[time-nuts] Aging 5065A ?

[Magnus Danielson]

Hi,

The existing syntesizer is rich on spurs. Very rich in fact. Fairly strong.

However these spurs is far enough not to cause much of a problem for the
clock, but they will help to demodulate some noise.

The noise being problematic is too far in for the spurs of the synthesis
to generate any real danger.

I replaced the 5.3 MHz syntesis with that from my HP3325B, which for
sure isn't free from spurs either, but it turns out that the far in
noise was filtered by the lock-up, so I ended improving the phase-noise
for the 6.8 GHz synthesis such that the stability out at 10000 s was
improving by a factor of 2.

So, don't over-do it with hunting spurs that ain't going to contribute
significantly to the noise and systematics of the box. Increasing
resolution is good. Enough bits to the DAC to keep the additional spurs
from non-linear mixing down, because that is what truncation of the
phase-accumulator state will do and mix the spurs from the current
phase-accumulator setting. Step the frequency setting and the spurs move
around and the mix products will also move around as with any mixer. As
long as that is linear enough, additional spurs will be sufficiently low
that we have other things dominating.

Cheers,
Magnus

On 2020-10-08 01:33, Bob kb8tq wrote:
> Hi
>
> Well….. the existing synth in the 5065 is no champion spur wise. Coming up with a 
> DDS that is “as good as” is not all that crazy.
>
> The 5 MHz reference is multiplied to 6.x GHz and then mixed with the synth. The close in
> phase noise of the multiplied signal is mixed with the synth. The output is the sum of the two.
> Going from 5 MHz to 6.x GHz gets you 20 log (N)  … a bit over 60 db added phase noise.
>
> The synth (at 100 ti 300 Hz offset) needs to be 60 db worse than the noise on the 5 MHz 
> before it even gets in the game. That’s not a really tough spec to meet.
>
> Bob
>
>> On Oct 7, 2020, at 6:18 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
>>
>>
>> kb8tq at n1k.org said:
>>> If you want to run a fixed / well regulated C Field, a DDS with (say) 48 bits
>>> would allow you to tune the device well past  parts in 10^-15.
>> I don't know how to think about a DDS in this context.
>>
>> I remember years ago thinking that a DDS was the greatest thing since sliced 
>> bread.  In the context of something like a PPS going into a PC for 
>> timekeeping, that's probably true.  You get long term accuracy and the PC 
>> can't see the short term issues.
>>
>> But then, somebody mentioned close-in spurs.  They get closer the more bits 
>> you have in the DDS magic number.  (What is that number called?)
>>
>>
>> Suppose I have a black box labeled "10 MHz" with a cable coming out.
>>
>> If you plug that cable into your ADEV measuring setup, can you tell if my box 
>> has a DDS in it?
>>
>> If you plug that cable into your spectrum analyzer, how good a setup do you 
>> need in order to see the spurs?  Do they get lost in the close in noise?  Or 
>> maybe the question should be how clean a signal do I need to start with before 
>> the spurs become visible?  Or what should I be asking?
>>
>>
>> -- 
>> These are my opinions.  I hate spam.
>>
>>
>>
>>
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