[evlatests] The 128 MHz comb zoo ..., take 2:

[Dan Mertely]

At the recommendation of Rick and Rob, I repeated my
search for the source of the 128*N comb line, this
time on ea26.  ea26 was reported by Rick to *not* show
the 23rd harmonic @ 2944 MHz.  The previous in-field
checks on ea10 were on an antenna which showed the tone
at 2944 MHz strongly.

Quick summary:

Although the L301 LO's on ea26 both show the 2944 MHz
harmonic at around the same dBc level as on ea10, the IF
output of the T302 LSC converter on ea26 was free of the
harmonic.

Details:

As with the tests on ea10 of last week, I first checked
the output of the S-band FE for the 2944 MHz harmonic.
As with ea10, it wasn't seen on ea26.  As a sanity check,
I swept 2300-2350 MHz to look for the XM satellite--it
was clearly seen in my 1 KHz RBW at an SNR of 10-15 dB.
The 2944 MHz harmonic was, however, not seen, not even
with a very narrow RBW (100 Hz),  and 10+ sweep averaging
to lower the noise level.

I then checked the output of the L301-1 (used to generate
IFA & IFC via the T302 downconverter).  The 2944 MHz
harmonic was clearly seen @ -59 dBm.  The main LO signal
was +9 dBm at 13184 MHz (S-band default A/C).  That's a
harmonic-free dynamic range of 68 dBc.

Next I checked the output of the L301-2 (used to generate
IFB & IFD via the T302 downconverter).  The 2944 MHz
harmonic was clearly seen @ -70 dBm.  The main LO signal
was +5 dBm at 13440 MHz (S-band default A/C).  That's a
harmonic-free dynamic range of 75 dBc.

As a reference, on ea10 last week we recorded the spur free
dynamic range of the L301-1 at 67 dBc, and that of the
L301-2 of from 64 to 69 dBc. (I hadn't measured the L301-2
output level, but would assume it was within the spec'd +5
to +10 dBm.)  So the 128*23 harmonic shows up at around the
same level down from the main signal on ea26 as it did on
ea10.

I also checked the L300 512 MHz comb reference output used
by the L301 modules for the 2944 comb line:  It was seen @
-69 dBm.  (Incidentally, although the 512 MHz comb is
"advertised" as a 512*N reference, the 128*N harmonics were
clearly seen in it's output in S-band at power levels not
much different than the 512*N harmonics!  Is this expected???)

After some confusion due to the L301-2 loosing lock and
generating 2, rather unstable LO's (one at around 8 GHz and
one at around 18 GHz), I was able to check the output of the
T302 downconverter module.  Surprisingly, the IFB & IFC
outputs were completely free of the 128*23=2944 MHz harmonic
(using a 1 KHz RBW), confirming Rick's results reported in
"The 128 MHz comb zoo ...".  As a sanity check, I swept
5-15 GHz in order to verify the passband, and saw the passband
hump clearly from 9.3-11.5 GHz, including numerous strong RFI
signals near the high (frequency) end (which I would presume
is our XM "beacon" signal again).

So, similar LO spur input to the T302 LSC converter generates
totally different spurious output, ea26 vis-a-vis ea10.  Either
the 2944 MHz harmonic is leaking in via some other path than
the LO, or the components or layout in the T302 converter vary
in their LO to RF-out isolation.

