Reducing noise on 5V rail from RAM on old computers - Page 1
Forum section: electronics / projects

author: llopis
author_rank: Contributor
timestamp: April 28, 2020, 04:21:14 pm
content: 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?

author: RoGeorge
author_rank: Super Contributor
timestamp: April 28, 2020, 05:14:47 pm
content: 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.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 28, 2020, 05:16:35 pm
content: 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.

author: llopis
author_rank: Contributor
timestamp: April 28, 2020, 06:08:09 pm
content: Quote from: RoGeorge on April 28, 2020, 05:14:47 pmRight. 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?

author: RoGeorge
author_rank: Super Contributor
timestamp: April 28, 2020, 06:31:09 pm
content: 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, preferablyLow 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.

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 28, 2020, 06:41:58 pm
content: I'd try LC filters to begin with (inductor in series + cap to ground).Since you seem to have specific issues with the video signal, I would just make sure the video circuitry gets a cleaner supply, and not bother with the rest.

author: llopis
author_rank: Contributor
timestamp: April 28, 2020, 08:53:09 pm
content: Thanks everybody! In addition to adding a couple more capacitors near the video chip (I didn't have any inductors on hand), I added some small caps (0.1uF) to the RAM and that helped a little.So back to my question about isolating the 5V rail of the RAM. The 5V come from a 7805 regulator and is fed through the whole board. Is there a reason not to put a second 7805 and use it exclusively for the RAM (isolating that track from the rest of the board)? As long as both regulators are powered by the same input voltage and ground, I don't foresee any problems, right? I believe the Commodore 64 does something like that powering its VICII chip. Any problems I should anticipate by doing something like that, or does its seem safe?

author: DaJMasta
author_rank: Super Contributor
timestamp: April 28, 2020, 10:01:39 pm
content: It doesn't make sense to isolate the rail from the RAM, you've got a whole digital system and an analog element, so it's much simpler/more targeted to filter/isolate the power to the analog output section from the rest of the digital lines.One way would be to cut the 5V rail to the analog portion and just have a second regulator for that section alone that gets its input from whatever the other 5V rail gets its source from (trickier if external or directly driven by a switcher). Another could be the LC filtering described but between the rail with all the digital stuff and the analog portion. If those aren't options, beefing up the bypassing (like you've already done) can likely still help. 22nF is under the normal go-to recommendation for a lot of chips, so an additional 100nF to 1uF cap is probably going to be helpful (and smaller ones are probably not beneficial given the lower speed of the chips on the board). Bypassing the analog portion could also be of some help, probably more with bulk capacitance (like an electrolytic) nearby, but it's probably going to be more effective to either isolate the analog section (its own regulator or LC filtering) or trying to reduce the noise at the source with additional bypassing there. And though it's been said, proximity matters for your bypassing, so as close as you can get to the power pin is best.

author: thinkfat
author_rank: Unknown
timestamp: April 28, 2020, 10:37:53 pm
content: The circuit around TR4 is actually a DC/DC converter (boost converter) that converts 9V from the power brick to +12V. The diodes are part of that functionality, not for noise reduction.