The part number has been garbled a bit, but the members of the family
are 74HC/HCT4046, 74HC/HCT7046 and 74HCT9046, the last one having a
significantly better phase detector. The on-board oscillator should not
be a issue, but on the '4046 and '7046, pin 5 held low disables the
oscillator. On the '9046, pin 5 shuts the whole chip down, but there is
a way to force the oscillator to a zero frequency.
73,
David N1HAC
On 2/22/22 8:09 AM, Bob kb8tq wrote:
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, [email protected] 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>_______________________________________________
time-nuts mailing list -- [email protected] -- To unsubscribe send an
email to [email protected]
To unsubscribe, go to and follow the instructions there.
_______________________________________________
time-nuts mailing list -- [email protected] -- To unsubscribe send an
email to [email protected]
To unsubscribe, go to and follow the instructions there.
_______________________________________________
time-nuts mailing list -- [email protected] -- To unsubscribe send an
email to [email protected]
To unsubscribe, go to and follow the instructions there.
