> >Re: Simrex - GLB Pre-Selector Verus
> >You're actually comparing two different boxes.
> n...@... wrote:
> Nope - comparing GLB vs. GaAsFET/pass cavity combo
Still not the equivalent box... you're still missing
the post active-device stages.
> > I have measured all the original GLB Pre-selectors
> > and their performance values are very realistic
> > (no surprises).
> Care to publish your results here?
Never thought about it much... those who were honestly
interested in the subject the last time we rehashed the
GLB active device topic here on the RB Group Emailed me
direct and we exchanged various notes, diagrams and
information.
Much of what I have is based on the Pre-selector with the
dual-gate mosfet, which I believe is similar to the BF-998
device.
>>>> GaAsFET preamp is ~$130. I don't think I've ever
>>>> paid more than $50 for a pass cavity, so the total
>>>> is ~$100 less than the GLB unit.
> >Reads like you're comparing a new Preamp with a used cavity
> >against the price of a new Simrex (GLB) Pre-selector. That's
> >not really fair...
> Perfectly fair. Used pass cavities in good condition
> are plentiful. Can't remember the last time I saw a
> used GLB or equivalent unit for sale, so I'm simply
> comparing what's readily available.
Yeah, but the numbers are off. To properly compare the
two you'd need to use more than one pass-cavity. At least
one additional cavity (min) following the active device
and to really be honest, more than one trailing BP Cavity.
> A single pass cavity usually has enough out-of-band
> rejection to be totally adequate on its own - no
> post-preamp filtering needed.
The post-preamp filtering can and does contribute in
the management (not necessarily the prevention) of high
signal levels issues.
I don't really care to detail out the above text theory but
I will say I feel it's a critical often over-looked (at
least in this RB Group - threads example) subject.
> Then again, the fact that post-device filtering is used
> in the GLB makes me worry about the actual selectivity
> ahead of that device. If there's only 1 or 2 resonators
> ahead of it, that's not much protection. A 1/4 wave bottle
> will provide much more rejection ahead of that first amp,
> and with less loss hence lower NF.
There are 2 resonators in front of the Active Device. The
higher Q of a 1/4 wave cavity is obviously better. The honest
to thyself person should determine the NF difference, which
is probably not a huge amount.
> IMO the Simrex amplified preselector is a space-saving
> compromise, nothing more.
> Bob NO6B
Sure, it's a compromise that works well for what they are.
I'd probably (and do) park a Simrex or GLB Pre-selectors in
front of less than bullet-proof receivers.
If the active device in your Simrex / GLB Pre-selector is
a decent GasFet layout, the only major difference is the
pre and post resonant circuits and how helpful/useful they
are (or are not) in your application.
s.
