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EVLA Front-End CDR On-the-Sky Tests April 24, 2006: System Performance Evaluation

In this report, we present the results of the EVLA Front-End CDR On-the-Sky Tests conducted on April 24, 2006. The tests included evaluating system temperatures, band variations with elevation, and polarization characteristics across different bands. The results indicate that the system performance met or exceeded specifications, with superior elevation performance in the L-band compared to the VLA. Bandpass stability and amplitude differential snapshots were also assessed. Overall, the tests demonstrate the telescope's readiness for scientific observations, ensuring the satisfaction of researchers.

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EVLA Front-End CDR On-the-Sky Tests April 24, 2006: System Performance Evaluation

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  1. EVLA Front-End CDR On the Sky Tests EVLA Front-End CDR – On the Sky Tests April 24, 2006

  2. Proof of the Pudding … • Engineering can’t rest until the scientists are happy! • To have happy scientists, the telescope performance must meet the basic requirements. • Extensive testing is ongoing to test on-the-sky performance. EVLA Front-End CDR – On the Sky Tests April 24, 2006

  3. System Temperatures • Hot/Cold load tests done at L, C, K, and Q bands. • For L and C, cold load was on sky – requires spillover assumption. • For K and Q, liquid nitrogen cold load permits full calibration. • For all bands, sky dips from 90 to 8 degrees done to measure spillover contribution. EVLA Front-End CDR – On the Sky Tests April 24, 2006

  4. Tsys Results: L and C Bands • L-Band (antenna 13,with prototype horn). • C-Band (antenna 14, using VLBA polarizer) • At 4850 MHz, Tsys = 23K EVLA Front-End CDR – On the Sky Tests April 24, 2006

  5. C-Band Variation with Elevation • C-Band Tip Results • Spillover contribution • varies very little for • elevations above 20 • degrees. • EVLA spillover below • 20 degrees greater than • narrowband VLA feed. EVLA Front-End CDR – On the Sky Tests April 24, 2006

  6. L-Band Elevation Dependence • EVLA L-Band system has greatly superior elevation performance! EVLA Front-End CDR – On the Sky Tests April 24, 2006

  7. System TemperaturesK and Q Bands • On antenna 14, a complete set of measurements were possible. • Tsys is for the zenith, under dry conditions. EVLA Front-End CDR – On the Sky Tests April 24, 2006

  8. L-Band Polarization • Requirements are set to give a spurious linear polarization of <5% • The major spurious polarization contribution is due to the sum of two complex `D’ terms: Rrl= I(Dr1 + D*l2) • The `D’ value is due to amplitude and phase imbalances. • The sum can be made close to zero with good design. (But small |D| is better!) EVLA Front-End CDR – On the Sky Tests April 24, 2006

  9. L-Band Results • Antennas 13-16 with VLA OMT. • Red = on-sky measurements • Blue = Prediction from Lab measurements • Spike near 1450 MHz due to ‘suck-outs’ in VLA OMT. EVLA Front-End CDR – On the Sky Tests April 24, 2006

  10. More Results • Antennas 14 x 16. • Higher polarization on-sky is expected, as contribution of antenna itself is not in lab measurements. • High values near 1460 MHz due to VLA polarizer, and will be eliminated with new design EVLA Front-End CDR – On the Sky Tests April 24, 2006

  11. Bandpass Stability • Requirements are that bandpass amplitude be stable to .01% over timescales of 1 hour, on frequency scales < 0.1% of the RF frequency. • Bandpass phase stability better than 0.007 degrees on same scales. EVLA Front-End CDR – On the Sky Tests April 24, 2006

  12. VLA AmplitudeDifferential Hourly Snapshots • VLA antenna 17 amplitude. • Ripple due to waveguide reflections. • Magnitude ~ 0.5% • Typical for all VLA antennas. RCP LCP EVLA Front-End CDR – On the Sky Tests April 24, 2006

  13. VLA Phase • Showing VLA ripple in phase. • Magnitude ~ 0.5 degrees. EVLA Front-End CDR – On the Sky Tests April 24, 2006

  14. EVLA Antenna 18 Amplitude Results • Amplitude stability excellent. • No sign of VLA’s 3 MHz ripple. • Full range is 0.4%. • Away from baseband edge, range is ~.05%. • Variation likely due to VLA baseband filter. EVLA Front-End CDR – On the Sky Tests April 24, 2006

  15. EVLA Antenna 18 Phase • Hourly observations of bandpass at X-band. • Mean bandpass removed. • BW is ~10 MHz • Phase peak range 0.2 degrees. • Away from baseband edge, phase range is 0.04 degrees. • Instability origin unclear, but unlikely to be FE. RCP LCP EVLA Front-End CDR – On the Sky Tests April 24, 2006

  16. Some Conclusions • Tsys meeting (or beating!) specs at all bands. • L-band elevation performance superior to VLA. • Other bands elevation performance acceptable. • L-band cross-polarization from hybrid looks good – but full frequency range tests needed with new OMT in place. • Bandpass shape/stability much better than VLA, but not at spec level yet. • Limiting factor likely VLA’s baseband filters. Final tests must wait for WIDAR correlator. • Scientists are (or will be) happy! EVLA Front-End CDR – On the Sky Tests April 24, 2006

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