Chinese AD9850 DDS modules – questions - Page 1
Forum section: electronics / projects

author: microbug
author_rank: Frequent Contributor
timestamp: July 08, 2014, 09:42:17 pm
content: Hi,One of my ongoing projects is an AD9850-based (see attached datasheet for AD9850) function gen (sine & square). The ICs on AD9850 modules you can get on eBay appear, by most accounts, to be genuine (seehere– apparently they made a big batch with the wrong output filter on).1) The AD9850 has a frequency range of 125MHz. Is it just incredibly noisy up there or is there some other reason for the 42MHz low-pass filter? I have read the datasheet but I am afraid my analog knowledge when it comes to 'aliased images' from the D/A converter is nonexistent.2) Am I missing something or is the logical thing to do just to stick one of these modules in my design, use the (unfiltered) ZoutB output (sine) and put in a direct copy of the filter from the datasheet reference design (I have heard that filters are horrible, in general, when they get complex, but I don't have to design it myself)? I suppose the comparator would still be running from ZoutA though...3) Failing 2), could I (and should I) desolder the AD9850 from the module and integrate it into my design on its own?4) Other general comments?Thanks!

author: rob77
author_rank: Super Contributor
timestamp: July 08, 2014, 10:42:16 pm
content: i have built a dual channel signal genartor with those cheap boards - arduino nano + 2x AD9850 board + 4x4 keypadthe boards are working well (if your expectations are low enough), basically it's good enough for playing around with various circuits, but the output is not clean enough for any serious work. but what would you expect from a board sold at $9 ?regarding accuracy - i don't have a freq counter, but the 2 boards are off by 8Hz @ 1MHz - measured by 2 channel scope - both AD9850 outputs set to 1MHz each connected to one channel on the scope - scope trigger on 1. channel (first DDS board) and stepping the 2nd board by 1Hz till the second waveform stopped "traveling".in fact those little boards are worth of the $9-$10

author: retrolefty
author_rank: Super Contributor
timestamp: July 08, 2014, 11:07:36 pm
content: I've used one as the local oscillator for a 40 meter (7 MHz) CW receiver controlled by a quadrature encoder running on a standalone 328P chip, software developed with the arduino IDE. I've heard no spurs or artifacts from the DDS module and is rock steady as DDS usually is. Certainly a cheap way to develop sine waves from audio to low RF frequencies.

author: rob77
author_rank: Super Contributor
timestamp: July 08, 2014, 11:08:30 pm
content: forgot to mention the filter on the boards is 42Mhz - the story about boards with incorrect filter is not true (at least not for the boards i have - signal fading out quickly above 42MHz - checked with FFT)and need to mention that the amplitude of the sine output is dropping with frequency - so you'll need some output stage with AGC.in fact i think those boards were built with rejected AD9850 chips (or simply the AD9850 is not good enough) probably it would be worth of looking after the AD9851 instead.

author: ve7xen
author_rank: Super Contributor
timestamp: July 09, 2014, 12:07:16 am
content: Quote from: microbug on July 08, 2014, 09:42:17 pmTheclockis up to 125MHz. The output with the maximum Freq setting is 1/2 * Fclk, or 62.5MHz. I believe the filter is just a complete copy of the 42MHz filter used on the AD reference design in the datasheet. I'm not sure why AD chose that cutoff frequency for their evaluation board, seems a bit odd unless the performance of the device gets much worse above that.QuoteYou will probably want some kind of output amplification(attenuation) / offset stage to make a useful signal generator. I would also recommend using an external comparator or digitally driven comparator voltage, since the sharp comparator edges get coupled into the sine output, at least on my board.QuotePossibly, but it's not easy without the proper tools. The development boards give access to most of the pins, if you need to you can remove the filters and implement them off-board.I did some AF measurements and have some other comments on this thread:https://www.eevblog.com/forum/beginners/2-%27s-analog-mux-chip-ad9850-dds-at-low-freq%27s/

author: dannyf
author_rank: Super Contributor
timestamp: July 09, 2014, 01:00:41 am
content: QuoteThe output starts to attenuate just before 10Mhz and beyond 20Mhz, it is not usable.Yes, the typical filter, while "over-engineered", is engineered to the wrong roll-off frequency.QuoteI would just use the module - much simpler and the wrong roll-off frequency has minimum impact.