how can i reduce interference from FM radio in my measurements? - Page 1
Forum section: electronics / testgear

author: JPortici
author_rank: Super Contributor
timestamp: March 17, 2017, 05:53:49 pm
content: some 10km near from here there are two radio stations (plus i think repeaters from others) that transmit in the 95 -105 MHz RangeSince it is near the resonating frequency of your standard 1.5m lenght probes, it is basically impossible to measure something at 1mv/divHow am i sure that this is the case? well, i used to keep one slot of my 7000 mainframe free and fold the probe cable inside when i could.. and thenvoilàno more ~100MHz oscillation on screenUnfortunately i can't cover my bedroom with tin foil..One solution i thought of was to make a bandstop filter centered at 100 MHz and 10 MHz bandwidth, connect it between the probe and the scope input but i'm not sure it's the best way.Another was a selectable lowpass filter (think 7A22) because i think i would only need to use that setting only for noise measurements and i'm not interested of noise over the audio range (in some cases) and a couple of MHz for power supplies. Needless to say, oscilloscope BW limit is useless in this case.Suggestions?

author: MrW0lf
author_rank: Frequent Contributor
timestamp: March 17, 2017, 06:09:12 pm
content: Quote from: JPortici on March 17, 2017, 05:53:49 pmBandstop maybe, not bandpass?http://janielectronics.com/index.php?route=product/product&path=33&product_id=125"FM Notch Filter 88-108 MHz band reject filter for HAM RTL SDR receiverLocal 88-108 MHZ broadcasters can be so strong that may overdrive your SDR receiverThe filter design, an optimized Chebyschev 7-th order band-stop"Recently bought this (was on eBay also). Not tested yet.

author: JPortici
author_rank: Super Contributor
timestamp: March 17, 2017, 06:11:14 pm
content: of course, bandstopif one had to desing and use such passive filter, what input/output impedance should there be?i only have 1Meg probes and 1Meg scope inputs

author: james_s
author_rank: Super Contributor
timestamp: March 17, 2017, 06:19:15 pm
content: Faraday cage?

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 17, 2017, 07:26:03 pm
content: Although the probe cables might look like a nice antenna with their 1.5M length it should not enter you scope. What happens if you put the probe cable through a ferrite ring at the scope end with a few windings?

author: JPortici
author_rank: Super Contributor
timestamp: March 17, 2017, 07:58:18 pm
content: Hmm. I'll try to find a ferrite core and report back.I'd be inclined to say i agree with you. May i remind that i live rather near two (or more) transmitters?At my place of work, 10 km further away i have no such interference (same scope. Both places should inherently be faradray cages as AFAIR armored concrete is grounded)

author: AF6LJ
author_rank: Unknown
timestamp: March 17, 2017, 08:08:09 pm
content: I hate to tell you this; the only way to eliminate this is with a grounded screen room.I have the same problem, it makes making measurements below 50MV next to impossible.One of these days I will own a screen room.

author: MrW0lf
author_rank: Frequent Contributor
timestamp: March 17, 2017, 08:22:14 pm
content: BTW shielding is cheap and trivial nowdays:http://www.yshield.com/eu/shielding-paints

author: AF6LJ
author_rank: Unknown
timestamp: March 17, 2017, 08:49:07 pm
content: Quote from: MrW0lf on March 17, 2017, 08:22:14 pmI remain skeptical.The OP would be better off with a Con-Ex container, a couple of two meter ground rods and fingerstock on the door.

author: TheSteve
author_rank: Unknown
timestamp: March 17, 2017, 09:19:58 pm
content: Are you 100% sure it is the radio stations? At 10km the signal shouldn't be that strong. I would look for a source of local interference.

author: james_s
author_rank: Super Contributor
timestamp: March 17, 2017, 09:28:17 pm
content: I once lived across a valley from multiple broadcast towers. There was a station that played crappy dance music that would come in across several locations on the dial. I didn't have a decent scope back then but I suspect even the probe tip and the DUT would pick up a lot of interference from one that close.

