Increasing the isolation in the downstairs 260 MHz monitor ports.


Isolation of port1A after additional amp,pad added.

     On 23apr03 the isolation for the IF2 port1a monitor port was increased. After the increase it was still possible to affect the correlator bandpass by changing the cables on the monitor port. On 04sep03 the isolation was increased for IF2 monitor port 1a. The configuration for the ports were:
Port Configuration Loss 
port 1A before change 10dbPad-24dbamp(30dbRevIsol)-3db pad 19db
port 1A after sep03 change 10dbPad-24dbAmp(30dbRevIsol)-15dbPad-11dbAmp(16dbRevIsol) 36db
port 2A 11dbAmp(16DbRevIsol) 5db

    To test the new isolation two experiments were done.
    The first did a 900 second on/off with the correlator. The correlator was set to 25 Mhz bandwidth (over 1024 channels). After acquisition the data was smoothed by 9 channels to increase the signal to noise. The On was with a cable going from the monitor port to a 5 Mhz filter. The off was with nothing connected to the monitor port. This emulates someone connecting/disconnecting something on the monitor port during a normal observation. This was done to the modified monitor port (1A) and to an unmodified monitor port (2A). Without sufficient isolation, reflections from the 5 Mhz filter would cause ripples in the bandpass.
    The second test injected a sine wave into the monitor port and measured how much of it ended up in the correlator bandpass. This test used a 390 Khz correlator bandwidth with 1024 channels. The data was then hanning smoothed. This resulted in a resolution  of 760 Hz.
    The test results are shown in the plots:

    Double checking the reverse isolation for the sine wave test (assuming the losses through the splitters are the same)..
port1a: 16+15+30+10=71db reverse isolation
port2a: 16db reverse isolation
The difference is  55db.

The loss equation is (let T be the noise power in 1 channel,A be the amplitude of sinewave,L be the loss from the components above, X be the loss through the splitters back to the correlator, R be the measured ratio of sinePeak to T)
(A1*L1*X + T)/T=R1=.25db=1.07   Then  L1*X=T*(R1-1)/A1
(A2*L2*X + T)/T=R2=14.8db=30.2  Then L2*X=T*(R2-1)/A2

L2/L1=A1/A2 * (R2-1)/(R1-1)=100*(30.2-1)/(1.07-1)= -46db the expected difference was -55db so we're off by 10db somewhere...


    With the added attn,amp there should not be any problem using the monitor ports.
processing: x101/030905/