Receivers and power-line noise

[Andre' Kesteloot]

Hans-Joachim Brandt wrote:

> Dear Kevin,
>
> so far I have not encountered problems with rfi from the mains supply. But I
> like to block every rectifier diode with several nanofarad in all my home brew
> power supplies (this also will suppress the 50 Hz hum in direct conversion
> receivers) but I have also found that this is seldom done in commercial power
> supplies and therefore may cause problems, even from our neighbours.
>
> Concerning battery supplies I have also found that typical voltage stabilizer
> ICs like 78xx, even when blocked according to manufacturers recommendations
> (with 0,33 to 0,47 uF) will generate LF noise at the input side, unless the
> input is blocked more heavily with 6,8 uF to 10 uF low loss electrolytics.
>
> 73 Ha-Jo, DJ1ZB
>
> Kevin Ravenhill schrieb:
> > Hi all,
> >
> > This may well have been mentioned before on this reflector, so apologies to
> > those concerned if it's going back over old ground.
> >
> > For anyone plagued by power line "hash" when receiving on LF, particularly
> > when using wire antennas, it's worth checking the cleanliness of the DC
> > power supply which is being used to power the receiving setup.
> >
> > I have found out the hard way that the rectifier diodes in some power
> > supplies can generate truly enormous amounts of harmonic hash at these
> > frequencies. I have not checked too closely but I assume this is simply
> > being re-radiated through the transformer and into the mains wiring. There
> > is EC EMC legislation relevant to this aspect of power supply design, but
> > I'm not sure how effective this is in "special cases" such as our very weak
> > signal conditions.
> >
> > I have tried several power units ranging from simple regulated "plug-top"
> > types to more complex heavy current 13.8V PSUs specifically designed for
> > powering radio gear. I currently use a small double-insulated (i.e. no mains
> > earth connection) plug-top PSU to power my receiver. This was originally so
> > bad that it completely wiped out reception on my AKD HF3 below about 2MHz
> > (note that this PSU was actually intended for this receiver!). In this case
> > the problem was cured by the simple expedient of fitting a 0.1uF ceramic
> > capacitor across the transformer secondary feed to the bridge rectifier, but
> > this is not necessarily a cure for all situations. Lots of lossy ferrite
> > etc. on the mains side may be called for!
> >
> > My Daiwa 13.8V 12A unit is much better in this respect but does still
> > introduce a small level of extra noise. This is not really significant
> > enough to warrant modification though. A slightly cheaper grade of 13.8V 5A
> > PSU was tested and this proved too noisy to be usable without extensive
> > modification.
> >
> > Finally, I have a multi-output variable voltage lab-type PSU for bench work
> > which has also proved too noisy to power the LF gear as it stands.
> >
> > I suppose the surest way to eliminate this kind of noise would be to run the
> > receiving gear from batteries, which would also get round the problem of
> > noise directly coupled from the mains. I would be interested to know how
> > many other people have encountered similar problems with LF reception.
> >
> > 73
> >
> > Kevin, G1HDQ
> >
> >