it should be mostly destroyed, so you just got a few aeresol vapors to worry about, the main ingrediant is Azelaic Acid which is actually a solid which I think will decompose when heated, so its only when it bubbles/sputters, its not a corrosive like sulfuric acid or something, which will destroy in vapor form. Any corrosion should be fairly localized near the weld if there is alot of residual.Obviously you can minimize this problem by carefully painting the flux on with a brush, making sure the use the minimum amount, don't flood the thing like you are doing PCB soldering. More corrosive =/= better in this case.If you read the documentation you might want to go with a different version, the 120 is meant more for mechanical strength then anything.

author: coppercone2
author_rank: Super Contributor
timestamp: September 01, 2018, 03:42:28 am
content: lol shit, it has some HCl in it, I guess I got lucky. The flux is zinc/HCl, which is probably why it works so good. The cored stuff is the organic acid.Maybe I will cut open one of my projects that I used it heavily on and check it for corrosion ! Works fine after 1 year though. I did make most of the box and wash it and only use it to seal the smallest side piece though. Also a tip if you wanna do it.Still really useful because you can make a box any size you want from home depot stuff, I guess just don't use it to make the final unclean able seal if you dont want to deal with weird washing procedures or risk acid damage.***********************Oh, actually I am starting to renember. I think what I did was put solder on both sides using the flux, rinse it out, and just mate the solder coated bits together with regular electronics flux or no flux at all using heat alone. It connects to itself alot better then it does to the steel, especially if you wire brush it. I don't think I am that big of a yahoo to seal the thing with flux vapors.I don't think to take notes for mechanical assembly********An improvement I thought of now is that you can buy automotive high temperature paint from autozone, and paint your lid piece with a gap around the mating area that is covered with solder, to inhibit internal corrosion.Description:Why did I bother? Because I am tired of people being at the mercy of box dealers. Something you start towonderabout people using PCB as a mechanical enclosure !Also, you can bend a lip around the box, drill holes, solder on bolts under the holes, to make a removable lid of you don't need to closetohermatically seal the thing (at least liquid tight, I never tried doing a helium test on it or something). If you make the fold inwards you can make it without a ugly flange.I made this abomination of a drawing too, because you make make really nice looking boxes using only a angle grinder, solder and bolts/nutsU don't need to house ur PCB in a PCBHOBOBOX made of PCB bc u r not machinist & superwelder & rich.U don't even need2own a metal break! all can be soldered from flat sheet.Did ur angles not come out rite?*obviously welding it together will give you better magnetic properties at the corners. Gas welding is cheap and easy for sheet goods, but not as accessible as using solder. The solder is way more then strong enough in terms of tensile strength for anything you use indoors that does not have giant american men jumping on it. IMO it seems underused by hobbiests.Tensile strength will be at least 12,000 PSI, probobly 16000+ if you do the gaps correctly (i read numbers for stuff like staybrite harris solder as high at 25k). A braze will be 40-70k, a weld like 70-140k.The best epoxy is like 6000psi (industrial, properly used, probably won't be achived). Common epoxy is somewhere between 2-4000.electrical solder is like 6000. Of course these are rough numbers from memory. There might be certain unique high performance materials significantly outside of these ranges that I never heard of.