-Mert



Dan Mertely wrote:
> In search of the not so elusive 128 harmonic
> source, Ed Menne and I brought a spectrum analyzer
> (SA) up into ea10 @ BN8 this morning.  We chose as a
> target the 23rd harmonic of 128 @ 2944 MHz in
> S-band.  (The 128*23 harmonic was very strong in the
> RFI sweep, and, being an odd harmonic, eliminates the
> 512 MHz reference as a source.)
> 
> We had the operator (JC) set-up the system to
> "default S-band", which sets the L301-1 (for AC)
> to 13184 MHz, and L301-2 (for BD) to 13440 MHz.
> These 2 LO frequencies were verified at the antenna
> using the spectrum analyzer.
> 
> We 1st checked at the output of the Front End, to see
> if the 2944 MHz harmonic was being radiated out from
> some hardware then making it into the feed.  The
> answer is no.  There was NO 2944 signal coming out
> of the FE.  We set the spectrum analyzer to a very
> narrow RBW in order to maximize sensitivity (reduce
> noise).  The excess gain of the FE put the viewed
> noise level (NL) at -100 dBm (the  SA NL was intrinsically
> -120 dBm at this RBW).  Just as a sanity check, we
> looked for the XM radio signal down in the 2320 - 2345
> MHz range, and were able to see it.  So, the 128*N
> signal is not coming out of the FE.
> 
> We then checked the L305 reference generator module
> output and saw the 2944 MHz harmonic @ -87 dBm on the
> 128 MHz output (we tapped in on the spare output of
> the rack-mounted splitter).  (We didn't check the power
> level of the 128 MHz fundamental.)
> 
> We then checked the L301-2 output, verifying the set-up
> frequency of 13440 MHz (as mentioned earlier), and
> finding the 2944 MHz harmonic in the output @ -59 dBm.
> 
> Using 13440 - 2944 = 10496 MHz for the conversion, we
> then checked the output of the T304B LSC converter, and
> saw the 128*23 signal at the expected 10496 MHz.  The
> power level at this point was -87 dBm.  As a reference
> check, we tuned "up" (reverse spectrum) around 600 MHz
> to look for the XM radio signal in the 1st IF passband,
> and saw it clearly around 11104 MHz at -80 dBm.
> 
> Finally, we checked the output of the L301-1 LO.  The
> main output was at 13184 MHz @ +7 dBm, and the leaky/
> sneaky 128*23 (2944 MHz) harmonic was also seen in the
> output at -60 dBm.
> 
> 
> So:
> 
> 2944 MHz is seen at the L305 output, but low.
> 
> 2944 MHz is seen at the L301 outputs, but down 67 dBc.
> 
> 2944 MHz is seen in the IF passband at the T302 output,
> within 7 dB of the strength of the XM satellite (one
> of the strongest sky sources in S-band).
> 
> Since in the correlated RFI sweep data of 18 March, 2011
> the harmonic isn't clearly seen above C-band (except for
> one occurrence in Ka band at 128*230=29440), I (with
> input from Rob Long) would suspect some problem in the
> T302 mixer scheme.  (With some other cause for the single,
> strong harmonic detection in Ka band?)
> 
> I leave it to LO/IF to find the mechanism.
> 
> -Mert
> 
> 
> 
> 
> Rick Perley wrote:
>>     I've labored hard to try and understand the characteristics of the 
>> 128 MHz comb -- largely without success.  The characteristics vary by 
>> band and even by frequency within a band. 
>>     But a few general characteristics seem clear. 
>>
>>     A)  Harmonics of 128 MHz are seen in L, S, and C bands.  Only a 
>> single harmonic is seen in X-band -- at 8192 MHz.  This one probably has 
>> a separate origin within the T304. 
>>
>>     B)  The odd harmonics are usually, but not always, the strongest.  
>> Not all odd harmonics are visible, and some even harmonics are very 
>> strong. 
>>
>>     C) There is no spatial correlations for these harmonics -- the 
>> 'loudest' antennas (which vary by band -- see below) are randomly 
>> distributed.  This clearly indicates an internal origin. 
>>
>>     D) At L-band, antennas 9, 10, 14, 20, 24 and 26 appear to have no 
>> 128 MHz harmonics.  All the others have varying degrees of the  tone. 
>>
>>     E)  As S-band, the harmonics are generally much stronger than in 
>> either L or C bands.  But the situation varies remarkably (or 
>> depressingly, depending on your POV) depending on which harmonic we look 
>> at.  For example, for the 21st harmonic (2688), antennas 6 and 7 are 
>> 'clean'.  But for the 23rd harmonic (2944), antennas 16 and 26 are 
>> 'clean', while 7, 8, 9, 24, and 28 are strong.  For the 25th harmonic 
>> (3200 MHz), the situation is again different:  antennas 14 and 26 are 
>> completely clean, antennas 16 and 24 give only very weak birdies.  For 
>> the 27th harmonic, antenna 10 and 26 are now 'clean', while 8 is 
>> especially bad.  (I gave up trying to characterize the harmonics  higher 
>> than this at this stage ...)
>>
>>
>>     F)  At C-band, the situation is again different ... <audible 
>> sigh>.     The 32nd harmonic (4096 MHz) is generally quite weak, but is 
>> strong on antennas 10 and 12.   The same pair gives a strong harmonic at 
>> 4992 (39th).   But at 5504 MHz, (43rd harmonic), only antenna 10 is 
>> bad.   But (sadly), the situation deteriorates at 5888 MHz, where 
>> antennas 10, 14, 20 and 24 are all very bad, while 15, 27 and 28 are good. 
>>
>>     In summary, multiples of 128 MHz show up everywhere at the lowest 
>> three bands, but it is very difficult to identify particular 
>> commonalities. 
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