author: cncjerry
author_rank: Unknown
timestamp: March 17, 2017, 09:58:13 pm
content: I have the same issue with FM stations coming in on my probes, both spectrum analyzers and scopes see the same. At first I thought I had an issue with the scopes signal comp output but found out when I tested it that they were FM station specific. Hook your probes to an FM radio as the antenna and see what you get, they will work great as antennas.

author: TheSteve
author_rank: Unknown
timestamp: March 17, 2017, 10:14:02 pm
content: Sure a probe can be used as an antenna on a radio but when connected to a properly grounded scope it really shouldn't. The outside of the probe cable is shielded so the only portion that should operate as an antenna is the tip - which is only 1/4 inch long. That is one small antenna.Of course building grounding varies and so does the quality of scope probes.

author: MrW0lf
author_rank: Frequent Contributor
timestamp: March 17, 2017, 10:18:04 pm
content: Physics of this could be fascinating. Probably culprit is shield - very low resistance... due to skin effect there will be actually 2 conductors - shield inside + outside form closed loop shorted at both ends. Why not happily transmit it's favourite tune to signal conductor?+further impedance to ground is huge for 100MHz. For RF scope is effectively not grounded unless very special care is taken:

author: ebastler
author_rank: Super Contributor
timestamp: March 17, 2017, 10:32:39 pm
content: Quote from: MrW0lf on March 17, 2017, 08:22:14 pmThese paints seem to have the same target audience as tin-foil hats;I don't think they are meant for people who intend to actually measure the effectiveness of the shielding:Quote

author: MrW0lf
author_rank: Frequent Contributor
timestamp: March 17, 2017, 10:42:07 pm
content: Quote from: ebastler on March 17, 2017, 10:32:39 pmAnd what is this? Scientific grade remark?Are you questioning manufacturer spec, on what grounds?

author: ebastler
author_rank: Super Contributor
timestamp: March 17, 2017, 10:51:39 pm
content: Quote from: MrW0lf on March 17, 2017, 10:42:07 pmOnly on the ground of their stated application, which I quoted from the manufacturer's website; so not very scientific.Don't laugh so hysterically at this hour, you'll be waking up the kids.

author: AF6LJ
author_rank: Unknown
timestamp: March 18, 2017, 12:14:41 am
content: Quote from: TheSteve on March 17, 2017, 09:19:58 pmYes

author: JPortici
author_rank: Super Contributor
timestamp: March 18, 2017, 12:39:24 am
content: what is more local than TWO (or more) broadcast towers 10 km away? i repeat, these are strong enough to transmit with no issue over at least 100 km raidus.if you want i can make a video of me bending the probe cable inside a tek 7000 mainframe (a faradray cage) and the 100 MHz interference magically disappears as soon as a couple of cm of probe cable are inside the cage.

author: JonM
author_rank: Unknown
timestamp: March 18, 2017, 03:59:01 am
content: It's often helpful to measure the frequencies via FFT, if available, to verify that they match the FM stations. I often use FM as a signal source with a random length wire attached to the scope input which provides a rich spectrum. I did not expect to see much with a scope probe, but I do see three FM carriers. Adding ferrites to the probe cable knocked out one of them, but the time domain signal still has a lot of noise at 10 mV / div.Long ago an instrument I built always showed a peak for a local AM station. Endless grounding and shielding magic could not get rid of it and I had to ignore it in software.Even longer ago I had a US Navy surplus Copper screened room Faraday cage. It was used to keep EMI from laser capacitor discharges from interfering with detectors. With the particular geometry of the equipment we were able to keep the "side door" open and have no noise issues.

author: jonovid
author_rank: Super Contributor
timestamp: March 18, 2017, 04:27:09 am
content: Faraday cage?a DIY lab Faraday cage can be made from an old microwave oven. the door has a window in itjust add rf sockets on a plate were the cavity magnetron was removed from the oven case.

