[time-nuts] Function of cap to GND in isolation transformer circuit

Dave Brown tractorb at ihug.co.nz
Wed Jul 21 09:05:40 EDT 2010


Its to prevent 'earth loops'  and avoid issues with DC and low 
frequency AC on the coax screen - usually its found only on tx or rx 
end- not both- depends on the installation which practice is followed. 
Telcos tend to solid ground at the send end and cap ground at the rx 
end.  And yes- it can make things worse-again depending on the 
installation. If you have the option of hard ground or cap ground at 
both ends you can select which combination gives the best result. 
This is usually only an issue with long runs of coax cable.
DaveB, NZ

----- Original Message ----- 
From: "Joop" <lous at xs4all.nl>
To: "Joop" <lous at xs4all.nl>
Cc: <time-nuts at febo.com>
Sent: Wednesday, July 21, 2010 11:46 PM
Subject: [time-nuts] Function of cap to GND in isolation transformer 
circuit


> Hi,
>
> I noticed in several circuits that the 10MHz isolation transformer 
> in
> input and output circuits have a 6.8nF or 10nF capacitor to GND. How
> necessary is this for suppression of unwanted signals? Is the
> transformer itself not sufficient? I would expect common mode issues 
> to
> be a bit worse with the cap in place.
>
> The circuit I refer to can be seen here:
> http://www.uploadarchief.net/files/download/cap2gnd.png
>
> The first one is an output as described in the Efratom FRK manual, 
> the
> second one the input in the TADD-2 manual.
>
> Cheers,
>
> Joop
>
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