Friday, July 30th 2010 - Duly noted on my calendar! LOL
----- Original Message -----
From: skipp025
To: Repeater-Builder@yahoogroups.com
Sent: Friday, July 30, 2010 6:53 PM
Subject: [Repeater-Builder] Re: Simrex - GLB Pre-Selector Pre-Amplifier
Re: Simrex - GLB Pre-Selector Pre-Amplifier
> Kevin Custer <kug...@...> wrote:
> What you are missing is that ANY losses ahead of the
> first active stage add to the noise figure of the
> system - directly.
What we have here is... failure to communicate... (a line
from a famous movie).
Actually I was trying to high-lite the active device
Noise Figure comparison and the lower spec'd gain value
of (at least) the GasFET GLB was mostly from the insertion
loss of the Pre and Post Filtering.
> So, while filtering exists in the GLB device, so does
> loss, and this loss is more than what is experienced
> when using a quality large diameter cavity.
Sure... but again we are comparing a box to another box
and they are not the same device. Please allow me to paste
some of the text from a recent post (by me).
[pasted text]
"The Simrex (aka GLB) units are actually amplified
pre-selector assemblies, not just plain wide-band
Receive Pre-Amplifiers.
[end of pasted text]
> Many times Skipp you tell us there is no free lunch,
> and the same applies to the comparison of selectivity
> and loss between the GLB and a quality cavity followed
> by a good active stage.
Correct... and a Simrex GLB Pre-Selector should really not
really be directly compared to a cavity followed by a good
active stage. The Simrex GLB box is more of a true Pre-Selector
layout and contains post amplifier filtering. To better
equate a similar layout would have you add at least one
or more cavities after the active device. And yes we should
clearly acknowledge the hopefully obvious lower loss through
a higher Q Quality Cavity.
> Real world test. Take a Hamtronics receiver (no preamp)
> and do a basic bench sensitivity test to obtain a baseline.
> If you find something around -123 dBm your in the right
> ballpark. Now install a bi-polar GLB preselector/preamp
> in front and measure the sensitivity again - you'll
> find you have lost several dB of bench sensitivity - at
> least 3 or 4 dB. Take the same receiver and add a quality
> 1/4 bottle with a good preamp (your choice - something
> with 1.5 dB NF or less) and do the test again. Now, the
> receiver hears at -123 to -127 dBm (dependent mainly
> upon the quality of the preamp that follows) because the
> filter hasn't severely ruined the system NF ahead of the
> first active stage. Even though the GLB has gain, the
> noise figure of the design has already determined the
> sensitivity that will be realized by the receiver that
> follows.
The primary land mine in the above comparison is the "Now
install a bi-polar GLB preselector/preamp". Remember Simrex
and GLB offer or did offer a GasFet version of their Pre-
selector box.
Separate the above in the proper context and the focus
should be on the filtering in front of the same type of
active device. No one here should discount the higher Q
cavity will be the better spec. But again even a high Q
cavity with a same or similar active trailing device is
still not the same box as the Simrex GLB Pre-selector.
You should account for the Simrex GLB integrated post
active device filtering.
> The GLB preselector/preamp should not be considered
> for adding basic sensitivity,
Did anyone make that claim?
> because it's possible (depending on how good the receiver
> is to begin with) the opposite will happen -
You are correct.
> however, it will protect a receiver that lacks good
> front-end filtering, like the Hamtronics. Like
> everything, the situation helps to dictate what
> equipment will give the best results.
We agree and it's a Friday... Mark today on your calendar.
What more could anyone ask for?
> Kevin
cheers,
skipp
