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| Reducing noise on 5V rail from RAM on old computers |
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| llopis:
I'm working on a ZX Spectrum (128K model) and I'm noticing quite a bit of noise in the 5V rail, which is affecting the video output. I started by adding some extra bypass capacitors near the ULA (where the graphic signal is created) and it helped a lot. Still, I would love to reduce some of that noise at the source. I suspect it's mostly coming from the RAM (16 x 4164 DRAM). They each have a 22nF bypass capacitor, so I could do something similar and add something like a 1uF capacitor, but I was wondering if there's something better I could do. For example, the 5V line that goes to the DRAM is very easy to break off from the rest of the 5V on the board. Could I isolate it by connecting it with a diode for example, to prevent any signal from traveling backwards? I see some models of ZX Spectrums doing that with the 12V line (one line for the DRAM, another line for the video chip for example) by using a couple of diodes and a couple of capacitors (so I suspect it's not just as simple as throwing a single diode). Like this: Dealing with noise in circuits is something I know very, very little about. What would you recommend? |
| RoGeorge:
Diodes won't help. Put 0.1uF right near each chip, as close as possible (sometimes I was soldering them right on the chip pins, on the back of the PCB). Also, replace all the electrolytic capacitors, they are probably old, dry and high ESR. Usually it's the chips that are producing the noises/spikes on the power lines, not the power supply. |
| Vovk_Z:
We use small inductors (several uH) to isolate different parts from each other (but not diodes). Use Low-ESR electrolytes (solid polymer ones) and bulky X7R ceramics as a bypass instead of that old ones. |
| llopis:
--- Quote from: RoGeorge on April 28, 2020, 05:14:47 pm ---Diodes won't help. Put 0.1uF right near each chip, as close as possible (sometimes I was soldering them right on the chip pins, on the back of the PCB). Also, replace all the electrolytic capacitors, they are probably old, dry and high ESR. Usually it's the chips that are producing the noises/spikes on the power lines, not the power supply. --- End quote --- Right. That's why I was looking to isolate the RAM chips, so their noise wouldn't affect the rest of the board. Electrolytic capacitors are fine on this board, so that's not an issue. Where does the 0.1uF recommendation come from? Is it just a generally-used value? Right now those chips have 22nF. I was thinking of adding a 0.1uF or even 1uF in parallel. It seems that adding a bypass capacitor in parallel would be better than simply replacing the existing one, right? |
| RoGeorge:
Yes, adding another one would be better than replacing. The value is generic, can be anything of about 10...100nF low ESR, made for DC bypassing, like MLCC type. Sometimes you will see many values in parallel, 10nF, 47nF and 100nF soldered together, because each value has a resonance at a different frequency. 1uF is too big, OK only if this value is the only one you have available, but not recommended. If the RAM chips have +12V and -12V, too, (soem have only +5V), then add 0.1uF between -12V and GND, and +12V and GND, too, near each chip, like you did for the +5V and GND. Same on the +5V of the Z80, and near the video RF modulator, if there it has antenna out. If the electrolytic capacitors are the original ones, just replace them with the same capacitance and the same voltage, preferably Low ESR, as Vovk_Z told you, too. Don't waste time measuring the old electrolytics, they are very cheap to buy new ones, just replace them. Even if they seem reasonably good today, later they can leak or dry out. |
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