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

John B. Stephensen kd6ozh at comcast.net
Thu Jun 28 12:36:37 PDT 2007


It will be useful to see an actual PAVE PAWS signal in order to determine 
the level of spurious signals in the transmitter's output. If we are 
unlucky, we will see spurs in the PAVE PAWS signal that are higher than 
those generated in the receiver and it will impede our effort to mitigate 
its effects. If the signal is very clean, we may not be able to just play it 
back. The DACs on the USRP will have spurs that are 80 to 90 dB down.

73,

John
KD6OZH

----- Original Message ----- 
From: "Rivera, Juan A." <Juan.Rivera at gd-ais.com>
To: "Robert McGwier" <rwmcgwier at gmail.com>; <juan-rivera at sbcglobal.net>
Cc: "David Smith" <w6te at msn.com>; "Dave Black (Work)" 
<dblack at mail.arc.nasa.gov>; "Dave Black (Home)" <dblack1054 at yahoo.com>; 
<eagle at amsat.org>; "Samsonoff at Mac. Com" <samsonoff at mac.com>
Sent: Thursday, June 28, 2007 15:03 UTC
Subject: [eagle] Re: IMD, Phase Noise, a New Spur, and EMI suggestions


> 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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