coax pulse filter? - Page 1
Forum section: electronics / rf-microwave

author: Gaktyt
author_rank: Regular Contributor
timestamp: May 01, 2020, 07:21:28 pm
content: Hi.I hope I'm posting this in the correct place, I couldn't find a dedicated analog sub-forum.Anyway I have a single pulse in a 50Ω coax, the first part looks like below, the second part is about 2 periods of a >100Hz sine about 2vpp.The first part is preventing me from looking at the second part on the scope, how can I filter the first part out? I have simply no idea, and coax is completely new world for me.

author: radiolistener
author_rank: Super Contributor
timestamp: May 02, 2020, 12:38:00 am
content: if you mean distortion and echo, you can reduce it by adding proper 50 Ohm load on the coax end.

author: fourfathom
author_rank: Super Contributor
timestamp: May 02, 2020, 01:24:21 am
content: If you want to see something that happens after that initial pulse, look for a "trigger holdoff" function on your 'scope.

author: james_s
author_rank: Super Contributor
timestamp: May 02, 2020, 02:07:00 am
content: https://siglentna.com/video/use-trigger-holdoff-function-siglent-oscilloscope/

author: fourfathom
author_rank: Super Contributor
timestamp: May 02, 2020, 02:15:25 am
content: Quote from: fourfathom on May 02, 2020, 01:24:21 amI mis-spoke -- that should probably be "delayed sweep", although you might be able to fine-tune trigger holdoff to show you the later portion of the signal.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 02, 2020, 03:10:46 am
content: With an analog oscilloscope, "delayed sweep" would do it "standing on its head", but I'm not sure of how the equivalent function on a DSO works.I do think however, that you may be approaching this the wrong way, as that initial transition decaying back to almost zero volts looks very suspect to me.Have you tried using a long time/div setting (around 5ms/div) to verify that your 100Hz signal is actually making it through the coax?At such a low frequency, a properly connected coax cable should have very little effect.An open circuit connection could still have enough capacitance for the initial transition to pass through, whilst blocking the 100Hz.Do you have some way you can show us the waveform before it enters the coax, as it sounds extremely unusual?If it is a narrow spike followed by a very much lower frequency sine wave, it is hardly a "single pulse".Or is that the result of whatever is happening?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 02, 2020, 03:32:58 am
content: Try HF Reject or noise filter trigger mode. Or severely reduce the channel bandwidth, if this is done before the trigger?The other part being "sine" implies it's bipolar, try triggering above 0V.Tim

author: Gaktyt
author_rank: Regular Contributor
timestamp: May 02, 2020, 08:52:13 am
content: Thank you all very much for your valuable input, I sincerely appreciate it.Quote from: radiolistener on May 02, 2020, 12:38:00 amYes you are right, the pulse however has the same form, less the attenuation, with a 50Ω termination. So I surmise therefore that distortion and echo is not the case here.Quote from: fourfathom on May 02, 2020, 01:24:21 amYes you right, however on my scope the "trigger holdoff" function will not work properly when I'm in lower V/DIV range, which is required to view the ~2Vpp sine. I surmise the "trigger holdoff" function malfunctions due to the higher voltage of the first part.Quote from: T3sl4co1l on May 02, 2020, 03:32:58 amI don't believe it's done before the trigger.Quote from: vk6zgo on May 02, 2020, 03:10:46 amYes it does come through, however my two scopes disagree on the amplitude, the disagreement in amplitude only ceases when I'm in 10V/DIV, but then the 2Vpp sine is not distinct enough. I surmise therefore that scopes malfunction due to the higher voltage of the first part, which was discussed in a previous thread when I wrongfully believed it was a scope thing, and not as it turns out - due to my own inexperince. The entire signal has to fit on the scope screen for the scope to work as intended, I have learnt.Quote from: vk6zgo on May 02, 2020, 03:10:46 amI'm sorry I don't understand that, which effect on what?Quote from: vk6zgo on May 02, 2020, 03:10:46 amNo, it is the output of a PEARSON 4100 Current Transformer.Quote from: vk6zgo on May 02, 2020, 03:10:46 amYou are right, I don't know if I was technically in correct calling it a "single pulse", I only opted to call it a single pulse because the first part can not exists without the latter part, and vice versa.

author: Gaktyt
author_rank: Regular Contributor
timestamp: May 02, 2020, 09:45:58 am
content: I can add that I have tried to clamp the voltage by using two general purpose zeners cathode to cathode before it enters the scope, but that seems to distort the signal maybe due to diode capacitance, I'm not sure as I said, I lack the experience.

author: Gaktyt
author_rank: Regular Contributor
timestamp: May 02, 2020, 03:46:06 pm
content: Quote from: Gaktyt on May 02, 2020, 09:45:58 amI made the enclosure bigger and that seems to have the desired effect, below is the result with two 9.1V zeners, the blue trace is the reference without zeners.The subsequent small signal has now dropped to 100mV peak and decreases exponentially to 0V over about 8ms, there are no oscillations now, and it could very well be a actual representation of reality except in amplitude as the two scopes now only disagree 50% so I will try with smaller zeners.Due to my lack of experience I am therefore not aware of the pitfalls of this method, so if the expertise could give me their feedback I would be much obliged.