Recommendations for low-pass filter on BNC-BNC connector/adapter - Page 1
Forum section: electronics / projects

author: thanasisk
author_rank: Regular Contributor
timestamp: June 07, 2018, 03:45:43 pm
content: Hi,do you have any recommendations on BNC-BNC hardware low-pass filters (e.g. 200KHz or 1MHz) for limiting the bandwidth at the oscilloscope input?I would like to bandwidth-limit the input of my oscilloscope in order to perform low frequency signal measurements by increasing the ENOB (decrease the bandwidth of the noise).I am thinking of a setup likePROBE - BNC LPF filter - oscilloscope CH1

author: donmr
author_rank: Regular Contributor
timestamp: June 07, 2018, 04:32:34 pm
content: At those frequencies the parts are going to be rather large. Fortunately the wavelengths are long too. So you can put your filter in a box and connect it with cables. You don't need something small right at the scope.

author: tomato
author_rank: Regular Contributor
timestamp: June 07, 2018, 05:01:58 pm
content: Pomona makes small aluminum enclosures (~ 1.5 in. long) with male/female BNC connectors on the ends. They work well for home-brewed inline filters.https://www.digikey.com/products/en?mpart=3752&v=501

author: ejeffrey
author_rank: Super Contributor
timestamp: June 07, 2018, 05:07:01 pm
content: Minicircuits makes coaxial BNC filters down to about 2 MHz.https://www.minicircuits.com/WebStore/dashboard.html?model=BLP-1.9%2BBut a DIY filter in a project box should work just as well at those low frequencies.It also isn't usually necessary. Most scopes have internal bandwidth limits and averaging modes (or "enhanced resolution") that can filter out the noise.

author: thanasisk
author_rank: Regular Contributor
timestamp: June 07, 2018, 06:36:21 pm
content: Thank you all for the interesting responses.Oscilloscope BW limits are typically 20MHz, whereas when measuring low frequency signals at low sensitivity (e.g. 2mV/div) , by filtering e.g. 200KHz onwards the noise performance and ENOB goes up significantly according to my calculations based on the ADC ENOB formulas.I was thinking about DIY but first want to explore off-the-shelf solutions with published specifications.I also saw these:https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=8613and I am wondering about proper matching to the probe/DUT and to the oscilloscope, do you think I will need extra adapters?

author: ejeffrey
author_rank: Super Contributor
timestamp: June 07, 2018, 07:43:05 pm
content: Both the mini circuits and the thorlabs filters are designed for 50 ohm inputs, they will probably not work well with a 10x probe. But you shouldn't be using a 10x probe if you want low noise measurement of low frequency signals anyway. The thorlabs ones are meant to drive a high impedance output like a scope and will work correctly with a 1X probe. The minicircuits ones are 50 ohm output. They will still probably work on a 1 Mohm input, but they will have different bandwidth from the data sheet. Alternately, use 50 ohm mode on your scope or a tee and a 50 ohm terminator.I suspect you won't get the advantage you are hoping for in any case. On mV level signals, the noise of the scope front end and ADC are a big source of the noise.A filter in front of the scope won't reduce those, only noise present on the input signal. Scope BW limit and digital filtering can average out noise from the source as well as from the scope analog front end. If your signal itself has a lot of high frequency noise, then the filter will help. But averaging on the scope will get the same effect for $0.

author: thanasisk
author_rank: Regular Contributor
timestamp: June 07, 2018, 08:31:00 pm
content: I think I understand what you mean. I was hoping to implement a narrower scope BW limit than the 20Mhz one, however I suppose in all scopes the BW limit hardware is present after the input amplifier front end and before the ADC. By placing a filter at the input of the scope, I do not filter the noise of the input amplifier front end..Regarding the 1X probe and the use of the LPF, I guess it will be problematic since now the circuit will see an impedance of 50 Ohm (the LPF in) instead of the 1MOhm impedance of the scope.