Precisely timed pulses injected into the input of each receiver are used to calibrate the phase and group delay  through each interferometer terminal. The short duration pulses are generated at a 1 MHz rate directly from the output of the frequency standard. The pulses are injected into the receiver at a level low enough to produce less than one percent increase in system temperature, yet can be extracted during processing with a high enough signal to noise ratio to determine the phase of the calibration rails within 1 degree in 1 second of integration. The calibration system also includes precise cable measurement electronics and a pulse echo for clock synchronization

Rogers, A. E. E.

English

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PHASE AND GROUP DELAY CALIBRATION 
OF A VERY LONG BASELINE INTERFEROMETER 
BY EAST COAST VLBI GROUP 
A. E. E. Rogers 
Haystack Observatory 
ABSTRACT 
Precisely timed pulses injected into the input of each receiver have been used to calibrate the phase 
and group delay through each interferometer terminal. The short duration (less than 35 picosec- 
onds) pulses are generated at a 1 MHz rate directly from the output of the frequency standard. The 
pulses are injected into the receiver at a level low enough to produce less than one percent increase 
in system temperature, yet can be extracted during processing with a high enough signal-to-noise 
ratio to determine the phase of the calibration “rails” within 1 degree in 1 second of integration. 
The calibration system also includes precise cable measurement electronics and a pulse echo for 
clock synchronization. 
255 
https://ntrs.nasa.gov/search.jsp?R=19800020318 2020-03-21T17:50:01+00:00Z
256 RADIO INTERFEROMETRY 
INTRODUCTION 
Very long baseline interferometer (VLBI) systems used for geodetic applications have to be either 
extremely phase and delay stable or be accurately calibrated. This paper describes a phase and group 
delay calibration system with millimeter level precision. The calibrator consists of two basic sub- 
systems. The first is a tunnel-diode pulse generator; the second is a cable phase delay measurement 
subsystem. 
THEORY OF OPERATION 
The system utilizes the 5 MHz output from the frequency standard to produce short duration (<50 
picosecond) calibration pulses at a 1 MHz rate. These pulses are injected into the receiver through 
an input coupler and are detected by the VLBI processing system. The calibration pulses are gener- 
ated at a precisely known epoch within the 1 microsecond ambiguity. A component of the 5 MHz 
signal used to generate the calibration pulses is returned along the same cable and phase compared 
with the transmitted signal in order to precisely determine the phase delay (with 200 nanosecond 
ambiguity) in the cable driving the calibration pulse generator. The system block diagram is shown 
in figure 1. The reflected 5 MHz signal is modulated in order that it may be distinguished from 
other intermediate reflections. In addition, a component of the calibration pulse itself is returned 
along the cable and can be viewed on an oscilloscope to resolve the 200 nanosecond ambiguity 
present in the cable measurement subsystem. 
PULSE GENERATOR 
General Description 
The phase calibrator consists of a tunnel-diode pulse generator driven by a 5 MHz square wave. The 
pulses (both positive and negative) emerge from the tunnel diode and are gated with a diode atten- 
uator so that only one positive pulse per microsecond appears at the output. The block diagram of 
the pulse generator is shown in figure 2. 
Pulse Duration 
The tunnel-diode switching time t (at constant current) is given approximately by the following 
equation for constant a current drive. 
t = (VF - VP)*CV/(IP - IV) 
Where VF = Forward voltage 575 mV 
VP = Peak point voltage 90 mV 
IP = Peak point current 10 mA (increased to 16 mA in later units) 
DEGREES KELVIN AND DEGREES OF PHASE 
IV = Valley point current 1 mA 
