[hpsdr] Preselectors

[Larry Gadallah]

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
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