A digital tone extractor which monitors the phase integrity of a VLBI recording system in real time was developed. The digital tone extractor monitors phase calibrator tones injected at the antenna and tracks their phase as a function of time. It is capable of maintaining 0.001 cycle phase accuracy over the course of a VLBI experiment in accordance with the accuracy requirements of centimeter VLBI work. This real time VLBI system monitor is a safeguard against most instrumental breakdowns and operator errors

Parks, G. S.

Sigman, E.

English

NASA Technical Reports Server

USE OF A DIGITAL TONE EXTRACTOR 
FOR REAL-TIME PHASE ANALYSIS* 
Elliott H. Sigman and Gary S. Parks 
Jet Propulsion Laboratory 
California Institute of Technology 
ABSTRACT 
With demands for increased accuracy and reliability for very long baseline interferometry (VLBI) 
experiments, single station phase calibration techniques are becoming a reality. The presence of 
phase calibrator tones within the VLBI band-pass make it possible, for the first time, to monitor 
accurately the health of the entire instrumental data path. The digital tone extractor, designed and 
built by the Jet Propulsion Laboratory is exactly such a device. This real-time VLBI system moni- 
tor is a safeguard against most instrumental breakdowns and operator errors. 
*This paper represents one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under 
NASA Contract NAS 7-100, sponsored by the National Aeronautics and Space Administration. 
263 
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264 RADIO INTERFEROMETRY 
INTRODUCTION 
A digital tone extractor which monitors the phase integrity of a VLBI recording system in real time 
has been developed at the Jet Propulsion Laboratory. This digital tone extractor monitors phase 
calibrator tones injected at the antenna and tracks their phase as a function of time. It is capable of 
maintaining 0.001 cycle phase accuracy over the course of a VLBI experiment in accordance with 
the accuracy requirements of centimeter VLBI work. This real-time VLBI system monitor is a 
safeguard against most instrumental breakdowns and operator errors. 
IMPLEMENTATION 
The digital tone extractor interconnects with the VLBI recording system to extract phase calibrator 
tones from the digital data stream. A typical VLBI system with a digital tone extractor is shown in 
figure 1. 
K PHASE CAL TONES 
EXTRACTOR 
Lo-- DATA t - :ORU: 
u CLOCK 
I 
I ____-_ -------- 
Figure 1. VLBI hardware configuration. 
Stable tones from the phase calibrator are injected into the system at the antenna. From the anten- 
na, the signals go through the front end of the receiver and are down-converted to baseband by the 
IF converter. A one-bit A to D converter digitizes the baseband data, after which it is formatted for 
recording on the video recorder. 
DEGREES KELVIN AND DEGREES OF PHASE 265 
The digital tone extractor can monitor the digital data stream either before or after it has been re- 
corded on tape as selected by the SOURCE SELECT switch. When monitoring data recorded on 
tape, a decoder is used which is connected to the read-after-write head of the tape recorder. Along 
with the digital data, clock signals which are synchronous with the data are sent to the digital tone 
extractor. 
The digital tone extractor synthesizes two tones in phase quadrature at the baseband frequency of a 
phase calibrator tone. These synthesized tones are cross-correlated with the incoming data and inte- 
grated to provide sine and cosine correlations. From these two correlations, the residual phase and 
amplitude of a phase calibrator tone is obtained. 
A block diagram of the digital tone extractor is presented in figure 2. As shown, a programmable 
digital frequency synthesizer in the digital tone extractor synthesizes a tone at the baseband fre- 
quency of a phase calibrator tone. The clock that is brought in with the data is used to synchronize 
this synthesizer. Quadrature sine and cosine outputs of the synthesizer are cross-correlated with the 
incoming data, and the results are accumulated in two 3Zbit registers. With 32-bit resolution, the 
hardware is capable of maintaining 0.001 cycle accuracy for up to 1 second of integration time. 
