MEGA electrical measurements

5 years 10 months ago #1449 by toabrotherelectro
Hi -
I have built the pedalshield MEGA...works well. For educational purposes, I am interested in having students make measurements on the board. Potential measurements include:
- effects of Low Pass Filtering
- OP AMP Gain
- PWM signals

The filtering and gain would be done using input/output measurements and forming transfer functions.

My questions for the forum are:
- Where is best location to measure input signal to board. Is this FX_In on schematic?
- Rather than guitar input, can you hook up a noise generator at input jack? Has anyone done this? I am thinking that noise would be easier than transient guitar signals for forming transfer functions
- Has anyone made these types of measurements before? What advice/guidance would you give? Did you use oscilloscope, FFT box,...??

TIA for any guidance you can provide.

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5 years 10 months ago #1452 by Ray
Replied by Ray on topic MEGA electrical measurements
Hi,
These are the answers for your questions:

- Where is best location to measure input signal to board. Is this FX_In on schematic?

On the MEGA, the best point to measure FX_in may be on top of the input connector (the are is big and you can attach an oscilloscope probe easily):



- Rather than guitar input, can you hook up a noise generator at input jack? Has anyone done this? I am thinking that noise would be easier than transient guitar signals for forming transfer functions

as far as the levels are sensible you can do it without problems, the shield is designed for guitar signals (around 200mVpp) but you can go around line levels (3Vpp) without problems (but you would need to adjust the input stage level for that with the VR1 trimmer.

We did not try a noise generator, people use smartphones with signal generator apps.

- Has anyone made these types of measurements before? What advice/guidance would you give? Did you use oscilloscope, FFT box,...??

We never try to measure the transfer function of it but it should not be difficult. I can do it if you need it. The signal is heavily attenuated over 5KHz to avoid noise (many digital guitar pedals do this "trick", there is not much info in the guitar signal over 5K and the Arduino sampling rate is not hi-fi)
Attachments:

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5 years 10 months ago #1454 by toabrotherelectro
Ray-

Thanks so much. Really appreciate your guidance. Couple of follow up items:
1) I would appreciate you making some transfer function measurements. It would provide a baseline for me to compare students' measurements against. Consider making the input stage TF measurement to see op-amp gain and effects of (low pass) filtering
2) Can you provide any further information on (recommended?) smart phone apps for noise generation in the context of making TF measurements?

Thanks again for your support.
Steve

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5 years 10 months ago #1465 by Ray
Replied by Ray on topic MEGA electrical measurements

1) I would appreciate you making some transfer function measurements. It would provide a baseline for me to compare students' measurements against. Consider making the input stage TF measurement to see op-amp gain and effects of (low pass) filtering

I did some measurements today. They are limited because if I apply maximum gain to the input signal I end up having clipping and the TF gets messed up.
This is a clean signal going through the pedalSHIELD MEGA in bypass mode:


Nothing special up there, this image is just for reference, my measurement system is band-limited as you can see the signal rolling off over 20K and below 10Hz.


Input Stage Measurements:
Measuring from the input jack to the ADC (R6-C5 junction):
Adjusting with the trimmer to have minimum gain:


Adjusting with the trimmer to have more gain:


I wrote more and not "maximum" because if you apply maximum gain with this trimmer you will get the signal clipped and measurement will go wrong. Guitar signals are pretty small (100-200mVpp) so they won't clip.
You can see how the signal is low-pass filtered to improve the ADC aliasing.

Complete System Measurements:
Measuring from the input jack to the output jack:
Minimum Gain:



More Gain:


You can see how the signal is low-pass filtered even more. Many digital effects remove all content over 5KHz (have a look to the Boss CE2 Analysis: www.electrosmash.com/boss-ce-2-analysis the antialiasing and reconstruction filters are around 5/6KHz)

2) Can you provide any further information on (recommended?) smart phone apps for noise generation in the context of making TF measurements?

I use ARTA and a good external sound card (EMU 0404), I think that you can use cheaper sound cards for this. There is plenty of info online or in the DIYaudio forum. Sometimes I use signal generators apps from my phone to generate the input signal, there is plenty of them. Right now I have this one: play.google.com/store/apps/details?id=radonsoft.net.signalgen but again, there is plenty of alternatives.

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5 years 9 months ago #1468 by toabrotherelectro
Ray-

I am very grateful for the work and your reply. You are best!

Steve
The following user(s) said Thank You: Ray

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5 years 9 months ago #1481 by toabrotherelectro
Ray-

We have been making some measurements and would like to get some additional guidance from you. Let me explain what we tried (I apologize for wordiness)...
1. Put a sine wave into input jack
2. Using a multimeter, we could observe correct frequency by putting positive lead on spring for input jack tip (FX_in) and negative lead at GND location on board (left of foot switch when viewing top-down)
3. However, we see no frequency indication when moving positive lead to A0 (ADC input) with negative lead at GND location
4. Keeping positive lead at A0 and moving negative lead to +2.5V test point does show correct frequency at A0.
5. We cannot see any frequency indication with multimeter at any location on output side. This includes output jack spring location and DIGITAL PINS 6 and 7 for Arduino board.

I should also note that board appears to working properly. We are running the spectrum analyzer sketch (BTW...very cool!) and see a discrete response on OLED screen that follows the tone frequency. Trimmer seems to be moving tone amplitude up and down appropriately. We physically hear tone when plugging output of board into an amplifier.

Would like your thoughts specifically on...
1. Why we are not able to see frequency indication at any location on output (jack, PWM pins or elsewhere?)
2. Why are we not seeing frequency indication at A0 using negative lead connected to GND test point on board and conversely, why do we see tone frequency at A0 when connecting negative lead to +2.5V test point.
3. For clarity, please identify the specific measurement locations that you used to generate the frequency response plots in your earlier post in this thread.

Really appreciate your support and thanks for taking the time to help. We purchased a number of the MEGA kits for my upcoming class this fall as the basis for a project in an electrical engineering class.

Regards,
Steve

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5 years 9 months ago - 5 years 9 months ago #1483 by Ray
Replied by Ray on topic MEGA electrical measurements

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