Portsdown Power Supplies

Discussion about this major DATV Project. See also https://wiki.batc.org.uk/The_Portsdown_Transmitter
G8GKQ
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Portsdown Power Supplies

Post by 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

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Re: Portsdown Power Supplies

Post by 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

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Re: Portsdown Power Supplies

Post by 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

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Re: Portsdown Power Supplies

Post by 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

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Re: Portsdown Power Supplies

Post by G8GKQ » Fri Nov 24, 2017 3:51 pm

Hi David

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

Dave

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Re: Portsdown Power Supplies

Post by 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
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Re: Portsdown Power Supplies

Post by 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

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Re: Portsdown Power Supplies

Post by 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

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Re: Portsdown Power Supplies

Post by keng8vdp » Fri Feb 08, 2019 10:56 am

Hello All, Interesting read of all the above. Im just about ready to do my 1st test , once ive figured out how everything gets it power. So I dont make any mistakes and blow fizz something before ive even started. ( ive already damaged my pi touch screen and had to buy a new one ).
I would like to have an "idiots guide" to what gets powered from what ?
To make things easier for myself i made my wiring the same colours and the same as here >

https://wiki.batc.org.uk/images/5/53/KL ... 810%29.JPG
and here>
https://wiki.batc.org.uk/File:G4KLB_Por ... G_3067.JPG

I have 3 of the Buck supplies ready to go.
1/ As far as I read I "think" the Pi is powered by one of the Bucks and via that the screen. is that correct ?
2/ I "think" the ADF435x board gets its 5v and 0v from the GPIO board. is that correct ?
Although I doubt that very much as i see NO WIRE in the 5v output position on the GPIO 5 pin PLUG marked ADF4351 ( only 4 wires )
If it doesnt get its 5v from GPIO then should I power it from one of the Bucks ?
3/ I "think" the Filter modulator board gets its power from one of the Bucks ( set at around 8v ) then via its onboard 5v regulator. is that correct ?
4/ Im seeing a 3.3v here On GPIO board here >
https://wiki.batc.org.uk/images/1/12/Po ... nnects.JPG
I see that this is another voltage taken from the pi via GPIO. is this correct ?
5/ Im seeing 3.3 to 5v on the output RF switch that "appears" to come from the GPIO. is this correct ? and 3.3v on the 4 band decoder possibly also taken from GPIO ?

Forgive me but its not totally confusing but on the other hand its not completely clear either. Its open to mistakes for old bald fat blokes like me who easily gets muddled.
maybe theres only me in this state of fading mental capacity or perhaps theres others like me who need a bit of clarification.

Thank you for any replies and your kind understanding

Ken G8VDP

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Re: Portsdown Power Supplies

Post by M0YDH » Sat Feb 09, 2019 11:47 pm

Hello Ken
I've been in your shoes with Portsdown.
Many of us have bought 2 Waveshare screens. The safest bet is to get it cased up in a printed plastic surround [e.g. from radiogareth] ASAP and use a ribbon cable connector.

Colin G4KLB's Portsdown was very influential and I copied it too. It's a great piece of work and won a prize at CAT17 IIRC. Colin has a big shack with lots of desk space so can plug USB items in to the Pi without mounting them in the case. The connections on the side of the case have become a nuisance for me in my rack system. A lot of the precious space on the rear panel is taken up with buck converters. So later features like composite video/audio out and e.g. signal generator port don't feature. The band select board shown has no outputs. My first Portsdown has an accessory box for the powered USB hub to house the gear that didn't fit in. There's a further unit to do Rx/Tx changeover and amp selection.

You've found the interconnect diagram. :D I found it baffling at first! I looked at it today and its seems quite elegant. (Portsdown 2019 thatI put together today only uses 3 main groups from the breakout board - 2 if no attenuator - you're doing the full fat version ;)

The Pi needs a net 5.1 V across it from its own regulator - you are correct. A buck converter or LM338T home brew per the Texas Instruments data sheet do well. A small LCD voltmeter is handy as part of the set up so you can tweak it. Ripple filters can be added - my arrangements came from LM2756 data sheet. G4KLB's has these after each buck converter.
The ADF4351 uses one of the buck converters as it seems to need 6-8 VDC - see Wiki. I've soldered a wire to the back of the power connector housing. Only LE, CLK, DAT and GND come from the Portsdown.
I have a 7805 regulator for the LO filter (and I think FM board too) 5V. It could take supply from the same regulator as the Pi.
The 3.3V line from the breakout board does 4-Band decode and the RF switch.
The FM board is supplied from 12-13.8V supply and has on board voltage regulation. In any event do what G8GKQ says!

A Stripmaster and a crimp tool for Molex KK connectors will help enormously.
I hope this helps.
David M0YDH

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