[evlatests] T302 Linearity Test

[Keith Morris]

I measured this recently in the lab.  LO-filter.pdf shows the LO leakage 
out of the T302, with (red) and without (blue) the T304 input filter. 
The second file, noise-plus-LO.pdf, shows the filtered LO leakage 
superimposed on the nominal T304 input noise power.

I injected into the T304 a signal consisting of x-band noise plus the 
L301 at the levels shown in the plots.  For any of the most common L, S, 
or C-band L301 tunings, I was unable to measure any gain compression in 
the T304.



Paul Harden wrote:
> 
> Bob Hayward wrote:
>> If the non-linearity is somehow coming from the LO used in the T302 
>> leaking into the T304 and compressing the first RF amplifier, then 
>> switching in the Solar 20 dB Attenuator wouldn't affect the strength of 
>> the LO birdy (since the Solar Attenuator is in front of the T302 mixer). 
> 
> The T302 is an upconverter using high-side LO injection.  For C-band, 
> the LO is in the order of 16 GHz or higher.  All T302s received 
> extensive bench testing to ensure they were compliant to specifications. 
>   Test data sheets exist on each T302 module build.
> 
> The amount of LO power and birdies was one of the things tested on each 
> IF.  I believe the specs were LO leakage in the IF must be at least 40dB 
> below the LO power, and -70dBm minimum on the IF outputs.  The average 
> IF output power was around -35dBm.  I do not recall ever having a 
> problem with achieving the LO leakage/spurious specs.  There was almost 
> scarcely a hint of the LO in the IF outputs.  That was measured with the 
> 8-12 GHz IF filters on the T302 output (even though further downstream 
> in the system).  It seems to cause compression problems in the T304 
> amplifiers, the LO leakage would have to be tens of dB above the nominal 
> -35 or -40 dBm IF power.
> 
> Still, it may be a good idea to look at the C-band inputs to the T304 to 
> ensure nothing has changed, or we don't have LO leaking in from some 
> other source (bad cable or something).
> 
> Eric Chavez, who now takes care of the T302s, can better answer what 
> these power levels are.
> 
> Paul Harden
> 
> 
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-- 
Keith Morris
National Radio Astronomy Observatory
1003 Lopezville Rd.
Socorro, NM 87801
575-835-7060 (phone)
575-835-7027 (fax)
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