Hi Folks

I discovered PSK31 just before the sunspots faded out. I bought an interface
and worked QRP on 14.070MHz. Had a really good time too, its a fantastic
mode, especially for QRP and those who cant fit yagis into the homestead
(like me).

But a quick question to those more familiar with data modes.

If I connect the interface to my desktop computer, it receives well, but I
cant get clear transmit. It sends full scale white noise no matter how I
adjust the mic gain or audio levels. If I connect up the laptop, same TX
noise problem .... until I unplug the mains charger and run on batteries.
Then its perfect.

My laptop batteries last about 2 hours there days, so I'm limited to 2 hour
long operation until I sort the problem or get a higher capacity battery.

Any thoughts what's causing it?

cheers
Jack VK2CJC / MM0AXL
FISTS #9666
CW Ops QRP Club #753
Mid North Coast Amateur Radio Group
www.mncarg.org

Jack,

The first thing to check is that the interface is FULLY isolated.
Surprisingly, some commercial interfaces use isolation transformers on
only the RX line. Also, not all use fully isolated keying. Check the
manual, or disconnect all the computer inputs to the interface, and
then check with a meter between the interface case and the rig. Should
NOT read a DC short. (Or probably with the laptop connected without
the mains, check between a metal point on the laptop and the rig.
Haven't tried this, but it should work the same.)

If you find a DC ground, it can be easily fixed with an inexpensive
600 ohm audio transformer. The PTT is a little harder, but not much.



--
Alan
WA4SCA

Alan WA4SCA wrote:
Jack,

The first thing to check is that the interface is FULLY isolated.

I don't agree. If the problem was in the interface, it would work
equally badly with the desktop, laptop, or laptop on batteries. The
thing that Jack did to resolve the problem was to disconnect the power
supply from his laptop. This implies to me that the issue is RF getting
into the computer via the AC power line. Or in the case of the laptop,
it could be that the RF is getting into the system because the cable
between the power supply and the laptop is acting as an antenna.

That's the aspect that I would work on. It sounds to me like RF is
getting into the computers. When the laptop is running on battery, it's
sufficiently shielded so that it's able to operate. If the desktop has
a metal case, it might be that bypassing the AC feed would help.
Shielding the cable between the laptop's AC supply and the laptop might
help. Even moving the physical location of the various components may help.

One of the things that tends to poke a hole in this whole theory is the
fact that Alan is running QRP. If there were a lot of RF floating
around the shack, my idea would make more sense. On the other hand,
computer equipment is remarkably sensitive to RF at specific
frequencies, and Murphy suggests that that's where you're operating . . .

One has to start somewhere, and the presence of ground loops can, and
have, resulted in RF and not just audio problems. FWIW.


--
Alan
WA4SCA

Alan WA4SCA wrote:
One has to start somewhere, and the presence of ground loops can, and
have, resulted in RF and not just audio problems. FWIW.

The whole concept of grounding is probably fertile ground for
investigation for this problem.

Thanks for the input guys.

I am using a Buxcomm rascal interface (I had forgotten the make, but have
been reminded since), which apparently has excellent isolation.

The problem occurs equally on all HF bands, so is not really frequency
specific.

If its a stray RF problem, which I am very willing to accept, I'm surprised
that it effects two very differently designed computers in exactly the same
way.

Jack VK2CJC wrote:

If its a stray RF problem, which I am very willing to accept, I'm surprised
that it effects two very differently designed computers in exactly the same
way.

Do they perhaps have the same CPU chip, in spite of their very different
design?

"Alan WA4SCA" wrote ...
One has to start somewhere, and the presence of ground loops can, and
have, resulted in RF and not just audio problems. FWIW.

This is a FAQ in the audio world. It is a very common problem
with getting audio into and out of laptops. Their "brick" power
supplies are notoriously noisy. At AF frequencies, and likely RF,
as well. In the audio world, we use full transformer isolation in
and out. (Or else use an inexpensive USB-powered audio interface.
Likely cheaper than trying to isolate the PSK31 interface.)

"Steve Bonine" wrote ...
Jack VK2CJC wrote:

If its a stray RF problem, which I am very willing to accept, I'm
surprised that it effects two very differently designed computers in
exactly the same way.

Do they perhaps have the same CPU chip, in spite of their very
different design?

Not the CPU's fault. It is almost guaranteed to be the "brick"
power supply. Note that the problem goes away when you
disconnect the "brick" and run on the internal battery supply.

I bought a used "brick" power supply for this Dell laptop on
eBay and it came with an intermittent (3-pin U-ground) mains
power plug. So I chopped it off and replaced it with an after-
market, 2-prong plug and I don't have problems with ground
loops and RFI from the garbage power supply anymore.

Yeah, yeah, green-wire "protective ground", etc. etc. Well,
I don't see what exactly the "protection" here is doing, and
if the green-wire ground WAS supposed to be there for RFI
shielding purposes, then why does it *go away* when I
disconnect the green-wire ground? Riddle me that! :-)

Yeah, yeah, green-wire "protective ground", etc. etc. Well,
I don't see what exactly the "protection" here is doing,

Well it will be double insulated so doesn't really need an earth at all.
I'm surprised there was one.

Jack