Frequency Response Graph

Started by FirstTimer007, June 08, 2020, 06:33:20 AM

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FirstTimer007

Hi there!
Hoping someone might point me in the right direction.
I am wanting to create graphs like the pic attached, for my builds.
What gear would I need, and how would I go about it.
I'm a new owner of a (digital) oscilloscope and wave generator...



Thanks in advance,
Sean

niektb

My guess would be that the image you attached is digitally simulated in Matlab (at least the graph style is form Matlab)

You could use TINA-TI to create a digital simulation of your pedal and then create frequency response graphs inside that tool. Check out this tutorial:
https://www.diystompboxes.com/smfforum/index.php?topic=117459.0

amz-fx

Quote from: FirstTimer007 on June 08, 2020, 06:33:20 AM
Hi there!
Hoping someone might point me in the right direction.
I am wanting to create graphs like the pic attached, for my builds.
What gear would I need, and how would I go about it.
I'm a new owner of a (digital) oscilloscope and wave generator...

It can be done with just a scope and wave gen, but it is a slow and tedious process. 

Basically, you just set the wave gen for the low frequency of interest then measure the output level of the circuit with the scope. Next, change the freq gen to the next highest frequency of interest and take another reading of the output. Increase the freq again and measure, and repeat until you get to the highest frequency that you need to record.

The output level measurements can be put into a spreadsheet and a graph of the response can be added to a chart. As I said, slow and tedious.

This is the manual way to do it. However, there are software programs that can do a frequency response measurement automatically, and give you a graph in just a few seconds using the soundcard in your computer. The generated tone comes from the left output and the right input is used to measure the level (no scope or freq gen required). The software does the process of changing the frequency and making measurements automatically. ARTA can probably do it for you: http://www.artalabs.hr/news.htm

There are other methods for getting frequency response, and this is just one version.

regards, Jack

mozz

Syscomp USB scope will do freq sweeps and plot graphs for you.
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idy

Sometimes they call that a spectrum analyzer. Several lower tech alternatives are:

Some "old time" home stereo (or prosound) graphic EQs had a bar graph display and a white noise generator. Put the noise through a circuit, look at the graph. Low resolution, like 10 or 12 bands often...

Another cheap way would be to record the sound of white noise through a circuit, then open the file in a program like "Transcribe!" that has a "show spectrum" function.

FirstTimer007

Thanks for all your replies, I'll check out these alternatives...

Cheers!

ElectricDruid

I'm with niektb on this. Graphs like that aren't produced by putting actual signals into an actual circuit. They're the output from a simulation, usually while you're developing a circuit. As such, they represent the ideal output of the circuit. (You can also model the realistic responses caused by all the individual component variations, if you can be be bothered - "Monte Carlo analysis" - but I don't see people doing it in general)

I use LTSpice for that type of modelling job. In many ways it's dreadful software, with a stupid and quirky interface, but its free and very functional, so it repays the pain of learning how to use it. It produced all the graphs in this article about the Boss MT-2, for example:

https://electricdruid.net/boss-mt-2-metal-zone-pedal-analysis/

I didn't have an actual MT-2 to play with and to feed signals into. I just simulated all the various parts of the circuit so I could see and understand their responses. In many ways it is far easier than dealing with the real circuit, as long as you remember that not everything that works in the sim works in real life (sometimes the simulator uses "ideal" elements that can't exist in reality).

PRR

PC with decent sound card and RMAA will do extensive audio testing including frequency response. The free version is fine until you get utterly addicted.
https://audio.rightmark.org/products/rmaa.shtml
https://audio.rightmark.org/index_new.shtml
http://nwavguy.blogspot.com/2011/02/rightmark-audio-analyzer-rmaa.html
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FirstTimer007

Thanks ElectricDruid,

I have thought of LT Spice, but couldn't even figure out how to get the software, let alone install it or use it.... I must be stupid.

But in the scenario where I have built 6 identical pedals and the transistor's gain (as measured beforehand) varies from device to device - even though they are the same part number and from one supplier - I'd like to be able to graph each pedal, rather than a simulation with generic part values.

The pedals respond differently when I listen to them, and I'd like to be able to quantify the differences.

I don't know if this is actually possible, as I have only read a post recently on another forum where the poster stated he measured his commercial pedal after a gig... that is his graph that I copied into the OP.

Cheers


anotherjim

I've used RMAA for years and years. It is only as good as the soundcard on the computer of course and the lack of a proper line input on many devices is a problem -  you won't get good results with a mic input. But you can ask it to generate a test soundfile and use anything that can play a soundfile to feed your subject and anything that can record the result of that to a soundfile. RMAA can then analyse that recording. So if you have a decent field recorder or two, you can test any bit of audio gear like that.

ElectricDruid

Quote from: FirstTimer007 on June 08, 2020, 09:58:21 PM
Thanks ElectricDruid,

I have thought of LT Spice, but couldn't even figure out how to get the software, let alone install it or use it.... I must be stupid.
That bit is easy enough. Here's a link to the download page:

https://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html

Quote
But in the scenario where I have built 6 identical pedals and the transistor's gain (as measured beforehand) varies from device to device - even though they are the same part number and from one supplier - I'd like to be able to graph each pedal, rather than a simulation with generic part values.

The pedals respond differently when I listen to them, and I'd like to be able to quantify the differences.

Ok, that does require testing actual pedals, like Temol suggests. The signal generator and frequency plotting can either be hardware or software. A decent sound card like PRR suggests is cheaper than a signal generator and oscilloscope, and does the same job, at least as far as audio is concerned (the hardware will do a lot else, but you might not care about a response into MHz).

What you *would* be able to do in the simulation is to see what effect changing the gain of a transistor might have on the circuit's response. That can help you find out if a circuit is sensitive or insensitive to variations in part values. That's certainly useful to know.

temol

Please note - to get best results you cannot use "any" signal generator. You need two channels with syncing between channels, otherwise it's a pita to set up and use.

I'd rather learn how to use LT Spice, measure individual transistors, then modify existing transistor model (create several models) and run the simulations.
Or you can do both - LTSpice and oscilloscope, compare the results and then pick the easiest/most accurate method.

T


FirstTimer007

Thx everyone I've learnt a lot from you all.
Cheers Sean  ;D