[TenTec] RF susceptibility

[Dr. Gerald N. Johnson]

On Wed, 2006-11-08 at 15:07 -0600, Stuart Rohre wrote:
> Any susceptibility to RF that gets back into the rig, starts with the 
> quality of shielding (or not) of the headphone cords.  Many modern 
> headphones do not even use shielded cords, nor twisted cords like the 
> communications phones of yesteryear.  They have thin plastic housings 
> offering no shielding to the transducer coils.
> 
> In modern Audio output circuits, (of the transformerless type), you can not 
> put a bypass cap for RF on the speaker or headphone cord, lest it cause a 
> high frequency condition in the Wideband IC audio amp chip.

If that's a problem, the device needs better roll off control, but you
can get RF bypassing and HF roll off with a small resistor, say 8 or 10
ohms in series with the bypassing capacitor.

>   In the old 
> days, almost every lead leaving a chassis would simply have had  a disc 
> ceramic cap for RF suppression.  Ferrites cost more than ceramic caps, thus 
> are reserved to be added by the user "if needed".
> 
> Using balanced antennas, some distance from a properly laid out shack, 
> usually results in no RF in the shack problems.  Some folks are more 
> fortunate in this than others.  A shotgun approach to RF suppression ups the 
> price.  RFI work usually has to be reactive to the problem, if any.  (As 
> much as those of us who worked in RFI/ EMC would wish designers to think of 
> all RF issues at the design stage).
> 
> Stuart
> K5KVH 
> 
> 
> _
-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer