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)