LMX2820: SYNC (mode 2) feature in PFDIN external feedback mode (offset loop)

David Bender

Part Number: LMX2820

Hi,

We use the SYNC feature in mode 2 (fout is integer multiple of foscin, input MULT/Doubler off) with FULL ASSIST.

Using internal feedback (not using PFDIN) it works fine.

Using external feedback, RFOUT becomes unstable/noisy as soon as R1:PHASE_SYNC_EN is set.
This happens even before the PSYNC pulse is actually applied.
When the PSYNC pulse is sent, RFOUT looks fine (searching for target phase?) for about 130 us.
After that, it becomes unstable/noisy again.

Are there general limitiations with using the SYNC feature in external feedback mode (offset loop) configuration?

Thanks & Best regards
David

over 1 year ago

0 vicente flores prado over 1 year ago

TI__Mastermind 29542 points

Hi David,
Just to clarify, are you using full assist mode or Instant calibration?

If using instant calibration there are some restrictions, Fcal_EN should be low during VCO calibration & should be down without external feedback - once instant calibration is done then you can connect external feedback.

Please note we have not characterized using SYNC with external feedback. In theory it should work.

Regards,

Vicente

0 Andrei Gorevoi over 1 year ago in reply to vicente flores prado

Prodigy 30 points

Hi Vicente

David mentioned Full Assist mode. For this we are suing forced values of VCO, CAP and ISET. And PLL is working with frequency offset in feedback. So we apply equal frequencies to PFDIN (Bout VCO-offsetting signal) and OSCIN, achieve the lock and expect that at any Aout divider value its output signal is phase-aligned with PSYNC rising edge. But we see that when PHASE_SYNC_EN=1 (no PSYNC pulse applied yet) Aout is noise-like. Applying PSYNC pulse makes the frequency locked for few microseconds and then - noise-like again.

BTW. We can`t reach synchronization described in section 3.2 Category 2 SYNC Example https://www.ti.com/lit/ug/snau273/snau273.pdf?ts=1702494863982

Please, provide more information about settings for phase sync.

Andrei

0 vicente flores prado over 1 year ago in reply to Andrei Gorevoi

TI__Mastermind 29542 points

Hi Andrei,
Please provide a config (preferably .tcs) file for me to review to figure out why you can't perform a Cat 2 SYNC.

Regards,

Vicente

0 Andrei Gorevoi over 1 year ago in reply to vicente flores prado

Prodigy 30 points

6100/128 at Aout. Offsetting signal 6 GHz is used. IF 100 MHz is applied to OSCIN, reference 100 MHz - to PFDIN. It seems like PLL is locked - I see the 6100 MHz leakage from Aout. But the divider itself produces a noise.

Andrei

6100M_128div_freqoffs_psync.tcs

0 vicente flores prado over 1 year ago in reply to Andrei Gorevoi

TI__Mastermind 29542 points

Hi Andrei,
I will test this on bench.
Give me till end of this week.

Regards,

Vicente

0 Andrei Gorevoi over 1 year ago in reply to vicente flores prado

Prodigy 30 points

Hi Vicente

Any news?

Andrei

0 vicente flores prado over 1 year ago in reply to Andrei Gorevoi

TI__Mastermind 29542 points

Hi Andrei,
I reviewed your config and have some questions:
A N divider value of 154 with a PFD frequency of 100MHz cannot result in a VCO frequency of 7680
I changed it 154 manually and as expected the VCO frequency field switches to 15400 MHz and becomes red.

You require a fractional N divider to create a 7860 MHz VCO frequency. The value I used as 76 for the Integer and and I pressed the "large"" button for the fraction terms to have the GUI calculate the appropriate fraction

Second question, in regards to you asking about why a CAT2 SYNC not occurring is because both RFoutA & RFoutB are not integer multiples of OSCIN - CAT2 SYNC is actually not possible in your use case. Given they're not integer multiples this is actually a CAT3 SYNC where SYNC timing is critical.

Regards,

Vicente

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LMX2820: SYNC (mode 2) feature in PFDIN external feedback mode (offset loop)