[time-nuts] OT: xtal osc PN

Magnus Danielson magnus at rubidium.dyndns.org
Sat Sep 18 13:01:53 UTC 2010


On 09/18/2010 02:41 PM, francesco messineo wrote:
> First of all, thanks to John and Magnus for inputs and links, makes a
> very good start!
>
> On 9/18/10, Magnus Danielson<magnus at rubidium.dyndns.org>  wrote:
>> On 09/18/2010 09:48 AM, francesco messineo wrote:
>>> Hello all,
>>>
>>> sorry for the OT, but the electronic expertise of the group is too good
>>> :-)
>>>
>>> I'm looking for ideas and directions (articles and so on) to realize
>>> very good phase noise xtal oscillator, in the range 20-50 MHz for high
>>> performance frequency conversion. I would like to understand what
>>> circuits can be realized (not requiring too much professional and
>>> modern equipment, test eq. from the 70s-80s is ok) and what is the
>>> contribution of the active oscillator device, the xtal itself and the
>>> following buffers.
>>> Another idea that came on my mind was using digital oscillator (square
>>> wave, cmos) and then filtering for sine output, if this makes sense
>>> for a low PN point of view.
>>> Is there any way to measure the close-in PN of oscillators with an
>>> amateur setup?
>>
>> First of all I think you need to quantify what you mean by "high
>> performance frequency conversion" and what stability measures you are
>> seeking as there are many degrees of excessiveness to attempt, and many
>> of them may be well beyond what you need. Remember, we are time-nuts... :)
>>
>
> Ok, let's say as good as practically and economically feasible for
> "single" prototype and homebuilder. I already chosed not to use a
> Si570 because I really need only few (2-4) fixed frequencies and I'm
> assuming that carefully made xtal oscillators can beat the Si570 phase
> noise performance.
> The conversion is obviously for a receiver, not for  the classic HF
> bands, but  for the lower VHF amateur bands (50-70 MHz) where IMD3
> performance of the receiver has to be the best possible, as these
> bands are used for TV and radio broadcasts in many nearby countries
> around here.
> Of course a very good frontend BPF, amplifier and mixer are needed,
> but these are less of a problem for me to chose (and are simpler to
> evaluate with "standard" test equipment too).
> Unfortunately I know very few  low-VHF-nuts and very few of them (if
> any) realize their setup performance are so far distant from what can
> be achieved nowadays.

One solution would use a stable standard oscillator, say 10 MHz, and 
then use a bandpass filter to select suitable overtones for first 
mixdown. You can select several options for selection of overtones, but 
fixed LC-resonators comes to mind.

Another variant is to use a fairly low-noise VCO and then PLL lock it 
with wide bandwidth to a stable fixed reference (such as a 5 or 10 MHz 
TCXO or OCXO of your choice, possibly divided down to suitable 
step-frequency) as the PLL does some interesting things with phase 
noise... within the PLL bandwidth the reference phase noise will 
dominate where as outside of the PLL bandwidth the VCO phase noise will 
dominate. This comes in handy, and for such PLL applications you want 
the PLL to be wideband.

A third alternative is to again let a stable reference of choice drive a 
modern DDS chip, for instance AD9971 or so.

I am not a radio amateur, so I won't be able to say which is the best 
solution for your needs, but that is at least what I would be looking at 
if I where to build something like this.

The link to Enrico I sent you is more the knowledge of the field, but if 
you follow the links to Wenzel and Bruce stuff you have some designs to 
look at. I wonder if you really need to go deep into the field to get 
satisfied.

Cheers,
Magnus



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