[evlatests] More on System Non-Linear performance

[Jim Jackson]

Rick,

Rob and I are doing some tests  in the lab right now attempting to 
simulate the effects of a strong interfering signal on the T302.

Another question comes to mind - Since there are concerns about 
unexplained linearity, compression, gain expansion, interfering 
signals, etc... in L, S and C bands, should we really be testing the 
3-bit digitizers with C-band?  Maybe we need to move that testing to 
one of the high frequency bands.

Jim

At 02:30 PM 5/3/2011, Rick Perley wrote:
>     I reported a few weeks ago on an unexpected result, wherein
>observations of Cygnus A, which increases system power by a significant
>factor (factor of 4 at L-band, 3 at S-band, and 2 at C-band) caused the
>PDif values to noticeably decline, typically by 10% or so.  This should
>not happen in a linear system with fixed system gain.  Probably
>importantly, there is no such effect at X-Band.  (The effect has not yet
>been tested for at higher frequencies).
>
>     I now have more evidence of the effect, taken from the S-band data
>taken on 3C273.  This object is at zero declination, so it rises
>more-or-less in the east, transits through the geostationary belt, and
>sets in the west.  The observations spanned 2 through 4 GHz, and we have
>switched power in each of the 16 subbands (each 128 MHz) wide.
>Satellite radio lies within subband 3, while another strong emitter lies
>in subband 2.
>
>     The effects of XM radio are easy to see.  As the source approached
>the geostationary belt, PSum increased by up to a factor of 150 within
>subband 3.  The interferor in subband 2 is clearly also in the
>geostationary belt, as the power within that subband increased also, but
>'only' by a factor of about 15.  Unsurprisingly, the PDif values in both
>these two subbands are completely trashed during the time when the
>satellite signals are dominating.
>     In an ideal linear system, these effects should be confined to the
>subbands in which they lie -- so in our case, all the other subbands
>should show PSum and PDif values completely independent of what's
>happening in the trashed subbands.
>
>     But that's not what happened (sad to say).  All the PDif and PSum
>values in the 'good' subbands show strong perturbations which reflect
>the increased power in subbands 2 and 3.  In all cases, the PDif and
>PSum values decline, by up to 30%.  We are seeing strong evidence for an
>overall gain compression.  The effects are the same in all subbands
>(other than 2 and 3).
>     That the compression is present in subbands 9 through 16 (3 to 4 GHz
>tuning, using the B and D IFs) is strong evidence (but not proof) that
>the problem lies upstream of the T304 module.
>     The 150-fold increase in power in subband 3 translates to an
>increment of about 10 over the entire bandwidth at S-band -- about 10
>dB.  This is significant, but well within the specs for the signal
>chain.  Since the synchronous power measures are made before the
>requantizer, we can't blame the latter process for the effects seen.
>
>     We need to understand -- and correct -- the origin of this problem.
>_______________________________________________
>evlatests mailing list
>evlatests at listmgr.cv.nrao.edu
>http://listmgr.cv.nrao.edu/mailman/listinfo/evlatests