Signal purity of a 125MHz HCMOS/TTL Clock Oscillator  - Page 1
Forum section: electronics / projects

author: nergal
author_rank: Contributor
timestamp: February 10, 2016, 06:25:39 pm
content: Hi guys,The story is this. I am designing a small DAC/DDS module around the AD9851. I am using a 1125Mhz clock oscillator (http://www.mouser.com/ds/2/96/008-0256-0-786323.pdf) that provides the general source.The circuit works well, and all except for one thing. When I checked the EMI emissions by moving a coil of wire attached to a spectrum analyzer over the PCB.I saw pic related. The smallest left peak is the fundamental.https://goo.gl/photos/g8Da8TBNS78NFr1c9Now, thinking it has something to do with the output of the ad9851 I disabled the chip, but the the signals were still there unchanged. So I started poking around with an oscilloscope with FFT and got this directly on the output of the clock oscillator.https://goo.gl/photos/T212mmZJXyRiWy8E8Ever seen even harmonics coming from an oscillator? And how do you propose I get rid of them? (impedance matching, filter, other clock)?Thank you, and sorry for the bad English sentences.

author: awallin
author_rank: Frequent Contributor
timestamp: February 10, 2016, 06:44:33 pm
content: double check and then check again you don't have the REFCLK multiplier ON !?! That's one thing that could generate high frequencies.Unless you are doing something strange with the osc-output (clipping-diodes, other nonlinear components?).9 Vpp sine is a bit hefty for 3.3V logic?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 10, 2016, 07:14:50 pm
content: Quote from: nergal on February 10, 2016, 06:25:39 pmSure, asymmetrical duty cycle or rise/fall time is all that's needed. Granted it shouldn't be within 10dB of the odd harmonics unless it's really bad...What's your layout look like?Tim

author: nergal
author_rank: Contributor
timestamp: February 10, 2016, 07:34:49 pm
content: Hmm...The REFCLK multiplier supply is connected, but I am not activating it (at least not intentionally). Even if it is activated can it interfere to this extent knowing that the max frequency of the AD9851 IS 180Mhz?The part of the schematic is attached.Thank you for the rapid responses.

author: marshallh
author_rank: Unknown
timestamp: February 10, 2016, 11:19:15 pm
content: Off topic, but can I ask what pcb software you are using?

author: nergal
author_rank: Contributor
timestamp: February 11, 2016, 08:27:10 am
content: it's Proteus. I'm not quite fond of it, but I am somewhat obligated to use it.

author: AndyC_772
author_rank: Super Contributor
timestamp: February 11, 2016, 08:56:33 am
content: Quote from: awallin on February 10, 2016, 06:44:33 pmAbsolutely; something isn't right here.In any case, it's not really possible to get a meaningful picture of the true shape of a 125 MHz square wave using a 600 MHz scope, presumably with a passive probe. I'd be looking to use a scope with an active probe and at least 1 GHz, and even then, the presence of the probe will still distort the wave shape. Just not by as much.In the absence of an active probe, putting the scope in 50 Ohm mode and making a low-Z probe out of a BNC cable and a 1k resistor might yield a better result.Nevertheless I wouldn't expect to see such an apparently large amplitude, even with an ordinary passive probe, unless perhaps there's something very wrong with the probing technique in use.#include <std_rant_high_speed_probing.h>

author: nergal
author_rank: Contributor
timestamp: February 11, 2016, 09:15:50 am
content: The whole board is powered by 5v. You are right, once I set the oscillator to 50 ohms, the amplitude got to a more reasonable level of 4.5v.Unfortunately this is the fastest scope I have and I'm trying to find the source of those even harmonics. The spectrum analyzer is a 1.8GHz HP.I am going to see if I accidentally got the REFCLK multiplier to turn on. At this point, this is my best bet.Thank you.

author: AndyC_772
author_rank: Super Contributor
timestamp: February 11, 2016, 09:59:30 am
content: Try making a low-Z probe with a very short ground connection; at least then you've got the best chance of showing an accurate signal. It might be close enough to show the duty cycle, which in turn could explain the even harmonics if it's not 50/50.

author: nergal
author_rank: Contributor
timestamp: February 11, 2016, 10:34:06 am
content: this is amazing... I can't believe this worked so well...Photos are attached.

author: nergal
author_rank: Contributor
timestamp: February 11, 2016, 12:11:44 pm
content: however, as you can see, i'm still getting those even harmonics, and the duty cycle seems on spec...hmmm...

author: AndyC_772
author_rank: Super Contributor
timestamp: February 11, 2016, 12:42:30 pm
content: I used a TDS3034B scope for many years; it's a nice bit of kit, very responsive and reliable, but I don't recall memory depth as being a virtue. In fact, IIRC, you have the choice of either 10k points or just enough points to fill the screen.I think you're carrying out an FFT over a small, and non-integral, number of cycles. Hence you're seeing harmonics that don't actually exist in the real signal, but instead, they're just an artefact of the small FFT window.Try making sure you have the longest record length enabled, then slow down the time base a few notches, so you at least get a larger number of complete cycles on screen. If the FFT changes significantly, then you know the harmonics are something to do with the analysis rather than the signal.

