W3JDR wrote:
So it's not a "mathematical fact" in the sense that any of us can look it up
and see how it was derived, it's your recollection of something you heard
and vaguely remember convincing yourself it could be true, right?


Andy replies:
That's right. But just because I don't remember how to derive it,
or can
give you the name of the paper that I read 25-30 years ago, it doesn't
mean it's wrong. So I would suggest you do your own research....

Whether it is true, or not true, IS a mathematical fact, and if you
are
capable of understanding the math proof, you are probably also
capable of proving or disproving it yourself on paper. Me, hell , I
always
had a good lab where I could try things out if I had doubt.....

It is probly easier, if you have access to a good lab, to rig up an
experiment to find out..... That way you wouldn't have to bother with
newsgroups to learn about this stuff.....

I would encourage you to try. If you come up with a good answer you
can be proud of, post it back here for us all to see. I am sure many
people here would be interested , since lots of time is often wasted
coming up with circuits whose purpose is doomed from the start....
such as using "hard limiters" to improve the SNR........

As far as the "tangential sensitivity", you can probly do a google
search and learn all about it.... If you haven't run into it yet, you
probably don't deal with OOK pulses like radar and stuff. There's no
shame in that.....

:)))) Andy in Eureka, Texas W4OAH

Andy writes

Well, I saved you some time by googling it myself. It took about 10
seconds
to be able to cut and paste the definition I have been talking
about....

Note that in this paper it says TSS is eight db +/- one db ....
and depends on several system factors, including the
actual detector used... From experience, the detector type,
and the absolute signal level it is detecting, is a BIG DEAL.
It needs to operate in the "perfect diode" region . Doing the
detection in the square law region buggers up the measurement.

That +/- one db is due to the accuracy that one operator can set up
the
scope versus the next operator that comes along. I use 8.5 db, and
I DARE anyone to set the same measurement up 10 times in a row and
get the same answer to less than +/- one db.

Agilent probly uses a math derivation to get 8 db to accomplish
some specific criteria, and did not specify a detector type, hence
it is a "mathematically perfect " answer. Furthermore, they use a
different
definition, but ORIGINALLY it was bases on the FIRST SENTENCE in
the cut and paste below...

The Agilent App Note that Bill Sabin refers to also came up on the
first page of hits, and you can go read it for yourself.

I hope this satisfies your curiousity. I had nothing better to do
today..... Now, I'm outta beer, and Jeff Foxworthy is on TV,
so 73s

Andy W40AH


***********CUT AND PASTE FROM GOOGLE HIT *************************

TANGENTIAL SENSITIVITY
Tangential sensitivity (TSS) is the point where thetop of the noise
level with no signal applied is level with thebottom of the noise level
on a pulse as shown in Figure 6.

Itcan be determined in the laboratory by varying the amplitudeof the
input pulse until the stated criterion is reached, or byvarious
approximation formulas.The signal power is nominally 8±1 dB above
thenoise level at the TSS point. TSS depends on the RFbandwidth, the
video bandwidth, the noise figure, and the detector characteristic.

TSS is generally a characteristic associated with receivers (or RWRs),
however the TSS does not necessarilyprovide a criterion for properly
setting the detection threshold. If the threshold is set to TSS, then
the false alarm rate israther high.

Radars do not operate at TSS. Most require a more positive S/N for
track ( 10 dB) to reduce false detectionon noise spikes.
SENSITIVITY CONCLUSION
When all factors effecting system sensitivity are considered, the
designer has little flexibility in the choice ofreceiver parameters.
Rather, the performance requirements dictate the limit of sensitivity
which can be implemented by theEW receiver.1. Minimum Signal-to-Noise
Ratio (S/N) - Set by the accuracy which you want to measure signal
parameters and by thefalse alarm requirements.2. Total Receiver Noise
Figure (NF) - Set by available technology and system constraints for RF
front end performance.3. Equivalent Noise Bandwidth (B ) - Set by
minimum pulse width or maximum modulation bandwidth needed
toNaccomplish the system requirements. A choice which is available to
the designer is the relationship of pre- (B ) and post-IFdetection (B )
bandwidth. The most affordable approach is to set the post-detection
filter equal to the reciprocal of theVminimum pulse width, then choose
the pre-detection passband to be as wide as the background interference
environmentwill allow. Recent studies suggest that pre-detection
bandwidths in excess of 100 MHz will allow significant loss of
signalsdue to "pulse-on-pulse" conditions. 4. Antenna Gain (G) - Set by
the needed instantaneous FOV needed to support the system time to
intercept requirements.

*****************END OF CUT AND PASTE
***********************************