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

[David G. McGaw]

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, 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>_______________________________________________
>> time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe send an email to time-nuts-leave at lists.febo.com
>> To unsubscribe, go to and follow the instructions there.
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe send an email to time-nuts-leave at lists.febo.com
> To unsubscribe, go to and follow the instructions there.