author: JonM
author_rank: Unknown
timestamp: March 18, 2017, 04:36:45 am
content: Quote from: jonovid on March 18, 2017, 04:27:09 amHmmm, I will be throwing out a large microwave oven soon, however I don't think that my scope will fit inside. It could be useful for something else though.

author: JonM
author_rank: Unknown
timestamp: March 18, 2017, 04:54:51 am
content: Thinking about it a bit more. The scope probably does not need to be inside the microwave cavity / Faraday cage. Connecting the scope to the DUT via feedthrough BNC connectors could be effective. The shielding of the scope would be an extension of the Faraday cage.I wonder what wife trouble I would cause by saving a microwave oven carcass? (Especially since we are in downsizing and moving mode).

author: SeanB
author_rank: Super Contributor
timestamp: March 18, 2017, 07:51:56 am
content: If you are using an old microwave you will need to bond the door much better than the original quarter wave stub filter in there, as that only works at 2.4GHz. Best is a good length of finger stock on all edges, or at least a broad braided strap ( flat, not round) with a good metal to metal contact at the top and bottom hinges. As well you will also need to bond the internal mesh in the door to the frame as well, it often is not well bonded and is capable of acting as an antenna at other than the operating frequency. Best is a stainless steel microwave, as those at least have metal covers that are easier to bond to ground than plastic, but you will need to strip the oxide off the bonding points with sandpaper.

author: David Hess
author_rank: Super Contributor
timestamp: March 18, 2017, 11:15:29 am
content: There are a few of things which can help:1. Add common mode filter chokes to AC power cords and probe cords.2. Place a large metallic ground under the surface of your workbench.3. Use a power line isolation transformer to move the power line ground closer to your workbench.

author: edpalmer42
author_rank: Super Contributor
timestamp: March 18, 2017, 05:55:42 pm
content: I also see local FM stations as a 'fuzz' on my scope screen. Here's what I do:1. Use the scope's 20 MHz filter when I can.2. Use double-shielded coax cables. My favorite type of cable is one that has one foil shield and one braid.3. Get a metal cookie tin - the biggest one you can find - and turn it into a faraday cage for your breadboard by installing coax feedthroughs or connectors and holes or maybe feedthroughs for single leads. Be sure to remove any paint or lacquer around the edge of the top that might interfere with RF shielding when you put the top on.Unless you live underneath the antenna, this should solve the problem, as long as your circuit is small enough to put in a cookie tin. It's also cheap and easy!Ed

author: JPortici
author_rank: Super Contributor
timestamp: March 19, 2017, 07:59:39 am
content: I am still sure the noise is picked up by the probes.an fft on any probing shows a plateau centered at 100 MHzan fft on the terminated input does not, hence the noise isn't picked up by the scope. (50 ohm termination into 1Meg input... is it still valid?)(can provide screenshot)I had been looking around the room for an usb cable with those snap on ferrites at one end. i'm sure i have a couple of those somewhere, but i don't know where i put'em.

author: SeanB
author_rank: Super Contributor
timestamp: March 19, 2017, 09:08:18 am
content: Just grab some ferrite toroids from old PC power supplied and break them in half if the probe does not fit through the hole, and then superglue to put back together. Makes them worse ferrites, but here it really is not a worry. One each end of the probe cable, and some foil to short the tip to the grounding ring ( or a BNC T with a terminator on it) so see how much attenuation you get without and with the ferrites.your pickup could also be on the ground lead, and there the only solution is the big biscuit tin or the modified microwave. Microwave has the advantage that you can at least put the door facing away from the transmitter if you need it open.As a test just a roll of chicken wire fence ( the cheapest one you can buy locally) wrapped around the bench, with a piece on the floor tied to the sides, will show you if there is any worth in doing a bigger faraday cage, and should be under $20 for the roll.

