[time-nuts] 4096 PLL - how to switch off on-chip VCO

[ghf at hoffmann-hochfrequenz.de]

Hi!

I'm building a clock source for my LMX2594 15 GHz PLL.

I want to max out the LMX2594 phase comparator frequency;
that means 300 MHz for fractional operation. The VCXO is a
100 MHz ECOC-2250 crystal oven by ECS because I have it,
and it's tripled.

Dumping its output into a pair of 1G125 line drivers gets me
this spectrum: (100 MHz....png) The 300 MHz is quite prominent
with not much of a loss. I dumped it into a 300 MHz filter,
3 poles, C-coupled, then a sot-89 MMIC and another 3 poles.
That cleaned up the harmonics quite good. Sorry, there were
no 300 MHz SAW-filters available. All obsoleted.
The filter is 6 Murata 0603 SMD inductors and fixed Cs. Still a
bit to the low side; that can be fixed.

BUT - there is a problem. The 100 MHz can be locked to an external
10 MHz reference, and the 10 MHz is modulated onto the 300 MHz.
I do not want this to be multiplied to 15 GHz.

There is a 74lvc163 counter that is visible as well. (100/10 = 10 MHz)
The PLL chip is a 74lv4046, the phase comparator part. When the
reference is != 10 MHz and the prescaler is running, OMG, grass like
on an African steppe. Only the zebras are missing.
Without ext ref: sx3fQ8.png

I don't think that the VCXO has thus a large modulation bandwidth
and I'd like to replace the 4046 with a 9046, but I've found no
way to switch off its internal oscillator. Having not-vanishing loop
gain at lock is fine, but getting the 8046's undefined on-chip VCO
on the output spectrum would be no improvement.

Any ideas?

Cheers, Gerhard

p.s.
How can the spectrum analyzer (89411A) encode such sharp screen dumps in 
just 3 KB?

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