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<mag...@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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