Portsdown Power Supplies

Discussion about this major DATV Project. See also https://wiki.batc.tv/The_Portsdown_Transmitter
G8GKQ
Site Admin
Posts: 594
Joined: Sun Mar 22, 2009 1:21 pm

Portsdown Power Supplies

Postby G8GKQ » Fri Nov 24, 2017 9:43 am

Ian asked on FaceBook about Portsdown Power Supplies. I'll answer here as this is the best media for technical Q&A - FaceBook is great for photos and success stories.

I recommend the use of buck converters to supply 5.2v to the RPI, LCD (through the RPi), ADF4351 module, LO filter RF Output switch and 4-band switch. No extra filtering required. I use a linear converter (the 7805) to supply 230 mA to the Filter-modulator board (needs heatsinking).

You can use a buck converter to supply the filter-modulator board, but the switching ripple does introduce some detectable sidebands on the transmitted signal. These have very little adverse effect, but offend me! We have tested on-board switching regulators, and the pads are there for L10 to provide extra filtering. Again, they introduce low-level sidebands that offend me, but are probably not significant.

The most important thing about the power supplies is that the RPi should be fed with a low impedance 5.2v supply which does not drop voltage under load. Most commercial micro-USB leads should be cut at 5 cm from the plug and connected to real wiring going to the buck converter.

Hope that helps

Dave, G8GKQ

M0DHP
Posts: 30
Joined: Sun Sep 24, 2017 3:25 pm
Location: Thames Ditton IO91TJ

Re: Portsdown Power Supplies

Postby M0DHP » Fri Nov 24, 2017 10:45 am

G8GKQ wrote:I use a linear converter (the 7805) to supply 230 mA to the Filter-modulator board (needs heatsinking).

You can use a buck converter to supply the filter-modulator board, but the switching ripple does introduce some detectable sidebands on the transmitted signal. These have very little adverse effect, but offend me!


Dave, I've gone for 7805 on the filter mod board fed with 8V (to reduce power dissipation in 7805) from a buck converter (down from 12V overall supply). Do you think the 7805 will mitigate the ripple from the buck converter?

Thanks and 73
Ray M0DHP

G8GKQ
Site Admin
Posts: 594
Joined: Sun Mar 22, 2009 1:21 pm

Re: Portsdown Power Supplies

Postby G8GKQ » Fri Nov 24, 2017 2:26 pm

Hi Ray

Yes, as long as you have over half a volt of headroom (I'd set it for 1v) above where it starts regulating, the 7805 should filter out the ripple nicely.

Dave

M0YDH
Posts: 67
Joined: Sat Dec 31, 2016 5:59 pm
Location: Wolverhampton IO82WO

Re: Portsdown Power Supplies

Postby M0YDH » Fri Nov 24, 2017 2:51 pm

Didn't you mean 9VDC max to the ADF4351 oscillator card Dave ? Mine definitely didn't work at 5VDC across it . See Fuse Rating topic elsewhere on this Forum.
Cheers
David

G8GKQ
Site Admin
Posts: 594
Joined: Sun Mar 22, 2009 1:21 pm

Re: Portsdown Power Supplies

Postby G8GKQ » Fri Nov 24, 2017 3:51 pm

Hi David

My ADF4351 surprised me by running OK on 5v. 9v is probably safer.

Dave

G4FRE
Posts: 10
Joined: Fri May 26, 2017 6:57 am
Contact:

Re: Portsdown Power Supplies

Postby G4FRE » Fri Nov 24, 2017 9:43 pm

Be careful with regards to the dropout voltage of the 7805. According to the manufacturers data sheet it needs a minimum of 2V across it to maintain regulation (under all load conditions). From past experience you need 3V to be sure.

For this reason I set my switching supply to 8V output.

Dave
G4FRE

g0mjw
Posts: 330
Joined: Sat Sep 20, 2014 9:15 am

Re: Portsdown Power Supplies

Postby g0mjw » Sat Nov 25, 2017 8:47 am

I suppose it depends which board you have and which we are discussing as there are several.

The LO is based on an ADF4351 is a 3 - 3.6V device. A 3.3V regulator would appear appropriate. I think it takes 100-200mA depending on how it is configured. If the on-board regulator is AMS1117-3.3 (like in the $15 board from Banggood), these operate down to 1V dropout, maximum 1.3V. This is well suited to a 5V power supply. Running it of a typical 13.8V supply means the regulator could end up dissipating 2W and the SOT-223 package could not handle this. Running it of 8V reduces that to 1W but its still a bit marginal on dissipation. I would recommend 5V. As suggested above, it seems you can get away with running it off the same supply as the PI. However, if you have a different board with a different regulator, it may need more.

The filter modulator board uses a 7805. I simply used the larger heatsink and ran it off 12V. It seems to work. I originally had a DC-DC converter in there, but concluded it just wasn't worth worrying about the ripple. The old trick of an appropriate series power resistor will deal with any dissipation problems without generating extra noise. Also putting a series diode in-line with the 12V supply is a good plan as it will provide some protection against supply reversal. None of us ever wire our supplies incorrectly, but someone else might.

For the PI, I use a DC-DC converter set to 5.1V. It is wired directly to the PI, on the underside of the PCB bypassing the USB connection but not the polyfuse. Easy to do and this has completely fixed the issue of the PI complaining about undervoltage.

Mike

M5TXJ
Posts: 44
Joined: Wed Jul 05, 2017 8:34 pm

Re: Portsdown Power Supplies

Postby M5TXJ » Sat Nov 25, 2017 9:28 am

Has anyone actually measured the buck converters for ripple? I might stick the 'scope on mine later even though I have absolutely no problems, I have wired all power feeds as twisted pair as is my norm.

73 Dave


Return to “The Portsdown Digital Transmitter Project”

Who is online

Users browsing this forum: No registered users and 2 guests