HELP Choosing a design for first digital PI build

[gdane]

Received the board (it arrived a day before the VLF board I ordered domestically, thanks @algan!) and finally got started building today. Embarrassingly I’m stuck on the first step not seeing +/-5V on TP4/5. Where is the best place to ask for build help?

FYI, I’m using a supposedly equivalent ICL7660SCPAZ in place of ICL7660 because Tayda was out of stock. Could this be the issue?

Also curious if the Nano needs to be connected and programmed before voltages at TP4/5 are valid, or can they be tested without the Nano attached?

[Qiaozhi]

Received the board (it arrived a day before the VLF board I ordered domestically, thanks @algan!) and finally got started building today. Embarrassingly I’m stuck on the first step not seeing +/-5V on TP4/5. Where is the best place to ask for build help?

FYI, I’m using a supposedly equivalent ICL7660SCPAZ in place of ICL7660 because Tayda was out of stock. Could this be the issue?

Also curious if the Nano needs to be connected and programmed before voltages at TP4/5 are valid, or can they be tested without the Nano attached?

I assume you're building the Arduino Nano PI?
If so, ground (0V) is TP1, TP4 is +5V, and TP3 is -5V.
The +5V supply is generated by the Arduino Nano, and the ICL7660 is configured as a voltage inverter to generate the -5V supply.

[algan]

Hello, gdane, you can watch the videos, they are exactly for this board, you can also find the book on Amazon and buy it, it will also guide you how to assemble it, here are some YouTube videos for this board
https://www.youtube.com/watch?v=xh8a2hOa29I
https://www.youtube.com/watch?v=T8EStHy71c4
https://www.youtube.com/watch?v=Q3mMjHaDXkU&t=9s
https://www.youtube.com/watch?v=G1gUYHVw20k&t=9s

[gdane]

Yes Arduino Nano PI. Thanks for the helpful replies.

Trying to sleep last night, I remembered reading in ITMD about how the power supply is synchronized with the transmit pulse, so I needed to double-check the Nano. Must have been a firmware loading issue because I reloaded it and now I'm getting 4.88V and -4.88V.

[gdane]

Another couple questions about the Arduino Nano PI build that I haven't been able to find answers for in the books. I'm sure these are newb questions but coming from software that would describe me pretty well.

- Are 16V electrolytic caps OK or should they be 25V or 50V?
- For the other caps, are ceramic or film better, or does it really matter?
- I assume that R2 should be a 2W like R3 due to the footprint on the PCB but want to confirm.
- Finally, is +/-4.88V on TP4/3 OK/normal or does it need to be closer to 5.0V?

Thanks again for the hand-holding. I can't wait to have this working and be able to switch over to mcu adventures.

[Qiaozhi]

Another couple questions about the Arduino Nano PI build that I haven't been able to find answers for in the books. I'm sure these are newb questions but coming from software that would describe me pretty well.

- Are 16V electrolytic caps OK or should they be 25V or 50V?
- For the other caps, are ceramic or film better, or does it really matter?
- I assume that R2 should be a 2W like R3 due to the footprint on the PCB but want to confirm.
- Finally, is +/-4.88V on TP4/3 OK/normal or does it need to be closer to 5.0V?

Thanks again for the hand-holding. I can't wait to have this working and be able to switch over to mcu adventures.

16V rated electrolytic caps are OK.
The other cap types are not critical. You can use either ceramic or film caps.
R2 and R3 are both 2W rated.
The -5V supply will always be a lower amplitude than the +5V supply. This is because the voltage inverter's input voltage is 5V, and there's a small voltage drop at the output.

[gdane]

16V rated electrolytic caps are OK.
The other cap types are not critical. You can use either ceramic or film caps.
R2 and R3 are both 2W rated.
The -5V supply will always be a lower amplitude than the +5V supply. This is because the voltage inverter's input voltage is 5V, and there's a small voltage drop at the output.

Thanks for the advice Qiaozhi i. I've made good progress on my build. My damped (3R3) coil flyback voltage seems to match Fig. 4-9 from the book so that's promising (image attached). I've moved on to Step 10 and built the pre-amp, but I'm concerned by what I'm seeing on TP6. I was expecting somethiing like the "Preamp Output" waveform from Fig. 4-14, but what I'm seeing looks more like a pulse (image attached).
One thing to note is that I used a 20k resistor for R9 instead of 22k because it was all I could find. Is that going to be a problem, and could it explain the TP6 output?

