PT2399 Questions: Not your average questions

[Ripthorn]

Hi guys, I am trying to reconcile differences in a a circuit I have on the breadboard vs. the PCB implementation, and have two questions. Hoping you guys can school me well!

1. The breadboard circuit works exactly as I want. I have traced the schematic and compared to PCB. They seem to match. However, the delayed signal on the PCB shifts upward in pitch as it decays, while the breadboard exhibits no such behavior. Pin 6 has a fixed resistor and pin 2 only has the 47 uF cap. I have checked the voltage on pin 2 using a scope and it stays rock solid, so it doesn't seem to be the "pin 2 hack". I'm at a loss of what the reason is.

2. Pins 7 and 8 are filtering for the modulator/demodulator, as far as I know. It seems that these voltages should be pretty much the same regardless of circuit implementation due to just having a cap to ground from them. However, I see on my breadboard that my values range from 800 mV to 1.5V (I have four PT2399 chips in use) at those pins. The two pins match on each chip, but each chip has a different voltage level. How could this be? I looked on my scope and noted that the AC signal level changes in response to input stimulus, which makes sense, but the DC values on the breadboard are all different, whereas on the PCB they are all the same.

Any help here would be great. I know this gets to some of the nitty gritty of the PT2399, but I would love to understand.

[niektb]

How do you create your delay time variation? Have you routed some high-speed signals close to pin 6 (or the resistor?)

[anotherjim]

Power supply noise & distribution would need looking at. Have you fitted any ceramic bypass caps on the chip 5v pins?

[Ripthorn]

Hey guys, thanks for the responses.

1. I don't have any high speed signals on my board
2. The delay time is through a fixed resistor then through a pot, but just to ensure it's not the routing to the pot, I replaced the resistor/pot combo with a single fixed resistor tied to the PT2399 ground and I still get the pitch shifting
3. All PT2399 chips are locally decoupled from the power supply with a 100nF ceramic cap. The main 5V line has a 100 uF cap right by the regulator.

[anotherjim]

In the PT2399, signal and digital currents are mixed in the common 5v chip supply. The digital contains activity to to 20Mhz. This really can't be helped and the supply pin caps and mod/demod filters have to be relied on.
A key thing is the analogue and digital grounds pins 3 & 4. Although there is an internal resistive connection between the two, it always seems to work better if there is one 0v connection with 3 & 4 shorted together. Some old designs show pin4 with no external connection but this seems to affect the performance of the clock VCO and shorting 3 to 4 always seems to help.

[merlinb]

I looked on my scope and noted that the AC signal level changes in response to input stimulus, which makes sense, but the DC values on the breadboard are all different, whereas on the PCB they are all the same.

If you swap the positions of the chips, do the voltage measurements follow the chips?

[Ripthorn]

I did swap some chips around, it appears the voltages are relatively stable at each spot, even when chips get changed. I think next I'll swap caps around on the Pins 7/8. This is strange...

[Ripthorn]

Just wanted to follow up on this.

1. Turns out I had two resistors that were way off. It was a copy/paste error in my schematic. Checked against breadboard and it matched with what I came up with experimentally on the PCB.

2. This has been a red herring. Not sure why I am seeing it on my breadboard, but I don't see it on the PCB and still get similar performance between the two with the correct resistor values mentioned in 1.

Thanks for putting up with my stupidity!

[pinkjimiphoton]

Hi guys, I am trying to reconcile differences in a a circuit I have on the breadboard vs. the PCB implementation, and have two questions. Hoping you guys can school me well!

1. The breadboard circuit works exactly as I want. I have traced the schematic and compared to PCB. They seem to match. However, the delayed signal on the PCB shifts upward in pitch as it decays, while the breadboard exhibits no such behavior. Pin 6 has a fixed resistor and pin 2 only has the 47 uF cap. I have checked the voltage on pin 2 using a scope and it stays rock solid, so it doesn't seem to be the "pin 2 hack". I'm at a loss of what the reason is.

2. Pins 7 and 8 are filtering for the modulator/demodulator, as far as I know. It seems that these voltages should be pretty much the same regardless of circuit implementation due to just having a cap to ground from them. However, I see on my breadboard that my values range from 800 mV to 1.5V (I have four PT2399 chips in use) at those pins. The two pins match on each chip, but each chip has a different voltage level. How could this be? I looked on my scope and noted that the AC signal level changes in response to input stimulus, which makes sense, but the DC values on the breadboard are all different, whereas on the PCB they are all the same.

Any help here would be great. I know this gets to some of the nitty gritty of the PT2399, but I would love to understand.

its been my experience that if the + supply falls too low, even slightly,  as the 5 volt regulator starts to mess up the delay will start to repeat with the pitch becoming unstable and chasing up or down.

maybe check the power supply?

[pinkjimiphoton]

3. All PT2399 chips are locally decoupled from the power supply with a 100nF ceramic cap. The main 5V line has a 100 uF cap right by the regulator.

you may be going under current for the regulator.... that would make the pitch wander for sure. you may need to have a regulator for each chip. i'm likely wrong.

[anotherjim]

Bad 47uF caps on pin2 could also make the pitch wander. You'd have to be unlucky to have all them bad, but it can happen.
I've got an ensemble chorus working using x3 PT2399. They run off the same 5v regulator, but it's a TO220 7805. No heatsink & it doesn't get more than a little warm even though the supply volts in this case is +12v. I wouldn't try that with a TO92 78L05 though.

[pinkjimiphoton]

i'm betting its the regulator causing the issue. least common denominator.