Power levels for the alfa post amps.

oct 2002

        The power levels needed in the post amp system are computed using a Tsys, bandwidth, and fiber optic xmter noise temperature. The values used are:
 
Bandwidth  300 Mhz
Tsys  30 K
Dewar Gain 40 db
fiber optic xmter noise floor  (fiber xmter) -130 dBm/Hz

    Using -198.6 dBm/hz   (10*log10(1.38e-23) - 30(mW)) gives the following:
 
1. input to the dewar -98.6 dBm
2. input to post Amps (ignore losses) -58.6 dBm
3. noise floor fiber optic xmter over 300 Mhz. -45 dBm
4. Signal power needed to be 20 db above the fiber optic noise floor -25 dBm
5. gain needed in post amps 33.6 dB

  1. At the input to the dewar: 30 K Tsys and 300 Mhz bandwidth gives:  -198.6+ 15(30K) + 85(300Mhz)=-98.6 dBm  .
  2. With a dewar gain of 40 dB and ignoring the losses in any filters and cables.
  3. anacom fiber optic transmitter: ac231-2.5. -130Dbm/hz + 85 db (300Mhz)=-45 dBm. This is the closest part i could find to the AC321T-CA part that we are using. There spec sheet has -130 dBm/Hz for the output noise floor.
  4. This is so the fiber optics contributes less than 1% to Tsys.
  5. This is the total gain. It  includes the amplifier gain and any mixer, filter losses.
    The 33.6 db gain is the minimun gain so that the fiber optics does not add to the system temperature. The maxium gain allowed is determined by how strong the rfi is in the band. The fiber optic xmter starts to have trouble around 0 dbm. This will give us 25 db dynamic range.
    A table of gain,noise figure by component for the rf processor of alfa is listed below in the table:
 
element NF
dB		 temp
degK		 gain
dB		 cumGain
@input dB		 temp @
input K		 Tsys
Contribution K
sky .22 15.0 0 0 0 15.
dewar .22 15.0 40 0 15. 15.
filter1 2. 169.6 -2 40. 3e5 .02
circulator .3 20.7 -.3 38. 1.9e5 0.00
mga86575Amp 1.6 129.2 23.0 37.7 1.8e5 .02
mixer- hp iam8 15. 8880.6 15. 60.7 3.5e7 .01
coilcraftFilter 1. 75.1 -1. 75.7 1.1e9 0.00
2ndAmp 2.8 262.6 8.5 74.7 8.9e9 0.00
DirectCoupler 1. 75.1 -1. 83.2 6.3e9 0.00
fiberXmter 44. 7.3e6 0. 82.2 4.9e9 (-16.9dbm) .04
final 82.2 5.0e9  30.09 Tsys
    The first 3 columns are the noise figure, equivalent temperature, and gain of each device. colums 4 and 5 are the cumulative gain and temperature at the input to the device. The final column is the devices contribution to Tsys. The Noise figure value for the fiber is taken from  -130Dbm/Hz. Some of the vendors plots shows the noise figure as a function of fiber optical loss. The 44db could be as low as 38dB.
    The maximum allowed power input to the xmter is +10dbm.  The system above has -16.9 dbm at the fiber optic input. This would give about 27 db of headroom. Of course the 10 dbm max level may be where it melts , not where it goes non-linear.

Power levels at lband wide dewar.

 A spetrum analyz we placed at the output of the lband wide dewar and the spectrum was measured. This dewar has a gain of 40 db. The plots show how large a signal the postamps will have to deal with:     The plots from 18sep02 have peaks at -45 dBm using a 30Khz resolution filter. The may01 data has the 1350 Mhz faa radar going up to -22 dbm. Adding 40 db gain to -45 dBm should be ok. Adding 40 db to -22 dBm will saturate the system. The discrepancy probably comes from the different resololution bandwidths used. 18sep02 used 30 Khz while may02 used 3 Mhz. This is a factor of 20db difference. The power from the radars is on the order of 1 Mhz Bw so the 30 Khz filter width was not including all of the radar power in a single channel. The 3 Mhz resolution bandwidth was. The other problem is trying to measure a pulsed radar that is rotating with a swept oscillator system. You have to wait a long time to be sure that you sweep across the radar frequency when it is at its peak.
    The rf monitoring will be through a directional coupler before the fiber optic xmter. With a 20 db coupler, the power levels for the monitor will be centered on -45 Dbm.
 

Anacom 231T-CA transmitter

    We are using the anacom 231T-CA transmiter. Anacom  lists the 231W and 231-2.5  devices.These are tx/rcv pairs. Our xmter probably corresponds to the 231W-A which is the wide band low noise figure device (in 231W). They quote a 38 dbm noise figure with a 1 meter cable. The NF of the device depends on the optical loss (see the anacom design guide).
 

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