[time-nuts] SMD TADD-1 distribution amplifier

Charles Steinmetz csteinmetz at yandex.com
Sat Dec 19 01:59:45 EST 2015


Chris wrote:

>You are going to get at least a little circulating ground current because
>of power supply parasitics

There is a reason why some power transformers cost $385 and others 
with similar basic specs (voltage, current) cost $22.  Properly 
designed instruments, radios, medical and other equipment (even 
quality audio and video gear) that must interoperate in adverse 
circumstances use the expensive ones, for good reason.

>I agree that for the typical test equipment case where all the gear is
>running from the same power feed it likely should not be necessary.  But
>putting the connectors on opposite sides of the PCB is still just asking
>for trouble.

I think those are two different things (ground currents and RF 
currents).  If the shields of all of the incoming and outgoing signal 
connectors are bonded firmly to a small area of the metal enclosure, 
and the enclosure is an effective shield at the RF frequencies of 
interest, there will be very little to no RF current on the shields 
to be drawn across the PCB.  If the internal power supplies are 
single-point grounded there, as well, and the parasitics are kept low 
and balanced (you used the $385 transformer, and best design 
practices), no mains power-related current will be originated on the 
shields by the device.

Even a moderately complex instrument has signals coming and going not 
just to different areas of one PCB, but to several (or even dozens) 
of different PCBs that may use different power supplies.  I'm not 
saying it's a bad idea to put each PCB's IOs in one small area of the 
card, but that is just one way to get where you need to be.

To maximize the probability that the conditions above are met (in 
particular, to maximize the effectiveness of the enclosure as a 
shield), you can add ferrite common-mode chokes to both the internal 
and external coax cables feeding the IO connectors.  Use lots of 
fasteners to assemble the enclosure, make sure each one provides good 
metal-to-metal bonding (use masks when the various enclosure parts 
are painted), and use RFI gaskets, spring fingers, wire mesh shaft 
seals, etc. as needed.  Look critically at every hole you are forced 
to make in the enclosure, and use whatever means are necessary to 
make them RF-tight at all frequencies at which your circuitry might 
be vulnerable.

Best regards,

Charles





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