[TenTec] Paragon II RS-232 noise

[Dr. Gerald N. Johnson]

On Sat, 2009-01-24 at 13:10 -1000, Ken Brown wrote:
> Thanks to all for the suggestions for finding the cause of and fixing 
> the RS-232 noise in my Paragon II.
> 
> > Is the noise you are experiencing still there when you pull the RS232 
> > Interface Card?
> Have not tried pulling the card. Turning off the RS-232 communications 
> makes the noise go away, either by stopping it through the software that 
> runs it, or by disconnecting the interface cable, so I am sure removing 
> the RS-232 board would also stop it.
> 
> > Maybe just needs 
> > reinstallation to see if the card connector needs cleaning. A bad ground 
> > perhaps? Or a poor connection at the cabinet rear connector?
> >   
> > I'd bet on a "pin 1 problem" at one or both ends of the cable.

Pin 1 is chassis ground. Some devices have neglected to connect it and
that can compromise the cable shielding.
> >   
> > Sounds like the computer and the rig are not at the same AC ground  
> > potential. That is, if you measure the AC voltage between those  
> > grounds, you may be surprised to see tens of volts.
> I'll try that, however it is the same computer, same port, same cable 
> and same grounding and power connections that I had been using with the 
> Omni VI, which had no detectable noise at any baud rate setting. The 
> Paragon II has a fixed rate of 1200 baud, and each data burst lasts 
> longer than with 4800 or 9600 baud I was using with the Omni VI. That is 
> only four to eight times longer, so I would have still heard a buzz and 
> not just a click if the Omni VI generated a noise by the same mechanism.
> > Also look at the simple bonding recommended between the chassis of 
> > the rig and the chassis of the computer.
> Again, since the only thing that has changed is the radio, and none of 
> the cables or the computer have changed, I think any "pin 1 problem" or 
> grounding issue causing this would have to be inside the Paragon II.

Its certainly likely. Then there are RS-232 drivers and RS-232 drivers.
The rules of RS-232 require a certain rise time to minimize noise
generation. But there are "RS-232" drivers that will drive that rise
time with a 0.1 capacitor on the signal line. Without a slowing
capacitor, they have a very fast rise time but will work to 115.2 Kbaud
where the original RS-232 specification only expected serial speeds to
9600 or 19.2 Kbaud.

So you need to check the grounding on pin 1 for the cable shield. And on
pin 7 for the signal ground. The if that's not a complete cure you may
need to add some capacitance to ground on the signal pin coming out of
the Paragon II, say 1000 to 5000 pf to slow the rise and fall times and
so reduce the harmonics.

RS-232 signaling will work without pin 1 grounded, but if pin 1 isn't
grounded on either end the cable works more as an antenna. For RF
shielding pin 1 should go to chassis with a short lead at both ends of
the cable.
> 
> > Could be RFI coming from 1 or more places to there.
> Not a single RF or IF control in the rig makes any change in the noise 
> from the RS-232 communications, so it may not be RF noise at all. At 
> least it is not radiated RF noise getting picked up by RF or IF stages 
> in the rig. Only the AF volume and tone controls change the sound of the 
> noise. I'm more inclined to believe it is getting into the audio stages 
> though the DC power distribution, or due to a grounding fault or "pin 1 
> problem."

So maybe pin 1's ground is shared with the audio board's ground and
there's too much mutual impedance that is supplying the coupling.
> 
> Not going to get resolved until after the 160 CW contest. I'll let you 
> know what I find.
> 
> Thanks again for the suggestions.
> 
> DE N6KB
> 
73, Jerry, K0CQ