Power Supply Noise

Discussion about this major DATV Project. See https://wiki.batc.org.uk/The_Portsdown_Transmitter
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gordon
Posts: 27
Joined: Sun Jan 21, 2018 6:52 pm

Power Supply Noise

Post by gordon » Fri May 04, 2018 4:01 pm

Hi,
Just for curiosity, I decided to look at the power supplies with scope. My Portsdown is fed from a 18V battery from which is derived the various voltages to power it. The RPi is fed from an Ebay type regulator set to 5.1V. 3 78XX chips feed the other units with 12, 8 and 5 volts respectively.First I measured the 5.1V supple close to the RPi. It was showing around 100mV of a spiky signal which, my scope said, was at 200KHz or so.Next I look at the 8V (modulator) supply. Here is a more sinusoidal display, also about 100mV. Now for the shock. The input (18V) supply was showing 400mV of spiky noise. This puzzled me as , normally, regulators reduce noise considerably, especially when decoupled correctly.
I should add that the RPi was not in transmit mode when these measurements were made.I should also add that it all works perfectly so am I worrying over nothing?

Comments appreciated, thanks, Gordon

G8GKQ
Site Admin
Posts: 2798
Joined: Sun Mar 22, 2009 1:21 pm

Re: Power Supply Noise

Post by G8GKQ » Mon May 07, 2018 12:02 pm

Hi Gordon

As long as the 5 volts on the modulator board is clean, and the RPi is working properly, nothing to worry about. The eBay type supplies do feed a lot of noise back to their input.

Dave

M0YDH
Posts: 264
Joined: Sat Dec 31, 2016 5:59 pm

Re: Power Supply Noise

Post by M0YDH » Mon May 07, 2018 6:12 pm

I saw that Colin G4KLB had ripple filters after all his Portsdown voltage regulators [see Wiki] and copied what I found in the LM2756 datasheet. 100µH on output followed by 1000µF to ground. It didn't do any harm anyway....

73

David M0YDH

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