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

Heinzmann, Stefan (ALC NetworX GmbH) Stefan.Heinzmann at alcnetworx.de
Thu Jul 22 03:43:12 EDT 2010


The way these caps are wired does not isolate DC from the transformer. The purpose is obviously a different one. Two reasons spring to mind:

1. If the caps are wired close to the connector, and Ground means the metal enclosure, then the caps provide RF continuity between cable shield and enclosure. That keeps RF interference out of the circuit.

2. They reduce the common mode signal coupled into the circuit via the parasitic capacitance between the transformer windings. This should help isolation.

Cheers
Stefan


-----Ursprüngliche Nachricht-----
Von: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] Im Auftrag von Geoffrey Smith
Gesendet: Donnerstag, 22. Juli 2010 06:13
An: Discussion of precise time and frequency measurement; Geoffrey Smith
Betreff: Re: [time-nuts] Function of cap to GND in isolation transformer circuit

Joop,
The cap is isolate any DC from the transformer,  The manual for TADD-1 has
this iformation at the TAPR site has this manual
http://www.tapr.org/~n8ur/TADD-1_Manual.pdf. 


On Wed, Jul 21st, 2010 at 9:46 PM, Joop <lous at xs4all.nl> wrote:

> 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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> 




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