Phase calibration in prototype VLBI2010 systems Brian Corey (MIT Haystack Observatory) With thanks...
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Transcript of Phase calibration in prototype VLBI2010 systems Brian Corey (MIT Haystack Observatory) With thanks...
2012 October 22 International VLBI Technology Workshop 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
2012 October 22 International VLBI Technology Workshop 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
LNA
feed
2012 October 22 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
2012 October 22 International VLBI Technology Workshop 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
http://www.haystack.mit.edu/geo/vlbi_td/BBDev/023.pdf
5 or 10 MHz
sinewaveclipper
comparator
logic gate
switch
pulse gating signal
differentiator
5 or 10 MHz pulse train
2012 October 22 International VLBI Technology Workshop 5
Digital phase calibrator output power spectrum
2012 October 22 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.
2012 October 22 International VLBI Technology Workshop 7
Phase CalGenerator
0-31.5 dB
NoisecomNC3208
PulsarPS2-26-450-13S
Splitter
PulsarPS2-26-450-13S
Splitter
H-POL
V-POL
5 MHz+13 dBm
Input
Broadband Phase/Noise Calibration UnitRF Wiring Diagram
0.141" Dia.Super-Flex Coax
Typical
SMAFeedthru
(6)PCal + NoiseOutputs (2)
RF Tight Enclosure
0-31.5 dB
ElectronicAttenuator
Thermal Enclosure
ElectronicAttenuator
Broadband phase/noise cal box: RF connections
2012 October 22 International VLBI Technology Workshop 8
5 MHzDetector
Board
PCalGenerator
Board
Noise Source
PCalMicrowave
Switch
Phase cal generator, microwave switch, & 5 MHz detector
2012 October 22 International VLBI Technology Workshop 9
NoiseSource
TemperatureSensor
SignalConditioning
Board
0.141” Dia.Super Flex Cable
(Typical)
Phase CalGeneratorAssembly
DigitalAttenuators
Phase/noise calibrator assembly
2012 October 22 International VLBI Technology Workshop 10
Grooves ForRF Gasket
NoiseSource
SignalConditioning
Board
Phase CalGeneratorAssembly
TemperatureSensor
EMI FiltersRF Absorber
Material
Phase/noise calibrator in RF-tight inner enclosure
2012 October 22 International VLBI Technology Workshop 11
Insulation
RF Gasket
RF TightEnclosure
[A box in [a box in [a box]]]
2012 October 22 International VLBI Technology Workshop 12
Monitor & ControlConnector
5 MHzInput
Phase Cal + NoiseOutputs (2)
(On Rear Side) Thermo-ElectricUnit
Fan
Complete cal box assembly with thermoelectric unit
2012 October 22 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)
2012 October 22 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.
2012 October 22 International VLBI Technology Workshop 15
Westford intra-scan rms tone phase (deg) during May 16 session
x H pol
o V pol
1-second integratio
ns
2012 October 22 International VLBI Technology Workshop 16
Westford intra-scan rms tone phase (deg) – no Nx10 MHz tones
x H pol
o V pol
1-second integratio
ns
2012 October 22 International VLBI Technology Workshop 17
H-pol phase cal delays (ns) during May 16 session
Westford
GGAO
2012 October 22 International VLBI Technology Workshop 18
“Correcting” GGAO pcal phase (turns) for delay to isolate LO phase
Before correction After correction
- Φpcal = τRF ωRF + τIF ωIF + ΦLO
2012 October 22 International VLBI Technology Workshop 19
GGAO pcal-inferred LO phase variations and pcal delays (V pol)
up/down converter temperature
2012 October 22 International VLBI Technology Workshop 20
GGAO 12m pcal delay vs. az/el during May 16 geodetic session
2012 October 22 International VLBI Technology Workshop 21
GGAO 12m phase cal delay during 4 azimuth scans (band C)
2012 October 22 International VLBI Technology Workshop 22
GGAO 12m phase cal delay vs. azimuth
2012 October 22 International VLBI Technology Workshop 23
GGAO 12m phase cal delay vs. elevation (band D)