Si5351 Breakout Questions - Page 1
Forum section: electronics / rf-microwave

author: rheb1026
author_rank: Regular Contributor
timestamp: February 03, 2017, 06:08:53 pm
content: I picked up an Adafruit Si5351 breakout to do some prototyping. I was hoping to use this as a local oscillator in a receiver project.I've been able to program it and change frequency outputs, but once the outputs reach the upper end of the frequency range the waveform becomes less and less square and more sinusoidal until about 135 MHz it becomes an unrecognizable mess. Granted, I am using this on a protoboard... but my scope probe is stuck into the nearest pin next to the output with the shortest ground wire possible. I'm waiting on the SMA connectors to come in the mail (I unfortunately ordered those idiotic reverse SMA by accident)Does anyone have any experience like this? Is this just a poor layout problem with the breakout board? According to the data sheet the output impedance should be 50 ohms, but the traces on the board look awful small to me. I was hoping to be able to connect the output from this directly into a 74HC74 to get a quadrature output.Here are some screen shots from the scope. Sorry for the small pics I had to resize to get them to post. They show: 20, 40, 60, 80, 100, 120 MHz in that order

author: nugglix
author_rank: Regular Contributor
timestamp: February 03, 2017, 06:38:55 pm
content: Are you sure your scope can handle the harmonics that make a square wave square?

author: rheb1026
author_rank: Regular Contributor
timestamp: February 03, 2017, 07:01:02 pm
content: That's a great question and I should have though of that! I will check on my old 400 Meg analog scope and let you know!Edit: Unfortunately that was not the issue it seems. Very similar waveform on the 400 Meg scopeThis shows 130 MHz output on both scopes:And here's 120 MHz output, which is the highest reasonable looking waveform I can get from the breakout board:

author: nugglix
author_rank: Regular Contributor
timestamp: February 03, 2017, 07:39:52 pm
content: I'm not convinced.Seehttp://www.allaboutcircuits.com/textbook/alternating-current/chpt-7/square-wave-signals/Do the math again.120 MHz, 3rd harmonic: 360 MHz, 5th 600 MHz ...Plus distortion and filtering done by the breadboard.

author: rheb1026
author_rank: Regular Contributor
timestamp: February 03, 2017, 08:17:04 pm
content: Unfortunately I don't have a faster scope at my disposal, but yes I understand that the scope will not be able to reproduce the waveform accurately. I was just hoping it would be better than it is! This IC seems pretty popular among the home brew radio crowd. Surely people aren't just using it on good faith and not verifying it's working as it should? I would imagine someone out there must've checked out this breakout boardI'm sure the breadboard is contributing a lot to it, but again, I won't be able to tell how much until I get the right connectors in the mail.Thank you for your feedback!

author: awallin
author_rank: Frequent Contributor
timestamp: February 04, 2017, 08:45:39 am
content: FWIW this page has spectrum analyzer views. Just do an I-FFT in your head and compare to what you see on the scopehttp://www.ra3apw.ru/proekty/si5351-spectrum/The spurs are probably a feature of the (fractional-N?) PLL used to generate the output based on a 25-27 MHz input (xtal osc or external source).OTOH your scope-view suggest a strong sub-harmonic spur not in range of those SA-pictures linked above..if you want reasonably clean sinewave either a DDS (clocked at >>f_nyquist), or perhaps a better PLL-chip might do it (played with a LMX2541 dev-kit once and the output looked OKish).

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 04, 2017, 10:53:18 am
content: I think you worry too much, the 74HC74 doesn't care if you feed it with a sine or a square.

author: rheb1026
author_rank: Regular Contributor
timestamp: February 05, 2017, 03:40:23 am
content: I got the right SMA connectors in the mail and used a coax cable to input directly into the scope. It appears that the breadboard was causing all kinds of distortion at the higher frequencies, even probing as closely as I could. Where as before I couldn't get a clean signal above 120 MHz now I can get a clean sine wave up to the maximum output speed. The signals look much squarer now than previously, especially at the higher end. So partly the breadboard and partly the limitations of the scope to reproduce the higher harmonics

author: retrolefty
author_rank: Super Contributor
timestamp: February 05, 2017, 03:27:04 pm
content: Quote from: PA0PBZ on February 04, 2017, 10:53:18 amAre you sure? AC implies both positive and negative voltage. Are you sure this digital component is rated to handle negative voltage applied to its input pin(s)? A datasheet for this chip will answer this question.

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 05, 2017, 03:33:10 pm
content: Quote from: retrolefty on February 05, 2017, 03:27:04 pmI didn't mention AC, I mentioned sine. And of course a square wave can go negative too.