Phase calibration in prototype VLBI2010 systems

Transcription

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 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 feed LNA 2012 October 22 International VLBI Technology Workshop 2

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 < 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 < radian & delay error < 0.3 ps over 3 GHz 2012 October 22 International VLBI Technology Workshop 3

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

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

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 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 October 22 International VLBI Technology Workshop 6

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

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

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

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

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

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 2012 October 22 International VLBI Technology Workshop 12

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., 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 ( accumulation periods) User-specified fixed additive phase corrections by channel Pcal tone exclusion (e.g., to avoid known spurious signals) 2012 October 22 International VLBI Technology Workshop 13

14 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. Finding spurious signals 2012 October 22 International VLBI Technology Workshop 14

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

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

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

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

19 GGAO pcal-inferred LO phase variations and pcal delays (V pol) up/down converter temperature temperature (degc) :00 11:00 12:00 13:00 14:00 15: October 22 International VLBI Technology Workshop 19

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

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

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

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