I also wanted to ask for advice about how to incorporate RX DSP into the Arduino Nano PI design. What is the best place to connect the ADC on the PCB to acquire samples?
Note that ultimately I'll be incorporating a STM32 board that only tolerates 3.3V on its ADC input, so any advice about how to condition the RX signal for acquisition using this board would be greatly appreciated.

[Qiaozhi]

I'm not sure what I'm looking at in the attached images.
The coil flyback voltage voltage should be about -200V, but the channel amplitude is set to 2V.
Did you also remember to fit a jumper between pins 2 and 4 of PL4 so that you can set the minimum sample delay to 10us?

Edit:
I've just re-read your post, and you said "My damped (3R3) coil flyback voltage".
If you mean that you used a damping resistor of 3R3, then that's completely wrong. The damping resistor should probably be somewhere between 330R and 680R.
See Chapter 5, page 45, and also Fig. 1-5.

[gdane]

Qiaozhi Attaching images showing full measurements. Apologies in advance for my limited understanding of how to effectively use my oscilloscope. Hopefully this makes sense.

- Without the damping resister installed, the flyback is showing voltage range +24V and -224V.
- With a 3R3 damping resistor installed, the flyback is showing voltage range +5.76V and -3.36V.
- TP6 is showing +4.48V to -3.68V range in a pulse waveform with jumper on pins 2 & 3 of PL4 per the book.

Note that I adjusted R12 to remove the hump preceding the pulse, which is how I interpreted the instructions in the book:
"adjust R12 so that the preamp output settles to 0V"
Just doesn't seem like TP6 should show a pulse?

Flyback with damping resistor removed:


Flyback with 3R3 damping resistor:


TP6 with sample delay pot jumper installed:


Build so far setup to measure TP6:

[gdane]

Qiaozhi after the Nth time going back over the steps, I realized that I misread the suggested initial value for the damping resistor. I had installed a 3R3 instead of 330R.
I switched this out and now my flyback no longer has the nice form it did before. It also appeared to add a lot of noise to TP6.
Incidentally I zoomed in on the waveform of TP6 and saw something that me a bit more revealing. The waveform now faintly resembles the expected waveform, only with a lot of oscillation. What do you make of this?

*** EDIT: The TP6 waveform below appeared after I turned R12 all the way counter-clockwise. Not sure if that is minimum or maximum resistance.

BTW, the coil building instructions in the book were a little vague about the wiring at the coil. The best I could make out from Fig. 4-8 is that both the coil shield and one of the coil leads are both connected to the coax screen, and the other coil lead is connected to center core of the coax. Is that correct?

Flyback with 330R damping resistor installed:


TP6 with 330R damping resistor:

[Qiaozhi]

I suspect your problem is with the coil.
Read Step 8: "Build a test coil" starting on page 28.

[gdane]

Qiaozhi I followed the coil build instructions to a T.

- Used 24 turns of 14 AWG THHN Nylon insulated wire wound around a 9 inch diameter form.
- Wrapped the coil with copper foil tape leaving a 1-2 cm gap, with a drain wire on one side of the gap.
- Wrapped the coil in a layer of electrical tape to protect the foil and keep the bundle tight.
- Finally connected one lead from the coil to coax core, and connected both the shield drain and the other lead from the coil to the coax screen.

That last step is the one that wasn’t clear in the book. Can you confirm if it’s correct?

[Carl-NC]

Could be EMI (LED lights give me grief), also make sure there are no other coils nearby. A good way to determine if the ringing is caused by external sources is to pick up the coil and rotate it around in free space, see if the ringing changes.

[gdane]

I think you nailed it Carl-NC! I moved the coil as far away from my workbench as it would reach and that seems like it may have resolved most of the noise. Probably was either the LED lighting or the power supply. There is still some transient noise that flashes across the waveform. Wondering if that is to be expected?

[Carl-NC]

Yes, I often get that. It's EMI from somewhere. A way to determine external noise from circuit noise is to disconnect the coil.