[time-nuts] Re: 9046 really, not 4096 PLL - how to switch off on-chip VCO

[ghf at hoffmann-hochfrequenz.de]

Am 2022-02-22 16:40, schrieb 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

Yes, that's true. I saw it 100 msec after hitting "send", but too late.

> 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.


Most interesting! The data sheet says nothing about that. But it also
says nothing about the minimum low/high/cycle times of the phase
detector inputs, only the delays through the chip which is not of
much use when integrating to DC. Oh, and the size of a reel of these
thingies. I have complained at TI's data sheet feedback.

I got 10 dB better now with a buffer between 100 MHz CMOS osc out
and the feedback 100/10Mhz divider. I suppose anything better
requires a new layout with a shielding can for the 10 MHz stuff.
OK, it's just the work. JLCPCB delivers the board for next to free.

I also reduced the divider Vcc from 5V to 3V3. That alone brought 6 dB.
I abused the 3V3 regulator for the 100 MHz LVDS driver for this.
LVDS feature--;

The Vtune of the crystal oven was not the culprit, nor the 300 MHz
output amplifier, nor.....

Maybe the the two CMOS buffers that drive the first 300 MHz Filter
with their harmonic spectrum have some influence via Vcc, but I took
insane filtering measures which did not make a difference; it seems,
the backward isolation of the /10 divider input sucks.

> 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).

I've shown the picture of the 100 MHz CMOS spectrum.

I still remember the professor say: "Lady and gentlemen, now you 
understand
why CMOS never will be fast." He was so wrong. Nobody escapes the
CMOS steamroller.  Yes, we had 1 lady amongst 120 guys in Electronics 
101.

>> 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.
....
>> 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.

Unobtainium now. Even the normal 4046/9046 is best bought at 
Rochester's,
which is usually a bad sign for their future. The ADF4002 has to be
programmed via SPI, a drawback for a DUBUS ham radio publication.

Maybe I add support for the CVHD-950 el-cheapo-VCXOs (quite OK for the 
price)
and the Axiom AXE15NL low phase noise oscillator when I have information
if the mechanical picture is top or bottom view. :-)

I'll probably stay with 4046 and bleeding resistor. The 9046 needs 
unpublished
features; otherwise I could use the 2FF-phase comparator in a Xilinx 
Coolrunner2,
or the AD9901, also recreated in VHDL. Hams won't appreciate that 
programming.

pic: current status of redesign.


Cheers, Gerhard, DK4XP

-- 
Unix since V6 on a PDP11. The first one east of the pond.
These are my opinions. I ate spam.
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