[evlatests] UX Converter tests

[Paul Harden]

Ken Sowinski wrote:
> The first step should be to
> measure Y factor at the downconverter input under various T303 setups.

I would expect (hope?) this to show little difference.  The transfer 
switches establishing the T303 mode manipulates the IF outputs.  There 
is plenty of isolation between input RF and output IF due to the 
amplifiers, filters and mixers in between.  Still, it would be a good 
test to confirm lab isolation measurements.

> If we don't get the same answer for the two paths at the same sky
> frequency then we fix that or understand it before going further.

You will not beable to observe the same sky frequency with both the 
X-direct and Ku converted paths.

 From a couple of conversations today, it appears there's some confusion 
on how the UX converter works.  Hoping I'm not being redundant, I offer 
the following to ensure we're on the same "wavelength" (sorry):

A simplified block diagram is on my website space at:
http://www.aoc.nrao.edu/~pharden/EVLA/EVLA.html
Top table shows the sky frequencies/powers for each receiver
Block diagram and specs in the file "T303 Quick-Look Summary" (.pdf)

The frequency input to the UX converter from the receivers is 8-18 GHz. 
This full range is amplified by a low noise input amplifier.  From the 
output of the input amplifier ...

The 8-12 GHz portion is bandpass filtered, forming the 8-12 GHz X-direct 
IF path.

The 12-18 GHz portion is bandpass filtered and applied to the mixers for 
down-conversion to 8-12 GHz IF.  The exact 4 GHz bandwidth portion 
converted to the 8-12 GHz IF is a function of the LO-2 and/or LO-1 
frequencies.  Thus, there is no 8-12 GHz input power applied to the mixers.

The only frequency common to both paths would be 12 GHz, which would be 
ill advised for use as this is on the filter band edges, and likely 
where the input LO is residing.  (The input LO is doubled by the UX 
Converter, such that a 24 GHz LO is actually formed by a relatively 
"loud" +7dBm 12 GHz LO input).

In the block diagram, transfer switches S1 and S2 are shown in the 
"normal" mode ... IF-A/C being the 8-12 X-direct and IF-B/D the 12-18 
GHz downconverted product.  For example, this splits the input RF into 
two IF's for 8-16 GHz continuous coverage (8-12 on IF-A/C and 12-16 down 
converted on IF-B/D).

Changing S1 to the other state selects the second mixer as the source 
for IF-A.  This splits the input RF into two IF's for 12-18 GHz 
continuous coverage (12-16 IF-A/C and 16-18 portion on IF-B/D with 
proper LO settings).  *This mode is used for Ku-band only (12-18 GHz)*, 
as LO-1 is used by the other receivers (K, Ka, Q) for the 1st 
downconversion to 8-18 GHz.

Changing S2 to the other state is a diagnostic mode only.  In essence, 
it makes IF-A = IF-B.  You might find it useful for checking power 
levels/performance in parallel channels of the T304 downconverters, DTS, 
etc.

A few other items that might be of interest:
1) The mixers are "on" at all times, whether being used or not, or 
whether or not an LO is being applied, to present a constant load to the 
filters and input amplifiers.

2) Output IF power levels would differ, depending upon which 
x-direct/converted path is chosen and configuration of S1-S2.  When the 
UX Converters are built, we set the gains and attenuation to maintain 
the same IF output power +/- 1dB regardless of the mode.  This keeps the 
IF power to the T304s constant at all modes to keep from eating up a 
chunk of the ALC range otherwise.  Therefore, changing between modes, 
X-direct or converted, should not see much of a change in output power.

3) Specs dictate the maximum slope and ripple in the 8-12 GHz IF to be 
<3dB.  Most of the Norden MMIC converter decks are <2dB.  However, the 
power at 18GHz can be several dB lower than 8 GHz due to the gain 
rolloff in the receivers and higher cable losses at the higher frequencies.

I do not know how much gain roll-off there is between 8-18 GHz due to 
the receivers.

4) Lastly, internal cabling in the UX Converters has been changed a time 
or two to bypass the S1-S2 transfer switches to "hard wire" a different 
mode, or to fix an apparent polarization swap.  I can not attest to 
their current condition.  Hopefully they have all been restored to their 
designed configuration.

> M301
> We must verify that the M301 really controls all those switches in 
> the UX converter as we expect.

A test rack, absolutely identical to the antenna configuration, has been 
built to check out the M301, software, and all the hardware pending for 
imminent installation in antenna 24.  All future antennas will be fully 
tested via the executor controlling all system switches prior to antenna 
installation.  This will checkout everything, including the cables, to 
ensure we have a standardized and consistent installation hence forth.
The switch commanding and monitoring of the system switches is now 
working (on the test rack).  If a failure or error, hopefully we'll 
catch it in the lab before discovering it in the antenna.  (We really 
are trying to reduce the level of confusion on these things!)

> T304
> The big problem here is fringe rotation.  The sense of the fringe
> frequency will be reversed when the conversion path is used and I know
> of no provision in the software to allow for this.

The tuning is "inverted" in the converted paths; X-direct path is not.

Sorry for the robust reply, but I didn't want to see anyone wasting time 
analyzing corrupted data due to a confusion in the UX converter 
observing modes.

Paul Harden