Re: DDS spectral "purity" ?

K5ESS
 

Jim,

Very Interesting.  Looks like the ERA3 generates lots of harmonics if the AD-9850 data sheet is to be believed.  Also found the latter part of the sweep very interesting.  I’m assuming that the responses shown marching right to left are from aliasing but they shouldn’t be showing up at output frequencies below 40 MHz with an assumed 125 MHz clock.  Maybe some funny business going on between the aliased frequencies and the harmonics generated in the ERA-3?  I’d be interested in seeing comments or just speculation on the likely source of these signals.

Mike, K5ESS

 

From: PHSNA@... [mailto:PHSNA@...]
Sent: Saturday, July 09, 2016 5:04 PM
To: PHSNA@...
Subject: [PHSNA] DDS spectral "purity" ?

 

 

Steve and other have discussed the spectral purity, or lack thereof, from our inexpensive DDS modules and accessories. Playing around a bit today with a spectrum analyzer, I made a "home movie" of a sweep from one my my very early builds of a Type-2 PHSNA

PHSNA Type-2 (push-pull transformer output)
Type-2 AD-9850 module with on board filter removed
Standard 30 Mz version LPF on the PHSNA
The ERA-3+ running with a bit too much bias, at ~ 40 mA
Using the WinPHSNA app with 9600 baud Bluetooth communications
Sweep from 1 MHz to 40 MHz, with 100 kHz steps
Rigol DSA-815TG analyzer

SA vertical divisions 10 dB (top = +10 dBm)

SA horizontal divisions 10 MHz (0 to 100 MHz scan)

 

It's too big (12 MB) for Yahoo to allow into the Files or Photos areas, so here's the URL for it on my web page

http://www.n5ib.net/DDS_Spectrum.wmv

 

The cinematography will not earn me any Red Carpet time, but it's interesting to watch the wheat field march across the screen. Gives a good feel for the limitations of inexpensive test gear. Make popcorn, call the kids. It's rated "G" and is just over minute long.


Jim, N5IB

 

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