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Lbn cal values measured 18dec00

18dec00

     The cal values for lband narrow were measured on 18dec00. The standard lbn cal diode was compared to the standard diode. Using the standard diode value and the insertion loss the lbn cal value was computed every 25 MHz from 1200 MHz to 1600 Mhz. The setup was:
    The lbn cal was injected via a waveguide coupler into the waveguide that connects the orthomode transducer to the dewar. For this measurement, the orthomode transducer was removed and a waveguide transition was installed. A cold load (at 70 K) via a 4 foot cable was connected to a 20 db directional coupler. This coupler was connected to the waveguide transition. The standard diode was connected to 18 db of attenuation and then into the coupled port.
    The observing setup was 4 25 Mhz bands on the correlator with a 15 second integration time. The 4 step observing sequence was (15 seconds at each step): calOff, calOn, calOff, standard On. After each sequence the frequency was stepped by 100 Mhz to cover 1200 to 1600 Mhz. The entire frequency band was measured 3 times.
After the measurement, the insertion loss from the standard diode to the output of the coupler was measured over the frequency range 1200 to 1400 Mhz. The plots show the results of the measurements:
  1. Figure 1 shows the Cal values versus frequency after correction. The values printed are the averages for the 3 measurements at each point (the outliers for polA at 1300 to 1400 Mhz were excluded from the average).
  2. Figure 2 shows the  the standard diode in Kelvins versus frequency. The standard claims to have an ENR of 15.19 at 1 Ghz and 14.93 at 2 Ghz. The ripple comes from the measured insertion loss through the  coupled port of the coupler. The standard value is assumed from its calibration. Only the insertion loss was measured.
  3. Figure 3 shows  calOn/Tsys(caloff) for the lbn cal and the standard. The ripple in the standard is from the insertion loss. The repeatability of the measurements were on the order of a few percent (except for 1 set of standard measurements at 1300-1400 Mhz).
The insertion loss for the waveguide transition was measured by putting two transitions back to back and measuring the loss. This was about .2 db but had some ripple of up to .4 db toward higher frequency. We don't know whether this was from the two transitions together or from the single transition we used. I took .1 db as the insertion loss for the transition over the whole band.  Fits were made to frequency dependence of the attenuator/coupler and ENR of the diode. The standard in degK at the input to the waveguide was computed as:

    stdDegK=10^(diodeEnr(freq) + attnCouplerInsertionLoss(freq) -.1)*.1)*290.

The lbnCal was then computed as:

    lbnDegK= (lbnCalon-lbnCalOff)/(stdCalOn-stdCalOff)  *stdDegK

The measurement assumes the gain of the system remains constant over the 1 minute cycle. The repeatability of the measurements attests to this.

    The major uncertainty in the measurement is in the standard diode ENR and the insertion loss of the waveguide transition. The .1 db transition may be a bit low at some frequencies. This would overestimate the lbn cal value by a few percent (mainly at the higher frequencies).
lbn cal values measured 18dec
freq [Mhz]
calval polA [K]
calval polB [K]
freq [Mhz]
calval polA [K]
calval polB [K]
1212
1.12 
1.16
1412
1.83
1.83
1238
1.18
1.24
1438
1.89
1.92
1262
1.23
1.34
1462
1.92
2.01
1288
 1.33
1.43
1488
2.08
2.15
1312
1.48
1.61
1512
2.12
2.25
1338
1.57
1.72
1538
2.22
2.41
1362
1.70
1.76
1562
2.29
2.50
1388
1.78
1.85
1588
2.28
2.62

    The data has been included in the idl routines that return the system temperature. They can also be found in the file: /share/obs4/rcvm/cal.datR6.
Note:13dec01.. There was a mistake in the original computation of the cal value.. When interpolating the diode enr I used : delta(enr)/delta(freq) + freq0.. I inadvertantly used the enr at 2000 rather than 1 Ghz for freq0. The above values are the correct ones. The old incorrect values can be seen here: (cals with 6% error).

processing: x101/001218/lbncal.pro 

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