[Elecraft] Elecraft KX3 IQ out and spectral display pograms

[Alan]

 > 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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