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From: "Alan Melia" <alan.melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
References: <F4A5BDDE17774D58A6400E2EFA82F93E@AGB>
Date: Sun, 21 Aug 2011 23:26:01 +0100
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Subject: LF: Re: Minimum carrier  detect  level  -140 dBm   is that  good ?
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Hi Graham it may not enough to remove the sig gen output and terminate it.
You must also ensure that the braid is connected (in some way) to the aerial
ground of the receiver to ensure that there is no cable leakage through the
ground connection. It is probably ok But it depeneds on the quality of the
generator. Many Marconi generators have a ferrite choke on the coax to the
attenuator to ensure that signal does not leak past the attenuator. Ideally
use a good VHF attenuator outboard which has an "off position".  You then
set the same level with a mixture on internal and external attenuator
settings. (This really shows up poor case isolation) At these sort of levels
getting genuine test signal levels can be quite tricky. All the receivers
are good quality and would be expected to have a good performance but then
again this would be meaured/specified in quite a wide bandwidth compared
with the one you are probably measuring in.

As a guide after a lot of care I found I could just detect  a 5nV signal on
a waterfall above the receiver noise (AOR 7030) I cannot now be sure what
the effective bandwidth was but I had a 2kHz wide waterfall.....so I am
guessing it might have been about 2Hz. this would equate to around -154dBm
(I think) It obviously depends on the receiver noise which may vary and the
levels you quote would not have been unreasonable or have degraded the
signal in say a 500Hz bandwidth. I did this test prior to Spec Lab becomming
available when the waterfall s/w was a bit primitive, and I have not
repeated it.
To give a clue I think I was running Win3.1 on that machine, at best it
would have been W95. I think the program was FFTDSP45 on a floppy from the
US for $30 !!

Alan
G3NYK

----- Original Message ----- 
From: "Graham" <g8fzk@g8fzk.fsnet.co.uk>
To: <rsgb_lf_group@blacksheep.org>
Sent: Saturday, August 20, 2011 10:28 PM
Subject: LF: Minimum carrier detect level -140 dBm is that good ?


Minimum carrier  detect  level  -140 dBm   is that  good ?

Run into  a odd  problem  on 500Khz , neither the RA6790 (with LF front end
/ no  rf  pre amp)  or the  RA1778   will  show a  carrier on 504 Khz , that
the  Skanti R5003 , shows at 10/20 dB over the  noise  level  using
'spec-lab' as the indicator, R5003 , has  no  pre-amp , direct to  first
mixer. same  as the RA 6790

Using the  sig-gen  and  60 dB  atten , the  RA1778 minimum  trace level
is  -150 / -155 dBm (has  rf-pre amp) , The  6790, round  -140 / -145..
removing the  rf  cable  and fitting  50  ohm load to  gen o/p with  atten ,
removes the  trace  from the  rx, so  assume its  not  leakage

As  these  levels  are  well below the  ambient  noise  levels, it not a
sensitivity  problem .??. adding a  stepped attenuator  to the  Ae  signal
path , makes  no  difference, other than to  alter the  noise  level..

The   500  khz feed is  from the  atu  and the  vswr round 1:1.2 on tx .The
R5003  is the  same  lay-out , 40 meg 1st IF > 1.4 second (rx section  of
the  TRP5000)

Any  ideas  or  suggestions  as to  'how'  to  solve  the  problem ..

wide band  noise  causing the  roofing filter to  ring may  be?

Tnx -Graham