[ARC5] [Milsurplus] AD9850 DDS Freq. Syth Module

[Bruce Long]

I believe the concern about DDS spurs is overblown. ICOM uses the AD9951 in
their IC-7000 and IC-7200 for their synthesizer and I haven't heard
complaints about spurs.

I don't know how ICOM uses the DDS in their products but the standard way to clean up DDS spurs-  besides DDS dithering- is to follow the DDS with a narrow bandwidth analog Phase locked loop.  In this way the analog vco provides most of the phase noise performance while the DDS serves to stabilize the vco against drift and perhaps improves close in phase noise.

However if you have a fairly good LC VFO to start with then you can gain most of the positive attributes of a DDS-analog clean up loop architecture  by just adding the digital frequency lock loop of the type suggested in this thread.   For old gear refurbishment i like the idea of being able to retain the use of the existing dial a possibility pretty much excluded if you replace the original lc vfo with an added DDS


That said not all amateur projects need the ultimate in spurious performance and the price- size- performance of DDS solutions is often attractive.

A craftsman generally has a toolbox full of tools.  His expertise to a large part is being able to select and apply the best tool for any given task.

bruce






________________________________
 From: Mike Feher <n4fs at eozinc.com>
To: mstangelo at comcast.net; 'Bruce Long' <coolbrucelong at yahoo.com>; arc5 at mailman.qth.net 
Sent: Saturday, March 23, 2013 7:53 AM
Subject: RE: [ARC5] [Milsurplus] AD9850 DDS Freq. Syth Module
 
Hi Mike -

It all depends on the application. DDS spurs can be pretty bothersome,
especially in satellite communications. The biggest contributor is the
number of bits in the D/A converter that follows the accumulator. 73 - Mike 

Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731
732-886-5960 

-----Original Message-----
From: arc5-bounces at mailman.qth.net [mailto:arc5-bounces at mailman.qth.net] On
Behalf Of mstangelo at comcast.net
Sent: Saturday, March 23, 2013 10:35 AM
To: Bruce Long; arc5 at mailman.qth.net
Subject: Re: [ARC5] [Milsurplus] AD9850 DDS Freq. Syth Module

The spurs generated by the AD9850 abd AD9851 are not that objectionable.

If you're concerned about spurs and phase noise I would use a DDS which has
more resolution (more bits) in the D to A converter such as the AD9951.
Check out WA1FFL's kit:

<http://www.wa1ffl.com/>

I believe the concern about DDS spurs is overblown. ICOM uses the AD9951 in
their IC-7000 and IC-7200 for their synthesizer and I haven't heard
complaints about spurs.

<http://www.ab4oj.com/icom/trend.html>

Mike N2MS
----- Original Message -----
From: Bruce Long <coolbrucelong at yahoo.com>
To: George at MaierGroup.com, arc5 at mailman.qth.net
Sent: Fri, 22 Mar 2013 23:44:54 -0000 (UTC)
Subject: Re: [ARC5] [Milsurplus] AD9850 DDS Freq. Syth Module

George. I don&#x27;t know anything about the product you refered to but
years ago as part of my day job I freq locked a ocxo quartz osc to a gps
receiver using that general approach   It worked great with no big problems 

As an ex high performance receiver designer at wj I am uncomfortable with
the spurs generated bu Dds sources although I admit the dds spur level is
too low to worry about in most ham applications.  I like the idea of a good
lc vfo stabilized wIth a non PLL approach bruce
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