[evlatests] T304 Power Levels

[Steven Durand]

The EVLA base band down converters (T304) are designed to have an input 
of -38 dBm when the input attenuator is set to 8 dB.  The present Input 
Total Power Detector is not functional at this power so this level needs 
to be manually measured in the antenna to verify the input is really -38 
dBm.  (Or one could set the attenuator to 0 dB and use the Input Total 
Power Detector.  The Input Total Power Detector will read -21 dB when 
the input is -38 dBm.  Remember to set the attenuator back to 8 dB 
afterwards.)

The Output Total Power Detector has a functional dynamic range of 15 dB 
and is presently set to read from -20 dBm to – 35 dBm.  Unfortunately 
the samplers require an input between -34 to -38 dBm.  So this does not 
really work well either.  Travis is planning to spin a new board that 
will have a “set detector gain” adjust that will double the dynamic 
range of the Output Total Power Digitizer but this will not be ready for 
3-4 months.

One suggestion is to use the DTS samplers to set the levels.  This may 
have a few advantages because presently the DTS amplifiers and sampler 
characteristics are outside the level setting loop.  The Minicircuits 
amplifier we are using as the isolation amplifier at the input to the 
DTS module has a specified minimum gain of 10.4 dB at 2 GHz and 
"typical" value of 12.5dB but they do not specify a maximum gain.  The 
variation across the 1-2 GHz band is only about 0.2dB but we expect to 
see at least 3dB variation between units.  The Atmel sampler is 
specified to have an input full scale input of 500mV p-p with a 
tolerance of ± 2 percent so the problem is just with the amplifiers and 
maybe the cables.

So how should “we” set the power level into the samplers?