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" <[email protected]>
To: <[email protected]>
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
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