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. 2012 October 22 International VLBI Technology Workshop2  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 – Phase calibration in VLBI2010 LNA feed

3. 2012 October 22 International VLBI Technology Workshop3 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

4. 2012 October 22 International VLBI Technology Workshop4 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 5 or 10 MHz sinewave clipper comparator logic gate switch pulse gating signal differentiator 5 or 10 MHz pulse train

5. 2012 October 22 International VLBI Technology Workshop5 Digital phase calibrator output power spectrum

6. 2012 October 22 International VLBI Technology Workshop6 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.

7. 2012 October 22 International VLBI Technology Workshop7 Broadband phase/noise cal box: RF connections

8. 2012 October 22 International VLBI Technology Workshop8 5 MHz Detector Board PCal Generator Board Noise Source PCal Microwave Switch Phase cal generator, microwave switch, & 5 MHz detector

9. 2012 October 22 International VLBI Technology Workshop9 Noise Source Temperature Sensor Signal Conditioning Board 0.141” Dia. Super Flex Cable (Typical) Phase Cal Generator Assembly Digital Attenuators Phase/noise calibrator assembly

10. 2012 October 22 International VLBI Technology Workshop10 Grooves For RF Gasket Noise Source Signal Conditioning Board Phase Cal Generator Assembly Temperature Sensor EMI Filters RF Absorber Material Phase/noise calibrator in RF-tight inner enclosure

11. 2012 October 22 International VLBI Technology Workshop11 Insulation RF Gasket RF Tight Enclosure [A box in [a box in [a box]]]

12. 2012 October 22 International VLBI Technology Workshop12 Monitor & Control Connector 5 MHz Input Phase Cal + Noise Outputs (2) (On Rear Side) Thermo-Electric Unit Fan Complete cal box assembly with thermoelectric unit

13. 2012 October 22 International VLBI Technology Workshop13 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., 2 N 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)

14. 2012 October 22 International VLBI Technology Workshop14 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.

15. 2012 October 22 International VLBI Technology Workshop15 Westford intra-scan rms tone phase (deg) during May 16 session x H pol o V pol 1-second integrations

16. 2012 October 22 International VLBI Technology Workshop16 Westford intra-scan rms tone phase (deg) – no Nx10 MHz tones x H pol o V pol 1-second integrations

17. 2012 October 22 International VLBI Technology Workshop17 H-pol phase cal delays (ns) during May 16 session Westford GGAO

18. 2012 October 22 International VLBI Technology Workshop18 “Correcting” GGAO pcal phase (turns) for delay to isolate LO phase Before correction After correction - Φ pcal = τ RF ω RF + τ IF ω IF + Φ LO

19. 2012 October 22 International VLBI Technology Workshop19 GGAO pcal-inferred LO phase variations and pcal delays (V pol) up/down converter temperature

20. 2012 October 22 International VLBI Technology Workshop20 GGAO 12m pcal delay vs. az/el during May 16 geodetic session

21. 2012 October 22 International VLBI Technology Workshop21 GGAO 12m phase cal delay during 4 azimuth scans (band C)

22. 2012 October 22 International VLBI Technology Workshop22 GGAO 12m phase cal delay vs. azimuth

23. 2012 October 22 International VLBI Technology Workshop23 GGAO 12m phase cal delay vs. elevation (band D)