Phase calibration in prototype VLBI2010 systems

1. Phase calibration in prototype VLBI2010 systems Brian Corey (MIT Haystack Observatory) With thanks for contributions by: Alan Rogers, Roger Cappallo, Mike Titus, Chris Beaudoin, Jason Soohoo (Haystack) Irv Diegel (HTSI) Katie Pazamickas (ITT Exelis) and everyone else in the NASA-supported Broadband Development group

2. feed LNA Phase calibration in VLBI2010 • Primary function: Measure instrumental variations over time. • Digital back-ends have not made phase cal obsolete! • Phase cal needed in VLBI2010 to measure • LO phase drifts between bands • Phase/delay drifts in RF/IF analog electronics and cables/fibers • Increase pulse repetition rate from 1 to 5 or 10 MHz • Reduces likelihood of saturation in broadband system • Tones still strong enough to meet phase precision spec • Broadband pcal generator deployed in NASA VLBI2010 test-bed receivers at GGAO and Westford • Options for pcal injection point – International VLBI Technology Workshop

3. Specs on VLBI2010 phase cal performance • Multiple (at least 5) pcal tones within each baseband channel (~32 MHz BW) • Pcal phase 1-σ measurement precision <~ 1° in 1 second for each tone • Peak pulse power / P1dB < -10 dB • Pulse temporal stability – • < 0.3 ps variations that depend on antenna orientation • Allan std dev < 10-15 @ 50 minutes • On other time scales, ASD scales with typical maser performance. • Upper limits on time-varying spurious signals – • For spurs that do not vary with antenna orientation – • Sufficient condition: spurs < -40 dB relative to pcal • Necessary condition: delay error < 3 ps over 1 GHz and < 1 ps over 3 GHz • For spurs that vary with antenna orientation – • Sufficient: spurs < -50 dB relative to pcal • Necessary: phase error < 0.004 radian & delay error < 0.3 ps over 3 GHz International VLBI Technology Workshop

4. 5 or 10 MHz pulse train 5 or 10 MHz sinewave differentiator switch comparator clipper logic gate pulse gating signal Haystack “digital” phase calibrator • High-speed logic devices can replace tunnel diodes in older pulse gen designs. • “Digital” phase calibrator designed by Alan Rogers (Haystack) • 5 or 10 MHz sinewave input; output pulse train at same frequency • Output spectrum flatter than in tunnel diode design • Pulse delay temperature sensitivity < 1 ps/°C with no external temp. control • No support for cable measurement system • Circuit diagram and details available at http://www.haystack.mit.edu/geo/vlbi_td/BBDev/023.pdf International VLBI Technology Workshop

5. Digital phase calibrator output power spectrum International VLBI Technology Workshop

6. Broadband phase/noise calibration unit • “Cal box” developed by Honeywell Technical Solutions Inc (HTSI) and Haystack Observatory for broadband front-ends • Cal box includes • digital phase calibrator • noise source • 0-31.5 dB programmable attenuators on phase and noise outputs • noise and phase cal gating • RF-tight enclosure • Peltier temperature controller (ΔT < 0.2°C for 20°C change in ambient T) • monitoring of temperature, 5 MHz input level, attenuation, gating • Two identical RF outputs with combined pcal+noise • Equalizers for phase or noise cal signals can be added if necessary. International VLBI Technology Workshop

7. Broadband phase/noise cal box: RF connections International VLBI Technology Workshop

8. Phase cal generator, microwave switch, & 5 MHz detector 5 MHz Detector Board Noise Source PCal Microwave Switch PCal Generator Board International VLBI Technology Workshop

9. Phase/noise calibrator assembly Signal Conditioning Board Digital Attenuators Phase Cal Generator Assembly 0.141” Dia. Super Flex Cable (Typical) Noise Source Temperature Sensor International VLBI Technology Workshop

10. Phase/noise calibrator in RF-tight inner enclosure EMI Filters RF Absorber Material Phase Cal Generator Assembly Signal Conditioning Board Grooves For RF Gasket Noise Source Temperature Sensor International VLBI Technology Workshop

11. [A box in [a box in [a box]]] Insulation RF Tight Enclosure RF Gasket International VLBI Technology Workshop

12. Complete cal box assembly with thermoelectric unit 5 MHz Input Monitor & Control Connector Phase Cal + Noise Outputs (2) (On Rear Side) Thermo-Electric Unit Fan International VLBI Technology Workshop

13. Applying pcal phases to visibility phases in VLBI2010 • Traditional Haystack/WACO/Bonn processing uses 1 tone per channel. • Throws away information (e.g., channel instrumental delay) and SNR • Susceptible to severe phase corruption by a spurious signal • Channel-dependent baseband tone frequencies when channel separation (e.g., 2N MHz) is not integer multiple of pulse rep rate (e.g., 5 or 10 MHz) • “Multitone” phase cal mode in HOPS fourfit – • Uses all, or a user-defined subset of, pcal tones in each channel • Finds best-fit delay in each channel for each station • Corrects channel visibility phase with pcal phase calculated at center freq • Multitone usage options – • Adjustable time segmentation interval (1-9999 accumulation periods) • User-specified fixed additive phase corrections by channel • Pcal tone exclusion (e.g., to avoid known spurious signals) International VLBI Technology Workshop

14. Finding spurious signals • Look for classic amplitude-vs.-phase sinusoids in each tone • Compare amplitudes and phases in adjacent tones • Fit linear phase-vs.-frequency model to phases for all tones in a band and look at variability of residual phase in each tone over time. International VLBI Technology Workshop

15. Westford intra-scan rms tone phase (deg) during May 16 session x H pol o V pol 1-second integrations International VLBI Technology Workshop

16. Westford intra-scan rms tone phase (deg) – no Nx10 MHz tones x H pol o V pol 1-second integrations International VLBI Technology Workshop

17. H-pol phase cal delays (ns) during May 16 session Westford GGAO International VLBI Technology Workshop

18. “Correcting” GGAO pcal phase (turns) for delay to isolate LO phase - Φpcal = τRF ωRF + τIF ωIF + ΦLO Before correction After correction International VLBI Technology Workshop

19. GGAO pcal-inferred LO phase variations and pcal delays (V pol) up/down converter temperature International VLBI Technology Workshop

20. GGAO 12m pcal delay vs. az/el during May 16 geodetic session International VLBI Technology Workshop

21. GGAO 12m phase cal delay during 4 azimuth scans (band C) International VLBI Technology Workshop

22. GGAO 12m phase cal delay vs. azimuth International VLBI Technology Workshop

23. GGAO 12m phase cal delay vs. elevation (band D) International VLBI Technology Workshop