LMX2572: What to expect about far-off spurs

Hi,

These pictures were just a quick setup to illustrate the common behaviour we see with these PLL's, we've tried several strategies with both the LMX2594 and the LMX2572 and we get this kind of "sea of spurs" behaviour.

We were used to other manufacturer's PLLs fractional-N PLLs such as the ADF5355 series of Analog Devices which we used extensively in the past, which gives much more predictable spurs. With them, you get fractional and subfractional spurs and then the only far-off spurs that appear are multiples of the PFD, so you know what you're going to get.They are simple yet very very predictable

The LMX25XX series perform better in terms on phase noise and so far it seems like it gives lower spurs if you tame them properly, but our impression is that the spurs are too "unpredictable". You get the common suspects such as PFD, fractional, subfractional, reference cross talk, etc, but instead of getting lower and lower such that far from the carrier only the ones with stronger generation mechanisms remain above -100 dBm, you get a sea of low-level yet noticeable spurs like in the pictures, and sometimes there are extra far spurs that seem to be VCO-related (they appear at the same level depending on the VCO frequency, impervious to FPD changes and N variations) wich can get as high as -50dB

My question is wether this "sea of spurs" behaviour is to be expected, so that we can better at optimizing them away.

By the way, what do you mean by "irrational number" for the denominator? The denominator is always an integer so it can't be irrational by definition. I thought you meant prime, but 12345678 isn't prime. I've read stuff about numerator/denominator choosing strategies and some people say that making them co-prime helps as it forces the maximum amount of fractional randomization, yet they don't say they WILL reduce the spurs, but that it MIGHT reduce them under some circumstances. Could you please clarify on what would be the best strategy to choose the denominator?

Right now, as we want to move the frequency in 1Hz increments, in our own driver (not in TICS pro)  we just set the denominator equal to the fPD in Hz and the numerator as the remainder from fRF/fPD, and then we either expand the fraction, reduce it or add 1 to the denominator to create an unequal large fraction.

EDIT: I've tested and setting CAL_CLK_DIV as you said created even more far-out spurs in between the ones that were already present, which I guess is to be expected. Apart from that, I've tested that a significant portion of the spurs are still present even if you turn off the output with OUT_PD and set the output mux to Hi-Z. The level goes lower but several are still above -80 dBm. Not sure where these could come from really, it must be some sort of crosstalk or coupling but I don't get where it could come from.

The sea of spurs behaviour was observed by us before when we were working with LMX2594 and we did things like putting the chip inside an aluminium box with EMI filters in all data lines and a common mode choke in the DC feed line and we still got similar behaviour. I just want to know if this is normal or expected.

Hi Dean,

The thing about LMX2572 being better than LMX2594 only happens close to the carrier, where fractional spurs are normally slightly lower. Far off from the carrier we get a very similar behaviour.

Some of the spurs we see are reference related. We inject the reference differentially from an LMX04828 and that creates reference spurs at integer multiples of the reference frequency. Others, we don't know. Most of the stuff seems to relate to known spur generation mechanisms and can be optimized away, as we currently do. But the sea of spur behaviour is where we're not sure about the cause or if it's actually a feature of MASH synthesizers.

See, there are so so many spurs at so so many places that we think it has to be something inherent to the chip and not externally induced. 

We're using official EVAL boards which were modified to fit an ultra low noise, ultra high PSRR LDO for point of load regulation. We place the boards in custom-made aluminium boxes with passthrough SMA pigtails, two power banana inputs with an internal common mode filter and feed-thru capacitor and a passthrough signal connector with integrated EMI filters. We supply the thing with a dedicated linear PSU of a reputable brand (TTI). Then we screw the lid and the box is sealed with an EMI gasket.

The pictures I showed were under these conditions. We're now at a point where we've tamed the LMX synthesizers to a point where we've gotten it where we wanted them. We can optimize away most of the highest spurs tweaking the configuration and we've implemented a filterbank in our application to get rid of the far-off spurs below the carrier, which are the worst for us as we're amplifying the LO signal to saturation and injecting it to a mixer at 20dBm in a high-side injection configuration. But we'd still like to know if the behaviour we see could be normal or not under the circumstances described before fully commiting to the final design.

Could you please recreate our scenario (the pictures I posted before with an LMX2572 EVM with stock loop filter, using the settings I posted) and send a capture of the spectrum you get?

Regards,
David

Hello,

Could you try with the reference pro board at 125 MHz like we did? Our settings are meant for 125 MHz instead of 100 MHz and the output frequency should be 4301 MHz instead of 3441 MHz (which is 4301/1.25).

Regards,

David