How do you calibrate/ensure your readings using an soundcard scope? - Page 1
Forum section: electronics / testgear

author: Turbon
author_rank: Regular Contributor
timestamp: February 12, 2016, 08:39:26 pm
content: Hi.The subject has been growing in my mind the last months. How is this to be standardised so we can trust what we share? I have no better way than to check with an RMS DMM and calibrate each time to get them to show the same.Regards

author: cdev
author_rank: Super Contributor
timestamp: February 14, 2016, 04:37:29 am
content: A sound card is not a scope.A program called jaaa (JackAlsaAudioAnalyzer) on Linux might be a start on measuring an inputs' dynamic range.Another truly great program (for FFT, primarily) is baudline. Also free. Works on all three main platforms.Both will teach you a lot. Things you would not learn from a scope.But they are not oscilloscopes.Measurement done that way is basically best thought of as relative.

author: Messtechniker
author_rank: Frequent Contributor
timestamp: February 14, 2016, 08:13:23 am
content: Quote from: Turbon on February 12, 2016, 08:39:26 pmThe way to go. Use a calibration frequency where the DMM will be most accurate i.e. around mains frequency. The soundcard scope programme like this one:https://www.zeitnitz.eu/scope_enshould remember its calibration settings so you will not have to recalibrate each time. To protect the soundcard use simple buffer amplifiers having a gain of 1 on its inputs and outputs.Yours - Messtechniker

author: Ian.M
author_rank: Super Contributor
timestamp: February 14, 2016, 09:06:37 am
content: Unfortunately 50Hz may be on the lower slope of the soundcard's bandpass input filtering. I'd use the highest frequency that your meter's AC V ranges are specified to fully meet their accuracy spec. Also, unless you have a very good meter (and in that case why are you messing with a soundcard scope, as you are obviously not short of cash?), it may not have a low enough fullscale AC volts range for full accuracy measuring the low level signal the soundcard can accept, so you may need a precision OPAMP x10 buffer.It *may* be worth building a squarewave calibrator, based on chopping the divided and buffered output of a precision voltage reference with a dual analog switch arranged as a changeover switch between the Vref out and ground, followed by another buffer. With a static DC control signal to the analog switch, any offsets can be measured with a DC millivoltmeter and compensated for.

author: Turbon
author_rank: Regular Contributor
timestamp: February 15, 2016, 09:18:40 pm
content: Thanks for your thoughts all.Ian.M could you elaborate on your idea "It "may" "...The squarewave part.Regards

author: Ian.M
author_rank: Super Contributor
timestamp: February 16, 2016, 11:00:33 am
content: First you need a squarewave signal of known frequency, within the bandwidth of your soundcard scope. It would help if there were several frequencies available. The easiest option would be a CMOS4060oscillator/divider chip with a 32.768KHz watch crystal to set the frequency. That will give you a squarewaves from 2Hz to 2.048KHz with frequencies that are all powers of 2 (except 16Hz as the 4060 divider chain doesn't have that tap available as an output)You use the resulting squarewave to control a CMOS4053triple analog SPDT switch, with all its sections wired in parallel to reduce the on resistance. (Feed the squarewave to all its S pins, and its /E pin should be tied low.)You then need a DC voltage reference, with a switched attenuator and an output buffer. I'm not going to reinvent the wheel here - there are plenty of plans here and elsewhere on the web. Connect its +ve output to all the nY1 pins of the 4053, and the nY0 pins to ground, at the reference. That will give you a squarewave of amplitude equal to the voltage reference DC output, and frequency equal to the selected 4060 tap. As long as the 4053 is powered by >=10V the on resistance will be <100R. If you are going to drive low impedance loads, add a unit gain buffer using an OPAMP.Use a 12V power supply for all the CMOS logic The 4053 has a negative bias rail Vee, and you will get better results if you can bias it approximately 1.5V negative. DO NOT exceed 15V between Vdd and Vee. The buffer OPAMPs are best run between the +12V rail and the -1.5V rail. The easiest way of getting the -1.5V rail is to put 2 silicon diodes in series with the common pin of your 7812 12V regulator, and a 470R resistor from the common pin to its output to provide a minimum of 25mA through them. Use a PSU with a floating output connected between the cathode of the lower diode (which will be your -1.5V rail) and the regulator input. Your circuit's 0V rail will be taken from the regulator common pin. This setup is likely to give you a bit less than 1.5V depending on the diodes used.A 100K pullup resistor on S inputs of the 4053 helps you set it up as then if you disconnect the jumper you are using to select the 4060 output, the 4053 will output the reference voltage as a steady DC level that you can check with your meter. Ground the S pins to check the output goes to 0V.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 16, 2016, 04:31:47 pm
content: A CMOS logic chip will already give reasonably well defined output levels (GND and VCC). So you can get away without the extra CMOS switches. If you don't need a frequency calibration you could get away with a simple 74HC14 set up as a RC oscillator and a second gate to have an unloaded output. A square wave can also be used to check for phase shifts and limitations of the input.

author: Ian.M
author_rank: Super Contributor
timestamp: February 16, 2016, 05:01:00 pm
content: My experience of MSI and LSI CMOS logic is that the output levels can be quite noisy due to other parts of the device switching. Yes you *could* use a level translating digital buffer in place of the analog switch, but I was suggesting a circuit that would work up to at least 10V and right down to the millivolt level, which would be impossible to achieve with such a buffer.If you use a RC oscillator you will need a flipflop after it to divide it by 2 to guarantee a 50:50 duty cycle. An asymmetric waveform will be offset due to the action of the soundcard's input bias network and its coupling caps, so is definitely undesirable. That's two chips in place of one if you use a 74HC14. The 4060 can be configured for RC oscillator mode, but 32.768KHz crystals are so cheap new and can be salvaged from many types of scrap consumer electronics, so you might as well get crystal controlled frequencies from the 4060.