Re: AD9850/51 module pc boards

Steven Dick
 

LP2985 is an excellent part. go for it.
-Steve K1RF

-----Original Message-----
From: Terry VK5TM
Sent: Sunday, September 30, 2018 8:58 AM
To: PHSNA@groups.io
Subject: Re: [PHSNA] AD9850/51 module pc boards

Henning,

There are two decoupling caps right at the chip on the underside of the pcb,
I can't get them any closer without cutting the top off and mounting them on
the die :) <-smiley face
Another one next to the V+ pin on the underside and the 10µF next to the
other V+ pin.
Looking at the evaluation board, which has much longer supply lines, it only
has one more than this around the DDS chip, the others are associated with
the interface chips.

Re the supply to the xtal osc, the LP2985 is listed as a low noise regulator
and I believe should be more than adequate.
It is well decoupled and again is on the underside of the pcb.

I did some digging through Analog's forums and engineering notes today and
one thing that I noticed was that, in their own words, some of their
recommendations for decoupling, seperate ground planes etc was somewhat over
the top and simpler schemes made no noticeable difference.

Re oscillator phase noise, well you're already hindered by the limitations
of the AD9850 to begin with, so I can't see using ultra pure regulators and
massive amounts of decoupling are going to do anything useful. I could be
wrong, which wouldn't be anything unusual, but I don't think so in this
case. The obvious proviso is of course to use a good quality Xtal Osc
module.

The test will be to build a couple and compare them against the standard
module and rework it if necessary.

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