I thought I would use switched sections with various sections switched
by relays - each section shielded if necessary.
The DDS will give an output down to -50dBm or so under software control,
though spurious outputs are not necessarily attenuated by the same
amount, so it is probably not a good idea to use the full attenuation range
My board will be double sided unless I *need* to use a more expensive board

Richard

Rene Tschaggelar wrote:
What attenuator are you using ?
-100dBm is a lot of attenuation.

Rene

Richard Hosking wrote:

Many thanks for the replies
That has given me some ideas for a start

Richard


Richard Hosking wrote:

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.