K3 serial cable shielding / noise question

[Paul Fletcher]

Hi list,

Bit of a strange issue here with serial comms between the PC and the K3. First a bit of description about my set up: I use an external noise sense antenna in addition to the main station antenna to cancel out some nasty local electrical noise with a Timewave ANC-4 noise phasing unit. This is connected to the RX out / RX in jacks on the transverter interface (this is a great feature).

The issue I have is this: when I use a USB serial adapter I get no additional noise issues when the computer is polling the rig. However, my computer has a serial port which I would rather use. I made up a cable with pins 2, 3, 4, 5, and 6 straight through and the cable shield soldered to the shell of the d-type that connects to the K3. When I use this cable the radio picks up bursts of noise from the noise sense antenna (which is not good if I'm using it to null out other local noise).

This is obviously radiation from the serial cable but I'm at a loss as to why (incidentally also tried some ferrites on the cable and they made no difference).

Any suggestions?

Thanks Paul M1PAF

[Julian, G4ILO]

M1PAF wrote

Hi list,

Bit of a strange issue here with serial comms between the PC and the K3. First a bit of description about my set up: I use an external noise sense antenna in addition to the main station antenna to cancel out some nasty local electrical noise with a Timewave ANC-4 noise phasing unit. This is connected to the RX out / RX in jacks on the transverter interface (this is a great feature).

The issue I have is this: when I use a USB serial adapter I get no additional noise issues when the computer is polling the rig. However, my computer has a serial port which I would rather use. I made up a cable with pins 2, 3, 4, 5, and 6 straight through and the cable shield soldered to the shell of the d-type that connects to the K3. When I use this cable the radio picks up bursts of noise from the noise sense antenna (which is not good if I'm using it to null out other local noise).

This is obviously radiation from the serial cable but I'm at a loss as to why (incidentally also tried some ferrites on the cable and they made no difference).

Any suggestions?

Thanks Paul M1PAF

Do you get this noise if you connect an audio cable between the computer and the K3? This would help decide whether the noise is coming from the computer itself (e.g. poorly suppressed power supply) being coupled to the radio via the shield, or if it is being generated by the serial adapter in the PC.

[Paul Fletcher]

G4ILO wrote

Do you get this noise if you connect an audio cable between the computer and the K3? This would help decide whether the noise is coming from the computer itself (e.g. poorly suppressed power supply) being coupled to the radio via the shield, or if it is being generated by the serial adapter in the PC.

I'll try that. What makes me think it's the serial port is that connecting the serial cable doesn't increase the noise level, it's only when the computer is polling the radio (i.e. traffic over the cable). I've already addressed general computer hash and that's down to an inaudible level. I'm sure it's a screening issue and could be down to the quality of the screen on the cable I'm using.

Cheers,
Paul M1PAF

[Julian, G4ILO]

M1PAF wrote

I'll try that. What makes me think it's the serial port is that connecting the serial cable doesn't increase the noise level, it's only when the computer is polling the radio (i.e. traffic over the cable). I've already addressed general computer hash and that's down to an inaudible level. I'm sure it's a screening issue and could be down to the quality of the screen on the cable I'm using.

You are probably right, then. Looks like you need one of these http://www.coilcraft.com/rs232.cfm . Can't find a price anywhere, though.

[Thomas Norff]

Hi Paul,

i do have the same issue from the beginning.
Only connected to the computer makes no problems.
But as soon as the computer starts with data transfer you can hear that.

Disconnecting the interface stopped that 'noise'.
In addition the normal signal gets louder. Which could be the AGC...

What i haven't tried is to separate the usb/rs232 interface from the trx
with an additional cable.
Maybe the internal signals are what we notice ?

73 de Thomas, DM7TN

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of M1PAF
Sent: Thursday, March 20, 2008 2:06 PM
To: [hidden email]
Subject: Re: [Elecraft] K3 serial cable shielding / noise question



G4ILO wrote:
>
> Do you get this noise if you connect an audio cable between the
> computer and the K3? This would help decide whether the noise is
> coming from the computer itself (e.g. poorly suppressed power supply)
> being coupled to the radio via the shield, or if it is being generated
> by the serial adapter in the PC.
>
I'll try that. What makes me think it's the serial port is that connecting
the serial cable doesn't increase the noise level, it's only when the
computer is polling the radio (i.e. traffic over the cable). I've already
addressed general computer hash and that's down to an inaudible level. I'm
sure it's a screening issue and could be down to the quality of the screen
on the cable I'm using.

Cheers,
Paul M1PAF
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[Alan Bloom]

On Thu, 2008-03-20 at 01:39, M1PAF wrote:

> The issue I have is this: when I use a USB serial adapter I get no
> additional noise issues when the computer is polling the rig. However, my
> computer has a serial port which I would rather use. I made up a cable with
> pins 2, 3, 4, 5, and 6 straight through and the cable shield soldered to the
> shell of the d-type that connects to the K3. When I use this cable the radio
> picks up bursts of noise from the noise sense antenna (which is not good if
> I'm using it to null out other local noise).

You might try grounding the shield to the shell at the opposite
(computer) end.  It is normally recommended to ground the shield at only
one end, but you might try it both ways (computer end only or both ends
connected).

Al N1AL


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[Paul Fletcher]

M1PAF wrote

This is obviously radiation from the serial cable but I'm at a loss as to why (incidentally also tried some ferrites on the cable and they made no difference).

Problem solved. In spite of my "experience" ho ho - the percieved wisdom to ground the shield at one end didn't work (either end). Connecting the shield to the connector shells at both ends fixed the problem. Usually I wouldn't recommend this due to potential ground loop problems etc. but no more noise (and no ferrites needed either)

Thanks for all the suggestions.

Cheers,
Paul M1PAF