[time-nuts] OT: xtal osc PN
francesco.messineo at gmail.com
Sat Sep 18 14:12:57 UTC 2010
On 9/18/10, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> 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.
This is a neat idea, but works only for overtones of the standard,
some of my needed frequencies aren't overtone of 10 (or 5) MHz.
> 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.
this is also interesting, but again, isn't a PLL overkill for just 4
fixed frequencies? I don't mind building separate oscillators. However
the PLL approach could be interesting for other reasons (stability),
any pointer? :-)
> 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 best solution depends on many factors:
-) if there were many others with my same needs, we'd probably find
better to use a modern DDS (share the pcb making and someone who can
do the soldering of such packages), instead, so far the low-VHF people
aren't generally aware that better frontends are possible, you find
lot of work in the HF receivers and next to nothing in the lower VHF
where big signals and intermodulations come from broadcasts and not
from other amateur stations!
-) if much more than 4 different frequencies would be needed, then PLL
would be the best choice anyway;
-) if cost was not an issue, probably the best thing would be ordering
4 ready made OCXO from a respectable company :-)
> 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
Probably not that deep, I'm convinced that a well studied and known
good xtal oscillator circuit could already do the job, I'm just not
able to judge the circuit myself, so I must ask for other's advice
(while I try to setup my own PN test bed).
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