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

[Bob kb8tq]

Hi

Best guess:

Your 10 MHz (for whatever reason) is being turned into a fast edge
square wave. That’s dumping current spikes into the supply and ground
on your board. You have a really wide spectrum as a result ( usually 
well up into the GHz region). 

That “stuff” is going here / going there. Some of it is getting into your
amplifier / filter chain by one route or the other. It looks like modulation
because of the filtering in your multiplier chain. 

Since the PLL chip does the same thing ( when it generates 10 MHz 
into the phase detector, there are two possible sources of the fast edges
and current spikes. Yes this makes tracking things down a bit more 
fun. 

Either way, the answer is board layout / bypassing / decoupling related. 

If you want to go with a different “low frequency” phase detector chip,
the ADF4002 has been a go to part for quite a while. 

Bob

> On Feb 21, 2022, at 9:27 PM, ghf at hoffmann-hochfrequenz.de wrote:
> 
> 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?
> 
> <100MHz_from 2 * lvc125.png><300mhzmal2-sky17.png><sx3fQ8.png>_______________________________________________
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