The integration time is controlled by a precision clock which is externally programmable. The sine 
and cosine accumulations are periodically sent to a MODCOMP mini-computer through a micro- 
processor. The phase of the frequency synthesizer is also updated by the MODCOMP. In the MOD- 
COMP’s software, 64-bit arithmetic is used which enables the digital tone extractor to maintain 
12 X 1 O-6 degree accuracy over a 24-hour period. 
The hardware of the digital tone extractor is controlled by the microprocessor which can store the 
phase and frequency of the tone synthesizer and the sine and cosine accumulations at any time and 
retrieve them at a later time. When these numbers are retrieved, the phase of the tone synthesizer 
can also be updated by the microprocessor. With this capability, the digital tone extractor can 
monitor time-multiplexed tones such as are encountered in time-multiplexed bandwidth synthesis 
experiments. 
A typical output from a two channel bandwidth synthesis experiment is presented in figure 3. Each 
printout represents the results of 15 integrations of 12 seconds duration each. The average residual 
phase and average magnitude or amplitude are indicated in the last two lines. The slope of a line, 
which is a least squares fit to the 15 points, is indicated by the “Phase Drift.” The scatter from this 
line is shown by the “RMS Phase Jitter.” 
During a VLBI experiment, excessive phase drift or rms phase jitter can alert the operator to a mal- 
function in the VLBI recording system. An alarm could be activated by the digital tone extractor 
software to warn the operator when these numbers become excessive. 
266 RADIO INTERFEROMETRY 
trogrrmbl. 
0161ta1 rmq. 
Synthalrrr Qusdrstun) 
I 
Clock - 
16 6tt 
stop 
Rag. 
Tlw Epoch 
Clock 
Figure 2. Digital tone extractor. 
APPLICATIONS 
The digital tone extractor performs, in real time, the same process on phase calibrator tones as the 
VLBI correlator. Since the digital tone extractor can monitor what is actually recorded on tape, it 
can indicate in real time the performance of the entire recording system. Thus, it can warn of an 
equipment malfunction when it happens and can save an experiment. 
The digital tone extractor can also be used as a tool for debugging the VLBI system. It can look at 
data before and after it is recorded on tape to isolate faults in the recording system. In addition, a 
frequency synthesizer can be fed into a part of the VLBI system, and the phase can be monitored 
in order to isolate equipment with phase instabilities. 
In addition to VLBI applications, the digital tone extractor has other uses which are of interest at 
the Jet Propulsion Laboratory. For example, it can be used to track spacecraft tones. The software 
can be programmed with a polynomial for frequency in order to track the frequency fluctuations of 
a spacecraft. Alternatively, an iterative software algorithm could be used to make the digital tone 
extractor follow the spacecraft frequency fluctuations and extract the Doppler shift of the 
spacecraft. 
BASEBANDTONENUMBERl CH.l 
CH.2 
BASEBANDTONENUMBERl CH.1 
CH.2 
BASEBAND TONENUMBER 1 CH.l 
CH.2 
DEGREES KELVIN AND DEGREES OF PHASE 
PHASE DRIFT = -0.009 DEGlSEC 13@.11:42:36 
RMS PHASEJITTER = 1.096 DEG 
AVG PHASE = l52.01DEG 
MAGNITUDE = 0.1354195785 
PHASEDRIFT - -0.012DEGlSEC 130111:42:36 
RMS PHASE JITTER - l.192DEG 
AVG PHASE = -144.132DEG 
MAGNITUDE-D.0869244525 
PHASE DRIFT = 0.001DEGlSEC 130:11:46:36 
RMS PHASEJITTER = 0.662DEG 
AVG PHASE = l51.525DEG 
MAGNITUDE = 0.1355491886 
PHASE DRIFT = -0.003DEGISEC 13Q11:46:36 
RMS PHASEJITTER = 0.884DEG 
AVG PHASE = -145.476 DEG 
MAGNITUDE = O.DB68894D43 
PHASE DRIFT = 0.043DEGlSEC 13@.11:50:36 
RMS PHASE JITTER = 3.661DEG 
AVGPHASE= l54.289DEG 
MAGNITUDE=0.13661540671 
PHASE DRIFT = 0.035 DEGlSEC 13D.11350136 
RMS PHASEJITTER =3.345DEG 
AVG PHASE= -144.127DEG 
MAGNITUDE = 0.0872003583 
Figure 3. Digital tone extractor output. 
ACKNOWLEDGMENTS 
267 
The concept of the digital tone extractor was originated by J. L. Fanselow. His enthusiastic sup- 
port and suggestions have been instrumental in developing this device. 


Use of a digital tone extractor for real-time phase analysis

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