[eagle] Re: IMD, Phase Noise, a New Spur, and EMI suggestions

Rivera, Juan A. Juan.Rivera at gd-ais.com
Thu Jun 28 08:03:10 PDT 2007


Bob,

I'm only simulating a clean CW signal of the amplitude predicted by John
as worst case.  I've been thinking of a field trip up to Marysville to
try to capture actual PAVE PAWS using the Matt Ettus USRP board.  That
might give us some real world signals to work with.  But until then I
have no idea what PAVE PAWS phase noise would look like, or if the data
in the ATP adendum is correct or not.  It's derived from open source
information released during environmental impact proceedings for a
proposed PAVE PAWS upgrade.

Have you looked at the PAVE PAWS addendum in the ATP?  Do you think is
is accurate?  I want to make sure I'm on the same page with you.  Pages
50 through 53 spell out the bandplan, pulse duration for both modes,
repetition rates, etc.  The tracking mode pulses are 16 milliseconds in
duraction and chirp 1 MHz. Yes, that's a pulse, but it's extrememly
long.  There are four channels that look like they will have the most
impact - 434.4, 435.6, and 436.8 and 438.0 MHz.

Assuming that this information is accurate, and assuming that the
uplinked data decoder dies when the receiver phase noise I'm seeing
buries the uplink signal, then my thinking goes like this:

Based on the test results I posted the other night, a clean signal at
-43 dBm (PAVE PAWS) will bury a clean adjacent signal at -112 dBm (ham
uplink) when it gets within about 7 kHz.  So if we do the math - 16 mSec
to move 1 MHz = 0.24 milliseconds to move 15 kHz (the distance required
to move from one side of the uplink to the other) The rep rate for
tracking mode is 54 milliseconds so I would expect to see a .24 mSec
drop out every 54 mSec when illuminated with PAVE PAWS tracking mode
EMI, under worst case conditions.  I would think that if you bulit in
enough forward error correction to deal with that then you could uplink
right through PAVE PAWS, assuming that my simple test produced accurate
results.

Something else to consider is the recovery time when the receiver is hit
by a strong pulse.  I haven't looked into that yet but it could add some
time to the .24 mSec drop out.

Bottom line - Yes, I know my test is simpistic but it's the best I can
do with what I have available now.  If we can capture actual PAVE PAWS
using the USRP board then we can conduct much more sophisticated tests
using that digitized PAVE PAWS signal as the EMI source.

73,

Juan



-----Original Message-----
From: Robert McGwier [mailto:rwmcgwier at gmail.com] 
Sent: Thursday, June 28, 2007 5:01 AM
To: juan-rivera at sbcglobal.net
Cc: eagle at amsat.org; Bill Ress; Dave Black (Home); Dave Black (Work);
Dave hartzell; David Smith; Don Ferguson; Rivera, Juan A.;
Samsonoff at Mac. Com
Subject: Re: [eagle] IMD, Phase Noise, a New Spur, and EMI suggestions

Juan:

If we have had this discussion before,  forgive me.  I have misplaced
both the interaction and my memory.

What is the generator for the "Pave Paws" source?  I ask this because
the phase noise for the "Pave Paws" source does not seem to be included
in your consideration.


Pave Paws is a pulsed instrument.  It will not have its energy
concentrated at a single frequency.  Its energy will be spread
considerably and much of that energy will fall outside of the front end 
filter of the 70 cm RX.   A more realistic test would compute that a 
-70.2 dBm Pave Paw signal will put  (say) -90 dBm inside our passband
and further, it will be on/off.  Since it is on/off,  there will be gaps
where the amateur signal is not drastically impacted.  A receive system
on the ground, necessarily narrow band,  will provide processing gain. 
If we do a good job of writing code, and we do not run out of processing
cycles, it would be nice to do pave paws pulse detection and
subtraction.

My point here is, the pave paws situation is a great deal more complex
than your test is capable of revealing.

Bob


--
AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL, TAPR,
Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair "If you're going
to be crazy, you have to get paid for it or else you're going to be
locked up." Hunter S. Thompson



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