author: MrW0lf
author_rank: Frequent Contributor
timestamp: April 03, 2017, 06:12:14 pm
content: Do you need antenna for SDR when having scope probe around? Nope.Reception w/o ground lead: ok, probe 1X/10X setting has no effectReception w ground lead shorted to tip: good, probe 1X setting little betterReception w ground lead and tip attached to lab equipment ground: excellent, probe 1X setting little betterReception w tip attached to lab equipment ground: excellent, probe 1X setting little betterBut dont ever think about attenuating RF in your lab with cheap shielding paint etc, only crazy TFHs would do that

author: MrW0lf
author_rank: Frequent Contributor
timestamp: April 03, 2017, 08:43:53 pm
content: 30min experiment with shielding paint:Disclaimer: this is not proper by-the-book test and not done done according to manufacturer or any other spec, I claim nothing, WYSIWYGYSHIELD sample set ordered out of curiosity with some other gear:Lowtech cardboard box:After 10min and little amount of HSF74 it's now hightech farday cage:Putting scope probe antenna in it did not yield much attenuation. Think it just did became part of antenna that's all. However did try with DUT that fully fits in box and works on small wavelength compared to box size.No box, -86dBmOpen box, -91dBmAlmost closed box, -94dBmClosed (sort of) box, -97dBmHSF74 spec is here:http://www.yshield.com/eu/yshield-shielding-paint-hsf74-hf-lf-1-liter"Screening attenuation: One layer 39 dB, two layers 45 dB. The shielding attenuation is regularly tested in our own EMC laboratory. We have measurement setups due to the following standards: ASTM D4935-10, IEEE Std 299-2006, IEEEE Std 1128-1998, ASTM A698/A698M-07.""Restricted use on absorbent emulsion paints, wallpapers, etc., please check first! Not usable on gypsum based substrates!"So use of cardboard is not really suitable since it absorbs paint too much, conductivity turned out quite uneven from couple ohms per linear cm to tens in other places.Paint was quite ok to work with, no nasty smell whatsoever. Stains from the skin did come off just with warm water.Large scale experiment sometime in summer if all works out as planned...

author: Bud
author_rank: Super Contributor
timestamp: April 03, 2017, 10:33:07 pm
content: As the previous poster said, put snapon ferrite chokes on cabling entry points before trying anything else.

author: MrW0lf
author_rank: Frequent Contributor
timestamp: April 04, 2017, 07:46:34 am
content: Quote from: Bud on April 03, 2017, 10:33:07 pmSeemingly no effect on actual probe. Some effect when putting whole SDR into box with USB extension cable, and putting ferrite on USB cable.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 04, 2017, 10:35:53 am
content: Quote from: JPortici on March 17, 2017, 05:53:49 pm10km isn't "near"---you might expect this sort of thing if you lived next door.FM receivers are several orders of magnitude more sensitive than any 'scope.Back in the 1970s, we received a nice new 7000 series 'scope to replace our 545B.This was in the Transmitter building at a Australian CH2 TV site,so the CRO wasonly a few metres awayfrom 10kW transmitters at around 70MHz.We were upset that the trace was noisy looking.The CRO had one plugin bay left vacant, & we also found that poking the coax into that vacant space caused a reduction in trace thickness.It was eventually solved by getting a dummy plugin cover.It seems that RF was getting directly into the vertical plugin, & poking the coax in the hole acted to some extent as a shield,reducing pickup.Try shielding that "gap".Staff at Transmitter sites often have to read quite low level signals, & usually get by OK.

author: HighVoltage
author_rank: Super Contributor
timestamp: April 04, 2017, 10:50:41 am
content: I had a horrible interference problem in 2015, described here:https://www.eevblog.com/forum/chat/43-mhz-burst-noise-every-63us-in-the-lab-(how-to-find-the-source)/At the end, when it was all analyzed, we knew it came from the neighbors server power supplySo, you might be surprised, may be it is not the FM station 10km away.

