[hpsdr] Preselectors

[kd5nwa]

Would it be realistic to be using spread spectrum across different bands?


At 04:59 PM 3/13/2007, Larry Gadallah wrote:
>***** High Performance Software Defined Radio Discussion List *****
>
>Graham, Francis:
>
>I know it's getting out into the wilds, but one possible concern over
>preselector switching times and transients would be some form of
>spread-spectrum. Perhaps it is a sufficiently distant possibility that
>it can be ignored, but I've seen many specs for synthesizers where the
>lock time was critical in order to maintain the frequency hopping rate
>required for a FHSS system. If such a system involved a tunable
>preselector, needless to say it would have to be able to keep up :-).
>
>Cheers,
>
>On 3/13/07, Graham Haddock <grahamh at verizon.net> wrote:
> > ***** High Performance Software Defined Radio Discussion List *****
> >
> > Frank:
> > Comments below...
> >
> > FRANCIS CARCIA wrote:
> > > The Cubic claims it can change frequency in 10 ms. The problem
> > > with relays is transients generated as the frequency changes due to
> > > relays making and breaking. Even solid state switches could cause
> > > transients during wide frequency sweeps.
> > Yes, any tuning modification to a tuned circuit will generate some kind
> > of transient.
> > What are you trying to do?
> > What is it that you are worried about?
> > Is a ten ms. transient when you switch filters, or settings inside of a
> > filter, unacceptable?
> >
> > If you are using Mercury for a spectrum analyzer, then I recommend that
> > you turn
> > off the preselector and the Mercury's preamp.  In that configuration,
> > the Mercury
> > has an IP3 of +50 dBm and a noise figure of somewhere around 20 dB.
> >
> > If you are doing spectral analysis requiring the preselector and 
> preamp, the
> > program will need to pause and throw away data for a few 
> milliseconds, every
> > time you cross an internal tuning boundary inside the preselector.  I
> > think the
> > preselector can be designed so as to have no tuning boundaries
> > inside a given ham band, other than perhaps 80 meters.
> >
> > > Discrete filters,
> > >  A standard board could be built with two different applications
> > > depending on parts installed. One configuration for Ham band only and
> > > the other for the whole spectrum.
> > > I've seen a number of SDR posts in different places where guys build
> > > brick wall preselectors to improve performance.
> > > I think the preselector should be in a shielded box away from digital
> > > noise or I'm about to leard some cool PCB layout skills from a master. fc
> > It might be a shielded card that plugs into the Atlas bus, or a shielded
> > box that
> > mounts elsewhere.  We are not that far along yet.  I am still trying to
> > fit it into
> > the allotted space for an Atlas bus card.  The larger inductors we are
> > considering
> > are not self shielding like a Toko can.  Toroids still have some
> > external field.
> > No PCB layout will fix that.
> >
> > --- Graham
> >
> > >
> > >
> >
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>
>
>--
>Larry Gadallah, VE6VQ/W7                          lgadallah AT gmail DOT com
>PGP Sig: 616D 4E52 CF1F 3FEC FFFB  F11B 7DB9 C79A EA7E B25B
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Cecil
KD5NWA
www.qrpradio.com www.hpsdr.com

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