CV = Valley point terminal capacitance 1 pF. Thus t = approx 50 picoseconds. 
(30 picoseconds in later units) 
257 
The condition of constant current is not well maintained as the driving impedance is close to 50 
ohms so that the load line intersection point is somewhat below VF. The effect is to reduce the 
pulse voltage without having much effect on the switching time. 
L- -- PHASE COMPARATO3 B- &d------------------ ----- -l 
Figure 1. Delay calibration system block diagram. 
Output Power 
The output temperature (averaged over several MHz) is approximately: 
2(VL- VP)2/(RL*(w2t2/12 + l)*T*Kw2) approx 5 X IO4 deg K (at 10 GHz) 
where w/2pi = Center Frequency (Hz) 
T = Pulse Repetition Period 
RL = Load Resistance Seen by Diode 
K = Boltzmann Constant 
VL = Load line intersection voltage 
258 RADIO INTERFEROMETRY 
L 
5MHz 25MHz 
DIODE 
FROM - _ CLIPPER PULSE GEM ONE 
FRED. T LOW PASS FILTER t 
*+ 
ATTENUAtOR - 
STND I-IOGHx 
BIAS 
PULSE GATE GENERATOR ------- --------------- 
1 
;1 
CLIP 
! 
I 
OUTPUT 
m$L~~~w~cl 
AT INPUT OF 
RECEIVER 
L--,,-,----,-, ----A 
ON/OFF 
t PART OF CABLE MULTIPLEXER 
Figure 2. Delay calibrator pulse generator. 
CABLE MEASUREMENT SYSTEM 
General Description 
The delay through a cable changes with temperature (approx 1 X IO*-4 per deg C) and flexure. 
Pressurized cables also change their electrical length with pressure. The phase delay through a cable 
also varies in almost the same way as the group delay but with small differences owing to dispersion 
and multiple-reflection effects. 
Theory 
The transmitted signal VT(t) is reflected after modulation by m(t) so that the signal received back at 
the transmitting end is 
VR(t) = m(t)Re(exp(jwt)exp(-2jp)) Re = real part of. 
This signal is mixed with a version of the transmitted signal phase shifted by P and low-pass filtered 
to form 
VD( t) = m(t)Re(exp(-2jp)exp(jP)) 
which is mixed with the modulation and low pass filtered again so that 
Vo(t) = (m2(t))cos(2p - P). 
DEGREES KELVIN AND DEGREES OF PHASE 259 
Vo(t) is integrated and the loop closed on the phase shifter so that 
P = 2p + 27r(N + %) where N is an integer constant 
allowing the cable phase-delay p to be measured from the measurement of P. A high-resolution 
phase comparator has been developed for the measurement of P with picosecond precision. 
PICOSECOND PHASE COMPARATOR 
The signal whose phase is to be measured is mixed down from 5 MHz to 25 Hz using a local oscil- 
lator signal of 5 MHz - 25 Hz which is derived from the reference signal. The mixing operation re- 
sults in a time expansion of 2 X lo**5 which is sufficiently large that a digital time interval counter 
(such as HP5308A counter with D/A converter 53 11 B) operating at a 5 MHz clock rate can now be 
used to determine the phase of the 25 Hz signal with respect to the reference 25 Hz signal with 
quantization of only 1.8 X lo**-3 degrees. One count corresponds to 1 picosecond two-way phase 
delay or 0.5 picosecond one-way delay. Cycle averaging can be used to provide even higher resolution. 
Figure 1 shows a block diagram of the comparator. The outputs of the comparator are two 25 Hz 
square waves with relative phases equal to the relative phases of the 5 MHz input signals. Single 
sideband mixing and filters are required to minimize the presence of spurious image signals and 
harmonics. 
MEASUREMENTS OF CALIBRATOR PERFORMANCE 
Output power can be measured by comparing output with that of a noise diode. A typical output 
power level is 60 Kelvin at 7850 MHz when coupled into the receiver through a 30 dB coupler. The 
VLBI processor requires only a relatively weak calibration signal since the processor extracts the 
signal by correlating the signals from each station with a sine and cosine at the frequency of the cal- 
ibrator rail. With this method, a calibrator signal strength of 1 percent is sufficient for the phase to 
be determined within l-degree rms with 1 second coherent integration. 