author: JPortici
author_rank: Super Contributor
timestamp: April 04, 2017, 09:20:02 pm
content: In the screenshots you can see the plateau around FM radio (88-107 MHz), the other are peaks at 62.5 MHz, 125 MHz, etc. that can be safely ignored for the purpose of this postI tried with the ferrite on probe coax, didn't help. Actually it was for USB cables, i'm not sure if it's also effective in the area i needlet me grab a tin can...(by the way, thank to you all who have responded. i've certainly read everything)

author: AF6LJ
author_rank: Unknown
timestamp: April 05, 2017, 03:18:05 am
content: I think you have a screen room in your future.Or...You just have to live with it.

author: edpalmer42
author_rank: Super Contributor
timestamp: April 05, 2017, 05:22:52 am
content: I realized that I've never actually measured the effectiveness of my cookie tin so I did a test. I took a 15cm loop of wire, amplified the signal by about 30 db, and measured the received level of one of the stronger local FM stations. Then I put the loop into the cookie tin and put the top on. The result was about a 30 db drop as shown in the attached picture. Not bad for a cookie tin.Ed

author: JPortici
author_rank: Super Contributor
timestamp: April 05, 2017, 06:34:28 am
content: Quote from: AF6LJ on April 05, 2017, 03:18:05 am.. or a fm notch filter

author: MrW0lf
author_rank: Frequent Contributor
timestamp: April 05, 2017, 08:03:40 am
content: BTW I re-painted cardboard box yesterday. Now it "looks right" with matte finish. Attenuation clearly improved. Also I have FM notch filter mentioned earlier. Will test that and 3 scenarios: scope+DUT not shielded, DUT shielded, scope+DUT shielded. Think good test scenario would be measuring small values on 0.1ohm shunt resistor with max sample rate.Maybe even try controlled RF source, not stations.As for ferrites I measured inductance of single loop formed from probe. Despite inductance rising up to 4x, ferrites did not help with SDR outside of box and ferrites on probe. Helped with SDR inside box and ferrites on the USB cable. Maybe some RF ace can comment on physics of this? Will try to get more ferrites so effect more definite.Also, isnt there possibility that loading probe with ferrites will mess up actual measurement, especially with f>FM?

author: vk6zgo
author_rank: Super Contributor
timestamp: April 05, 2017, 08:58:25 am
content: Quote from: JPortici on April 04, 2017, 09:20:02 pmYou have "moved the goalposts" a bit,as now, you are using DSO, instead of the 7000 series mainframe, so does that do it, too?Or,are you feeding the 7000's Y output to the DSO?Re: ferrites.When I worked at a TV transmitter, we had a problem with faint, but definite vertical lines on the transmitted picture.It was not present at the Studio-Transmitter Link (STL) output,but was present at the output of the VDA following the STL.Looking at it with a Spectrum Analyser revealed that there were spurious signals at 800kHz & several of its harmonics.We tried some "ferrites" on the incoming coax,which reduced these radically."OK,let's get some ferrites for medium frequencies!"Well,they didn't do anything.When we looked more closely at the ones that did work, they turned out to be "iron dust" rings---not ferrite!In the end,it turned out that "800kHz & harmonics" were a mixing product of the various FM Transmitter signals both from our building & next door.The "iron dust" rings worked at 100MHz, ferrites didn't!

author: nctnico
author_rank: Super Contributor
timestamp: April 05, 2017, 09:17:51 am
content: Quote from: MrW0lf on March 17, 2017, 06:09:12 pmThis filter will require a specific input and output impedance to work (likely 50 Ohm) so you can't put it into any signal path and expect it to work.

author: JPortici
author_rank: Super Contributor
timestamp: April 05, 2017, 09:31:43 am
content: Quote from: vk6zgo on April 05, 2017, 08:58:25 amehr... maybe it wasn't clear before? it happens in both. with a DSO i was able to confirm that the interference was only in the FM radio band, via FFT.we both did the same trick with the empty slot in the 7k mainframeQuote from: nctnico on April 05, 2017, 09:17:51 amof course. i have a noobish question: if i wanted to make a bandstop filter..all my probes are 1 meg, all my scopes have 1 meg input. what should the input/output impedances be?or should i make 50 ohm feedthrough -> filter -> scope input

author: nctnico
author_rank: Super Contributor
timestamp: April 05, 2017, 10:07:27 am
content: I think the easiest solution will be ferrites around the probe cable or use the oscilloscope's bandwidth limiter. If you want to use the filter then you'd need a pre-amplifier as well. Further along that road you'll end up at a differential probe.

author: MrW0lf
author_rank: Frequent Contributor
timestamp: April 05, 2017, 10:08:52 am
content: Quote from: nctnico on April 05, 2017, 09:17:51 amDiff probe having 50ohm output => filter => (50ohm pass thru =>) scope?Or indeed construct something.

author: nctnico
author_rank: Super Contributor
timestamp: April 05, 2017, 10:11:47 am
content: Quote from: MrW0lf on April 05, 2017, 10:08:52 amYes, that would be the way if you want to use the filter pictured above (assuming it is designed for 50 Ohm).Edit: on a scope without real 50 Ohm inputs the scope's input capacitance is likely to mess with the filter characteristic as well. You'd need to check the setup thouroughly to see what the filter does (and doesn't).

author: PA0PBZ
author_rank: Super Contributor
timestamp: April 05, 2017, 10:17:55 am
content: I wouldn't use a filter advertised as "FM notch filter for SDR receivers" in a scope lead, ever. You have no idea about the bandpass characteristics of this filter for let's say 10KHz, but I can assure you it won't be pretty over the input range of the scope.

author: JPortici
author_rank: Super Contributor
timestamp: April 05, 2017, 10:20:47 am
content: Quote from: nctnico on April 05, 2017, 10:07:27 am1 and 2 don't seem to work. d'oh.

author: MrW0lf
author_rank: Frequent Contributor
timestamp: April 05, 2017, 10:25:13 am
content: Quote from: PA0PBZ on April 05, 2017, 10:17:55 amNo idea!?You shall see FR plot of this filter in the evening DC...120MHz.BTW $399.95, probaby cheaper than diff probe, more versatile...http://www.mathews-engineering.com/store/p1/High_Impedance_Buffer_Amplifier.html

author: dimkasta
author_rank: Regular Contributor
timestamp: April 05, 2017, 11:03:14 am
content: If you do not need the FFT, you can use two channels and subtraction on the Math channel to remove interference from your signal.Not sure what other stuff you need to measure though.

author: AF6LJ
author_rank: Unknown
timestamp: April 05, 2017, 12:13:21 pm
content: Quote from: JPortici on April 05, 2017, 06:34:28 amThe problem with the FM notch filter is it will effect the amplitude of the desired waveform, and will cause distortion in complex waveform that may have components that are above and below the notch filter's center frequency.

author: ebastler
author_rank: Super Contributor
timestamp: April 05, 2017, 12:19:24 pm
content: Quote from: dimkasta on April 05, 2017, 11:03:14 amYou would need to know the phase and amplitude of the interfering radio signal. Both will probably vary over time due to multi-path interference. And as there is no fixed phase relation between your signal of interest and the radio signal, their relative phase will be different for each sweep (trigger event). So I don't see any practical way to implement this subtraction scheme.

author: AF6LJ
author_rank: Unknown
timestamp: April 05, 2017, 12:23:16 pm
content: Quote from: nctnico on April 05, 2017, 10:11:47 amThe filter won't be perfect to say the least, add to that when you get far from the notch frequency those filters start doing screwy things. The frequency response goes all over the place especially at the low end.The notch filter is a bad idea if the OP wants meaningful results.The differential probe may eliminate the interference... But he is going to need one that is good to 200MHZ in order to limit the effects of high frequency roll off.I hope the OP has a fat wallet....

