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
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