> Why? It's an unbalanced interface!
Yes, that's the problem. The power supply in the computer probably has
bypass/filter capacitors between the line and the chassis. So with an
unbalanced connection to the KX3, there will be 60 Hz (and harmonics) AC
current flowing in the cable shields. Even a fraction of a millivolt
voltage drop in the shield is enough to cause a visible 60 Hz spur on
the panadapter display.
The voltage drop can be reduced by connecting a short, low-resistance
wire between the two chassis. I suggested making the connection on the
computer end to the audio connector shells to make sure you're
connecting to the ground reference of the sound card, which might be
different from other points on the chassis. That may have been bad
advice - I'm not an expert on the construction of a typical PC chassis.
But I think isolation transformers would be an even better solution.
That should completely eliminate hum caused by different voltages on the
two chassis.
The PX3 uses differential sensing on the I/Q inputs instead of isolation
transformers. In effect, the PX3 ground reference is the KX3 chassis,
rather than the PX3 chassis. That way, even if there is a 60 Hz voltage
difference between the two chassis, it won't cause interference.
> This is smelling VERY much like Pin One Problems.
For those who aren't familiar with the term, the so-called "pin one"
problem is when the ground pin on a connector is not connected directly
to the chassis but rather via a wire, PC board, etc. I believe the name
comes from audio XLR connectors, which use pin 1 as the ground.
The pin one problem normally shows up at RF frequencies because of the
inductance of the internal ground connection. But the issue under
discussion is 60 Hz interference, so I doubt that that's the problem.
Alan N1AL
On 12/20/2014 07:36 PM, Jim Brown wrote:
On Sat,12/20/2014 4:57 PM, Alan wrote:
Perhaps the problem is due to noise on the computer ground. The PX3
has differential amplifiers at the I/Q inputs.
Why? It's an unbalanced interface!
For the computer-based panadapter, it might help to connect the
grounds of the computer audio input connectors to the KX3 ground with
a short, large-diameter wire.
PLEASE study my tutorial on Bonding, previously linked. What should
be bonded are the CHASSIS of all the interconnected equipment.
Another possible solution would be to use audio isolation
transformers on the I and Q signals to isolate the grounds. That will
cause a notch in the panadapter response. The notch width will be
twice the transformers' low-frequency cutoff frequency.
This is smelling VERY much like Pin One Problems. The KX3 has Pin One
Problems at the connectors not screwed down to the chassis. The only
connectors screwed down to the chassis are mic connector and the BNC
antenna connector. On the PX3, two of the I/O connectors are not
screwed down to the chassis. THESE ARE PIN ONE PROBLEMS.
The ONLY proper connection point for a cable shield the is shielding
enclosure. When a cable shield is insulated from the shielding
enclosure and goes through an enclosure penetration to the circuit
board, that cable, and that equipment are UNSHIELDED, and the cable
shield conducts shield current into the equipment.
PLEASE study my material on The Pin One Problem. Many users would be
spared a lot of grief if gear was built without Pin One Problems.
k9yc.com/publish.htm
And please excuse my frustration about this. Thanks to the late Neil
Muncy, W3WJE, we discovered and fixed our Pin One Problems in the pro
audio world back in 1994. Ham gear is full of Pin One Problems, and
I've been trying to teach ham manufacturers about this issue for ten
years. I first discussed this with Wayne in Dayton in 2004 or 2005.
It's almost 2015, and Elecraft products are still being built with Pin
One Problems.
73, Jim K9YC
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