author: dimkasta
author_rank: Regular Contributor
timestamp: April 05, 2017, 12:46:14 pm
content: Quote from: ebastler on April 05, 2017, 12:19:24 pmI was thinking that the interference would be the same on both probes, so subtract them and you will have just the signal coming from the first.How about measuring the signal with two probes, reverse the trace on one, and use addition.The signal will be x2, while the interference should remain the sameAgain, I am trying to think "differential" with plain-jane probes.

author: MrW0lf
author_rank: Frequent Contributor
timestamp: April 05, 2017, 07:03:25 pm
content: Some tests onJaniLab 88-108MHz FMNF:SNA plot 25...2500MHz (RF Explorer 6G + RFE6GEN):In followingblue (ChA) is FMNF,red (ChB) is signal gen second channel, sync, with unfiltered signal(Pico 2408B + Siglent SDG2000X):FR plot 0...100MHz:Sine 10kHz:Square 10MHz:Sine 50MHz:

author: nctnico
author_rank: Super Contributor
timestamp: April 05, 2017, 07:28:03 pm
content: It would be nice to have a phase & frequency plot. I have the analyser but not the filter

author: MrW0lf
author_rank: Frequent Contributor
timestamp: April 05, 2017, 07:48:27 pm
content: Proof of concept:Signal gen (square, 8Vpp, 100kHz) => diff probe => wideband amplifier => FMNF => scope.100kHz because Aim-TTi WBA is bottleneck, only up to 1MHz.WBA would be not needed with 50ohm output diff probe.ChA is thru FMNF, ChD is direct.Tomorrow maybe look at dealing with some actual noise. But not with this crazy setup, math channel filter probably sufficient

author: MrW0lf
author_rank: Frequent Contributor
timestamp: April 05, 2017, 08:09:18 pm
content: Quote from: nctnico on April 05, 2017, 07:28:03 pmLook last image0...10MHz and50MHz image. Maybe get some idea. But made no effort to account for cabling propagation delay etc.PossiblySurely phase data all messed up... Will take couple points below and above stop band manually tomorrow.

author: AF6LJ
author_rank: Unknown
timestamp: April 05, 2017, 08:45:08 pm
content: Even after seeing the results of your tests MrWolf, I still believe the notch filter would do more harm than good.I am all for the differential probe set up..Rather it is a separate probe or two (more or less) matched probes and the scope operated in differential mode.

author: MrW0lf
author_rank: Frequent Contributor
timestamp: April 05, 2017, 09:01:30 pm
content: Quote from: AF6LJ on April 05, 2017, 08:45:08 pmQuite possibly, just doing some experimental physics for fun of it

author: MrW0lf
author_rank: Frequent Contributor
timestamp: April 06, 2017, 07:32:05 pm
content: Allright think got this filter thing sorted out now...Theory goes like this (some random notch filter):I do not have means for proper phase testing above 88-108MHz notch. So have to conclude on data below that. Findings:- propagation delay constant ~2.5ns up to ~50MHz. Easy to delete with channel skew.- makes good lowpass filter, if there is no content above notch-probablymakes good highpass filter, if there is no content below notch- indeed crap with "mixed content"So dunnothink if one is on top of situation very handy little device still, in 50ohm system of course...

author: AF6LJ
author_rank: Unknown
timestamp: April 07, 2017, 03:43:06 am
content: The real world is not so pretty....

author: MrW0lf
author_rank: Frequent Contributor
timestamp: April 07, 2017, 07:48:24 am
content: Quote from: AF6LJ on April 07, 2017, 03:43:06 amDo you mean user not being on top of situation?

author: AF6LJ
author_rank: Unknown
timestamp: April 07, 2017, 07:07:59 pm
content: Quote from: MrW0lf on April 07, 2017, 07:48:24 amNot at all filters are not perfect, resistors have inductance, caps have leakage current....and everything adds noise.The real world is gritty and full of traps.