Measuring of filter performances of 1 to 20 kHz filters - Page 1
Forum section: electronics / testgear

author: Ronald1962
author_rank: Contributor
timestamp: July 26, 2013, 01:15:05 pm
content: Hi folks,first of all sorry for my broken English. I promise to get better!!And I am not sure if this is the right topic of the forum for my question.Because of a new project I decided to realise, I will have to measurefilter performances of passive and active filters inbetween frequencyranges of 1 to 25 kHz.(gain over frequency curves - phase is not that important yet)How can I performe these measurements by using which measurementequipment do I need?I guess:1 x oscilloscope1 x kind of generator which sweeps the frequency (sine) from about 0 to 25 kHz.But is there a generator like this on the market?Do I have to consider the settling time of the filters by selecting the sweepfrequency (I hope this is the right denotation)?Sorry that I am not on the level as most of you are!!!!Thank you very much ind advance for your support.RegardsRonald

author: jpb
author_rank: Super Contributor
timestamp: July 26, 2013, 02:12:49 pm
content: The question is probably more appropriate to beginners classification.As it stands, the question is difficult to answer because you don't say what dynamic range you need.An oscilloscope is only 8bit or 256 levels so if you're trying to measure out-of-band attenuation that is at low levels you're going to be down in the oscilloscope noise.Getting a frequency/function generator that does sweeps is easy. A DDS one would give good frequency accuracy and be cheap as it only needs to go up to less than a MHz.The absolute amplitude accuracy might not be great but you only want to measure relative values before and after the filter.

author: Ronald1962
author_rank: Contributor
timestamp: July 26, 2013, 04:08:54 pm
content: Hi jbp,Thanks for your reply!Maybe Dave can shift this topic over to the beginners classification.What I'm trying to do is filtering out different 2kHz frames out of the1 - 20 kHz frame.To see if the filter is working properly, I'm looking for a good typeof measuerement without measureing e.g. every 10 Hz step by hand.My scope is a Tektronix 2225.Thanks again for your reply

author: robrenz
author_rank: Super Contributor
timestamp: July 26, 2013, 04:44:17 pm
content: Watch this video inthis post. It will work on an analog scope also you will just have to use a sweep speed fast enough to get a visible pattern. several posts after that also discuss this method.

author: madires
author_rank: Super Contributor
timestamp: July 26, 2013, 05:10:32 pm
content: Use a computer, soundcard and a function generator / spectrum analyzer software. The problem with standalone spectrum analyzers with tracking generators is that they don't support such low frequencies and special audio analyzers aren't cheap either.

author: PA0PBZ
author_rank: Super Contributor
timestamp: July 26, 2013, 05:31:47 pm
content: +1There is some nice software around that generates the tones and measures the output, and afaik there is even some free software to do this.No need to get or use equipment other than a pc, and since you are posting here I assume you have one.

author: G0HZU
author_rank: Super Contributor
timestamp: July 26, 2013, 10:51:14 pm
content: One thing to note is that you should make sure your test equipment terminates the filters correctly in the correct design impedance.I would be tempted to use a decent PC soundcard but there are analysers that can measure this range. eg I can remember using the old HP4195A 500MHz spectrum/network analyser to set up some audio filters many years ago. A nice old analyser but very slow to sweep... For some fun I remember setting it up to display a sweep time of 25 years once.It actually said '25 years' for the sweep time on the screen...

author: Smokey
author_rank: Super Contributor
timestamp: July 27, 2013, 12:15:03 am
content: I'd have to second the Analog Discovery suggestion for this. Assuming it can do the voltage ranges you need. Here is a site where a guy did some bode plots:http://www.jensign.com/Discovery/bode/index.html

author: jimmc
author_rank: Frequent Contributor
timestamp: July 27, 2013, 10:59:50 am
content: If your computer has a decent sound card then how about Visual analyser?http://www.sillanumsoft.org/prod01.htm.Free and no additional hardware required although simple buffer amplifiers might be useful to protect the soundcard from nasties.Jim

author: MarkL
author_rank: Unknown
timestamp: July 29, 2013, 06:45:30 pm
content: Another possibility is this 2-ch USB scope/waveform generator from Syscomp Design for US$189:http://www.syscompdesign.com/CGR101.htmlIt will do gain and phase plots out of the box at 1Hz to 2MHz, with settable linear or log steps. The GUI runs on Windows/Linux/Mac and is open source, so you can tweak the analysis if it doesn't quite do what you want.If you're looking for professional grade measurements, this isn't the device for you. It's oriented towards student/entry-level, but may fit your need to do simple 1 to 25kHz filter characterization.

author: KJDS
author_rank: Super Contributor
timestamp: July 29, 2013, 07:16:58 pm
content: Quote from: MarkL on July 29, 2013, 06:45:30 pmThat's only a ten bit ADC, which might be ok but probably won't be enough to measure the stopband of a decent filter.

author: nctnico
author_rank: Super Contributor
timestamp: July 29, 2013, 09:13:41 pm
content: For this frequency range I recommend a function generator (sine wave) and an AC voltmeter. Preferably an AC voltmeter with 2 channels so you can see the input and output levels. From there it is easy to determine the filter attenuation (in dB ofcourse!).

author: macboy
author_rank: Super Contributor
timestamp: July 30, 2013, 01:36:27 pm
content: There are many ways, but I would use a signal generator (sine wave) and a good True RMS meter.Many good DMMs have a dB function. Send the input signal (unfiltered) to the DMM, set it to manual range, and turn on the dB function. Then zero the reading (usually using the "ref", "null" or "zero" button). Now the input signal is 0 dB and any attenuation or gain is directly read off the display. Put the filter in the path and adjust the frequency of the source, taking measurements.If your meter does not have a dB function, then just measure the voltage of the input signal (do NOT zero the reading) and measure the voltage at various frequencies. You can then convert to dB manually using dB=20*log10(Vout/Vin). Put the measurements into a spreadsheet for easy calculation and plotting.

author: Ronald1962
author_rank: Contributor
timestamp: August 03, 2013, 12:10:09 pm
content: Hi guys,Thank you very much for your replies!!!!!As soon as I'm back in life again (flew plus a heavy throat infection),I will check yor proposals and let you know about my choice.Thanks again!!!!Ronald

author: EEVblog
author_rank: Unknown
timestamp: August 03, 2013, 12:38:23 pm
content: Quote from: Ronald1962 on July 26, 2013, 01:15:05 pmThe right tool for the job is a Dynanamic Signal Anayser