On Feb 4, 3:55 am, Andrew VK3BFA wrote:
On Feb 4, 10:13 pm, Leon wrote:

On 28 Jan, 11:27, "Richard" wrote:

I have an ex AM PMR radio that I wish to use on 144Mhz. I need a PSU feeding
24V @ about 300mA.

Are switch-mode powers supplies alright or are they too noisy to be used for
receivers? TIA.

I use a 12V switcher with my FT-817D, it's very quiet.

Leon

You were lucky - you got a quiet one. And its the luck of the draw,
unfortunately.

How come we have all been conned into these switchmode things? - by
their very nature, without a lot of effort, they are noise generators.
Their cheap to make, thats their only advantage - everything else is
part of the spin cycle. And if it works on 2m and above, fine - you
will just be adding to the already horrendous RF pollution from
thousands of the bloody things already on HF - the suburbs are getting
unbearable, S9 plus noise, every night, on 80m.....160 aint to crash
hot either...

I refuse to have to work on a power supply to get it quiet enough to
be used with a radio. I am interested in the radio, not fixing
shortcomings in design of things that , by their very nature, are
unsuitable to be used near radios. That sucks. Linear technology is
far easier and quieter...

Andrew VK3BFA.

There's another advantage of a switching supply that is very important
to some of us: efficiency. The receiver I've been working on runs
from 5VDC, but I have to supply power at 1.0V, 1.2V 1.8V, 2.5V and
3.3V, and each one of those uses a switcher. A linear regulator to 1V
from 5V will result in a supply that's 20% efficient. The switcher
I'm using for that supply is about 87% efficient under its operating
conditions. If I tried to run all my supplies with linear regulators,
I'd have to supply over twice the power; it would come close to
violating the power rating of the 5V supply, and power dissipation in
the receiver would result in excessive heat rise.

But even with all those switchers, the only spurs greater than about
-145dBm (antenna-input-referred) and greater than 1.5MHz are from
digital circuits, not the power supplies. -145dBm is, I believe, 20dB
_below_ S1 by usual definition of the S-meter units. Even the power
supply fundamentals are only at about S1. And the supplies are not
themselves shielded. (The receiver RF circuitry is, of course, as is
the whole receiver module.)

I would be much more circumspect about using a switching supply in a
homebrew receiver that was going to be used where power dissipation
wasn't an issue, but then I'd do a lot of things differently than I
have for such a receiver. It wouldn't keep me from using switchers;
I'd just look at the whole design in a different way. On the other
hand, finding ones that would be adequately quiet for a 144MHz
receiver should not be difficult at all.

Cheers,
Tom