What spur level to expect from SI5351 and AD9851??? - Page 1
Forum section: electronics / rf-microwave

author: erikka
author_rank: Regular Contributor
timestamp: August 19, 2019, 05:32:09 pm
content: I need some help on what spur level to expect from a SI5351 and AD9851The only tool I have is my own build spectrum analyzerI started to establish a baseline at 10MHz with a good old analog signal generator, the RBW is in reality 40kHzLooks clean apart from the Phase noise due to the very high first IF of the spectrum analyzer.Then the SI5351 through an attenuator to ensure the signal level is not above the analog generatorLots and lots of spurs. The amount depends on the output frequency where 10MHz is still somewhat good but at higher frequencies it gets really bad.Then a AD9851 with very weird things happening, sometimes I get a clean picturebut each scan is different and sometimes it is as worse as thisSo is all this some imagination from my spectrum analyzer or are these spurs really present?Is there someone that has done similar measurements?

author: fourfathom
author_rank: Super Contributor
timestamp: August 19, 2019, 05:41:05 pm
content: Quote from: erikka on August 19, 2019, 05:32:09 pmYour SI5351 results look about right. I have some similar plots I will search for and post (if I can find them).

author: radiolistener
author_rank: Super Contributor
timestamp: August 19, 2019, 06:17:13 pm
content: I'm using adafruit module, bought here:https://www.aliexpress.com/item/32817431908.htmlIt uses 25.000 MHz XTAL.Here is my result for 10 MHz output (taken with ADC at 1 GHz sample rate with 100 MHz analog bandwidth):

author: fourfathom
author_rank: Super Contributor
timestamp: August 19, 2019, 07:32:53 pm
content: Here are some plots I did for a ham club presentation, where I was showing how the SI5351 might be used in a telemetery project (30-meter ham band, JS8Call mode). These are just examples of what I saw as I played with the device, no attempt yet to seriously characterize it. This chip has an impressive phase-smoothing jitter attenuator, but the spurious levels can dramatically change with the divider settings. When I say "unlikely to cause actual problems", that is in my anticipated usage. Your needs may be more stringent.I used a Signalhound SA44B (up to 4.4 GHz) spectrum analyzer for the measurements.

author: rf-messkopf
author_rank: Regular Contributor
timestamp: August 20, 2019, 08:15:46 pm
content: I did some measurements on a Si5351 module by QRP Labs and posted the results on the German mikrocontroller.net forum:https://www.mikrocontroller.net/topic/458531. Links to the QRP Labs website are in that posting.The QRP Labs Si5351 breakout board sits on top of a control board with an Atmel microcontroller, and there is virtually no power supply filtering between the boards. So I don't know how many of the spurs inhttps://www.mikrocontroller.net/attachment/375224/SI5351_1.pngare due to interference from the controller board or the display.I did not bother to isolate the Si5351 board from the controller and display since I mainly wanted to check the phase noise. If you want to know about spurs in the Si5351 output and give me some time I could repeat the measurements with the Si5351 board separated from the controller.

author: Bud
author_rank: Super Contributor
timestamp: August 21, 2019, 05:10:16 am
content: If you got your parts from eBay, anything may happen. To eliminate that variables use a reputable parts source.Now, i recall AD publishes spur specifications for their DDS in datasheets. Just refer to AD9851 datasheet, i found them accurate. Also AD has somewhere on their website a software that calculates spurs. You may want to search for it , install and play with it.

author: radiolistener
author_rank: Super Contributor
timestamp: August 25, 2019, 12:40:36 pm
content: Quote from: rf-messkopf on August 20, 2019, 08:15:46 pmI also didn't bother to isolate si5351 from controller, the picture I posted above is taken when si5351 was powered from STM32 blue pill module with no filters for power supply at all. STM32 works at 72 MHz. But as you can see, si5351 output is pretty clean.

author: rf-messkopf
author_rank: Regular Contributor
timestamp: August 25, 2019, 04:39:29 pm
content: Quote from: radiolistener on August 25, 2019, 12:40:36 pmOkay, I did another quick measurement, this time with the Si5351 breakout board detached from the controller and display after the registers of the Si5351 were written. The output was fed through a DC block and a 10 dB attenuator directly into the spectrum analyzer.I measured at both 10 MHz and at 100 MHz output frequency. At 10 MHz the signal is much cleaner, with spurs below -80 dBc, in agreement with your measurements. At 100 MHz the largest spur is at about -45 dBc.

author: fourfathom
author_rank: Super Contributor
timestamp: August 25, 2019, 04:55:41 pm
content: Quote from: rf-messkopf on August 25, 2019, 04:39:29 pmBe sure to take a close-up look at the 10MHz output. The spur characteristics can change dramatically depending on the VCO PLL feedback divider and the output divider settings, especially if you use the fractional division capabilities. One of my plots up-thread (titled "4.jpg") shows the output with a 1KHz analyzer span. In this particular case there are *lots* of discrete spurs down about 75dB to 110dB. Changing the dividers can move things around a lot.

author: rf-messkopf
author_rank: Regular Contributor
timestamp: August 25, 2019, 05:32:51 pm
content: Quote from: fourfathom on August 25, 2019, 04:55:41 pmHere you go. I do not know the divider settings the Si5351 is running with, because it is controlled by the QRP labs firmware. I think they will have optimized for spurs. If I'm bored at some point I'll flash the µc with my own software and have a play with the dividers.