Fred gave you a very nice bunch of ideas.

For data lines, you might consider using differential signalling. You
can apply common-mode choking fairly easily, without disturbing the
data. But if you put capacitance to ground, you'd have to be very
careful to keep it balanced, and to not have so much it causes signal
degradation.

We fairly regularly deal with high isolation out to 100MHz and a bit
beyond, and it hasn't proven to be terribly difficult. We have things
like switching power supplies, whose noise would cause lots of
problems if it got into the wrong places. One of the key tricks is to
keep loop areas very tight. Another is to be very sure that there is
a return path for everything, and that the return path is close to the
outgoing path. That's really the same as keeping loop areas low.
There is a lot of advantage to using multilayer boards and SMT parts.

Not long ago, I revamped a board that had a heavy cast shield "clam
shell" bolted around a critical area of the board. (It digitizes
wideband analog inputs, and has lots of digital communications going
on not far from the analog stuff.) My new design uses a simple
snap-on folded metal shield, and I'm not even convinced it needs
that...and the spuriouses are at least 20dB lower than the old board,
down in the -120dBfs region as I recall. In laying out the board, we
simply paid a lot of attention to keeping things very tight, taking
advantage of SMT parts and differential signalling. Think of
everything as an antenna that can radiate and/or pick up signals, and
minimize their ability to do that. And realize that current in any
copper conductor causes a voltage drop that you must avoid sharing
with some other circuit you don't want the signal getting into.

I gather you want a stepped attenuator on the output of your DDS. You
may well want to have the attenuator itself be a separate board with
some decent shielding between sections, and essentially DC controls
(to relays??) to control the steps. Common mode choking on the RF
signal cable should be helpful.

Cheers,
Tom

Richard Hosking wrote in message .au...
Dear all
I want to design a DDS board which has attenuators on the output to
provide a low level output (-100dBm)
I have a venerable HP8640B which can give a calibrated output to -137dBm
To achieve this HP have gone to extraordinary lengths to shield the
oscillator attenuator/output amp circuits and any control lines - I note
there are at least two stages of bypassing/low pass filtering with an
intermediate shielded section

My question is: how do I get data and power lines into my DDS chip (in a
shielded enclosure) and prevent RF leakage out which will limit the
useful minimum level out from the DDS board?
I presume I will have to use a buffer of some sort for the data lines
and extensive bypassing on the power lines. I want reasoanbly quick
update speeds for my DDS, which is a serial port, which will mean data
rates in the MHz region.

Thanks

Richard