author: dmills
author_rank: Super Contributor
timestamp: February 11, 2016, 12:45:46 pm
content: Unfortunately the scope FFT does not seem to have a calibrated scale so it is hard to tell what the levels are.CMOS clocks running at any speed often have poor waveforms in this sort of sense as the source impedance differs between the pull up and pull down phases, not helped by the decoupling on the osc involving at least two vias in the high speed loop....I take it that this board does have a SOLID (Not split by tracks) ground plane under everything?Decoupling high speed circuits should place the HF caps such that they connect to the power and ground pins directly, not via a loop involving other layers.I might see if your actually seeing radiation from the power traces rather then the signal ones, it would not exactly be the first time.Regards, Dan.

author: nergal
author_rank: Contributor
timestamp: February 11, 2016, 05:33:13 pm
content: I am almost sure that the harmonics are there because I can see them on the spectrum analyzer.This is a two layer board and the bottom plane does have signal and power traces. If I am seeing radiation from another trace, haw can it radiate higher frequencies? The 125Mhz is the highest frequency on my board (the AD9851 is deactivated).

author: rfeecs
author_rank: Frequent Contributor
timestamp: February 11, 2016, 11:19:56 pm
content: Why are you concerned that your digital clock has harmonics?Of course it does. You should be surprised if it didn't. If your waveform isn't perfectly symmetrical it will have even harmonics. So it does.If you are concerned about EMI emissions then you will need some shielding. Put the thing in a metal box.

author: uncle_bob
author_rank: Unknown
timestamp: February 12, 2016, 12:48:00 am
content: Quote from: rfeecs on February 11, 2016, 11:19:56 pmHiIndeed, if you look at the spec on your oscillator, it is a square wave clock. It probably also has horrible phase noise and jitter. It is not the sine wave / low noise / low jitter source you should be using for something like this. Yes, the "right part" is a *lot* more expensive.Bob

author: dmills
author_rank: Super Contributor
timestamp: February 12, 2016, 09:37:54 am
content: You could well be seeing radiation from the power trace that supplies the oscillator, @250MHz, 500MHz and up it does not take much loop area (And your decoupling is less then ideal, depending on the bottom copper layout possibly very much less then ideal) ,high speed clocks without solid planes under them never end well.Spin another prototype on a 4 layer board with a solid ground plane (and a power plane) and see how you go, but I concur that cheap microprocessor clocks are seldom wonderful from a RF designers perspective.My default starting point these days is a 4 layer board unless the thing is going to be totally trivial, mostly for reasons of edge rates.73 Dan.

author: dannyf
author_rank: Super Contributor
timestamp: February 12, 2016, 11:13:39 am
content: '1125Mhz clock oscillator 'That's kinda fast, isn't it?

author: uncle_bob
author_rank: Unknown
timestamp: February 12, 2016, 06:02:07 pm
content: Quote from: dmills on February 12, 2016, 09:37:54 amHiThe next interesting point on the PC board (I fully agree about 4 layers) is thickness. Often people go for big thick boards in order to "make them rugged". With high speed stuff, you have just put big long leads on everything with that 0.096" thick board. Most modern guidelines suggest layer spacing of 0.01" or less. With a 2 layer board, that gets pretty flimsy. With a 4 layer board you are up to 0.032" thick. That's not exactly hefty, but it is quite usable. The cost impact of doing a board at 0.01" or a bit less on layers is not significant.Bob

author: dmills
author_rank: Super Contributor
timestamp: February 12, 2016, 08:29:51 pm
content: Close spacings also make getting sane trace impedances very much easier and actually improve radiated emissions even if you change nothing else.There is a reason layer stacks are a thing.....50R trace on 1.6mm board is inconveniently large a lot of the time, but I have actually gone to a 2.4mm monster thickness board in an RF power application to reduce the dielectric stress and to allow a thicker trace size (it was a 1.5KW LPF bank).73 Dan.

author: nergal
author_rank: Contributor
timestamp: February 13, 2016, 03:13:50 pm
content: Wow... thanks for the replies ..The final version of this project should pass the FCC/EC. Once all the modules are assembled and tested, The final product will be on a 4 layer board. As for shielding, I think a metal can over the DDS and the crystal should solve the eventual emissions.Thank you.

author: uncle_bob
author_rank: Unknown
timestamp: February 13, 2016, 03:20:05 pm
content: Quote from: nergal on February 13, 2016, 03:13:50 pmHiIf you are going to have a setup like this *and* have to pass EMC testing .... do yourself a favor. Get an EMI guy involved early. In most companies that do this right, he is the one who has the final say on any part of this sort of thing. They do it that way because they have a lot of experience at doing it wrongThere are a *lot* of things to look at ....Bob