On 09/18/2010 04:12 PM, francesco messineo wrote:
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.
What frequencies you actually needed was not clear to me, so I assumed
that 30, 40, 50 and 60 MHz would be useful frequencies, in which case it
would be a simple and not very complex approach. It has been used in
frequency counters for ages to mix-down the signal and then count the
beat frequency.
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 simple PLL is not that complex these days. As long as you have fairly
high comparator frequency after dividing down the VCO and reference you
could get away fairly easilly. Standard programmable dividers in the TTL
family and a single chip for phase-comparator will work fairly well.
There is gazillions of examples among hams for this approach.
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!
Maybe the DDS board from the DMTD project would fit your needs?
-) if much more than 4 different frequencies would be needed, then PLL
would be the best choice anyway;
Actually, a DDS beats PLL on number of frequency any day of the week.
-) if cost was not an issue, probably the best thing would be ordering
4 ready made OCXO from a respectable company :-)
Certainly, it would be easy, but you would not learn anything.
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.
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).
John's and Bruce's pages alongside Wenzel should be the place to start then.
Cheers,
Magnus
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