The stability of the calibrator was measured by observing the phase of the frequency rails from the 
calibrator in the receiver video output. The overall sensitivity of the phase calibrator to tempera- 
ture changes was measured by cycling the temperature of the calibrator from 30 degrees C to 40 
degrees C. In addition, an attempt was made to thermally isolate the individual components within 
the calibrator and so measure the individual coefficients. The results of these measurements are 
shown in table 1. The sensitivity of the calibrator to input ac voltage changes was undetectable be- 
ing less than 1 ps per percentage voltage change. 
260 RADIO INTERFEROMETRY 
Table 1 
Temperature Sensitivity of Phase Calibrator 
25 MHz low pass filter 
Diode clipper 
Wideband amplifier 
Tunnel Diode 
Overall sensitivity 
(measured separately) 
+l.O+/-5 ps/deg. C 
+lO+/-2 ps/deg. C 
-3 +/-1 ps/deg. C* 
-2 +/-1 ps/deg. C* 
6 +/-2 ps/deg. C 
*A negative coefficient indicates that an increase in temperature decreases the phase delay through the 
device. 
TESTS OF THE VLBI RECEIVER AND CALIBRATOR 
In order to acquire high quality VLBI data, it is useful to test the performance of each interferom- 
eter terminal. An adjustable line in the calibrator cable allows testing of the cable measurement 
system and can be used to change the phase of the calibrator. Since the frequency of the calibra- 
tion rail being observed is known, a complete test of the system can be made by comparing the 
change in cable length from the cable measurement system with that computed from the change in 
fringe count observed when the adjustable line is changed. The coherence of the calibrator and re- 
ceiver local oscillator can be measured from the ratio of power in the calibrator rail to the total 
power contribution of the calibrator. The fractional coherence is given by 
(Ts/T~al)l’~ Acal 
where 
Ts = system temperature 
Teal = effective temperature of the calibrator from total power 
Acal = measured correlation coefficient of calibrator. 
Complete tests of the calibrator performance can be made by injecting a common noise source into 
two terminals to form a “zero baseline.” In this configuration, receiver local oscillator phases and 
IF cable lengths can be changed. If the calibration system is working correctly, these changes 
should produce no changes in the calibrated delay measurements. In addition, the calibrator cable 
length can be changed, and the observed delay changes compared with the corresponding changes in 
the cable measurement readout. Figure 3 gives a summary of the calibration technique. 
DEGREES KELVIN AND DEGREES OF PHASE 
REASON FOR USE: 
TO CALIBRATE DRIFTS IN GROUP AND PHASE DELAY THROUGH: 
LOW NOISE AMPLIFIERS - WITH TEMPERATURE AND MAGNETIC FIELD 
LOCAL OSCILLATORS - WITH TEMPERATURE 
CABLES - WITH TEMPERATURE AND FLEXURE 
METHOD : 
INJECTION OF SHORT DURATION PULSES (5.35 PS (1 CM)) INTO 
RECEIVER AT A RATE OF 1 PULSE/MICROSECOND 
DETECTION METHOD: 
EXTRACTION OF THE PHASE OF EACH CALIBRATION IIRAILn 
IN THE DATA STREAM FROM EACH STATION 
SENSITIVITY: 
WITH A SIGNAL OF ONLY 1% (OF TOTAL POWER) PHASE CAN BE 
MEASURED TO WITHIN 1 DEGREE R,M,S. IN 1 SECOND 
PERFORMANCE: 
1) PRECISION 1 PS (0,3 MM) 
2) TEMPERATURE COEFF. 6 PS /'C 
EXPERIMENTS THAT DEMONSTRATE PERFORMANCE: 
1) CALIBRATION SEEN TO IMPROVE RESULTS IN EARLY EXPTS 72-74 
21 HAYSTACK-WESTFORD BASELINE REPEATABILITY 5 MM 74-76 
3) ))ZERO BASELINE TESTSn - SHOW AGREEMENT BETWEEN 
MEASURED DELAYS, CABLE LENGTH ELECTRONIC READOUT 
AND LINE STRETCHER MECHANICAL READOUT TO BETTER 
THAN 3 MM 76 
4) RESIDUALS OF LONG RASELINE EXPTS EXAMINED 
WITH AND WITHOUT CALIBRATION CLEARLY SHOW 
THE EFFECTIVENESS OF THE CALIBRATION SYSTEM 78 
261 
Figure 3. Phase and group delay calibration. 


Phase and group delay calibration of a very long baseline interferometer by East Coast VLBI Group

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