The design and performance of a prototype real-time baseband signal combiner, used to enhance the received Voyager 2 spacecraft signals during the Jupiter flyby, is described. Hardware delay paths, operating programs, and firmware are discussed

Howard, L. D.

English

NASA Technical Reports Server

TDA Progress Report 42-60 September and October 1980 
Prototype Real-Time Baseband Signal Combiner 
L. D_ Howard 
Radio Frequency and Microwave Subsystems Section 
This article describes the design and performance of a prototype real-time baseband 
signal combiner, used to enhance the received Voyager 2 spacecraft signals during the 
Jupiter flyby. 
I. Introduction 
Signal enhancement obtained by arraying the 64-meter and 
34-meter antennas at each Deep Space Communications 
Complex (DSCC) is an operational requirement for both 
Voyager Saturn flybys. Arraying is performed by a Real-Time 
Telemetry Combiner (RTC) Subsystem located at each 
64-meter site. This subsystem resynchronizes and sums the 
baseband signals from each antenna site. The output signal-to-
noise ratio (SNR) is given by SNRRTC = L SNRinputs as shown 
graphically in Fig. 1. 
This article describes the prototype Voyager RTC which is 
based on the theoretical work by R. Winkelstein (Ref. 1) and a 
successful research and development combiner demonstrated 
by H. Wilck (Ref. 2). 
The RTC design characteristics are: 
(1) Combine baseband signals for the 64- and 34-meter 
antennas without degrading the optimal combined 
signal-to-noise ratio (SNR) by more than 0.2 dB. 
(2) Operate in an unattended mode. 
(3) Control and monitor functions performed remotely 
from a Data System terminal or locally via dumb 
terminal. 
(4) Identical hardware for both 34- and 64-meter signals, 
so that external delay mechanisms could be eliminated 
and hardware would be interchangeable. 
(5) Capable of open-loop tracking when signal is too weak 
to provide useful correlation feedback. 
(6) Provide for automatic signal acquisition, tracking, lost 
signal detection, and reacquisition. 
(7) Provide self-test, calibration, and diagnostics. 
(8) Provide for compensating relative delays of 2 to 115 
microseconds. 
(9) Bandwidth to be equal to or greater than 10 mega-
samples per second. 
(10) Alignment tolerance plus or minus 30 nanoseconds 
cumulative tolerance over entire track. 
Signal power degradation due to signal misalignment in the 
delay channels requires an alignment accuracy of 1.5 percent 
of a subcarrier cycle to keep source degradation less than 
0.1 dB. With a maximum subcarrier rate of 500 kHz this 
becomes a ±30 ns alignment tolerance. Because of the require-
ment that the RTC track open loop, without alignment error 
feedback from the correlators, the ±30 ns tolerance must be 
cumulative over the entire track. 
145 
https://ntrs.nasa.gov/search.jsp?R=19810004716 2020-03-21T14:38:20+00:00Z
II. Implementation 
Signals from the 64-meter and 34-meter station receiver 
outputs pass through identical hardware delay paths in the 
RTC (Fig. 2)_ Baseband signals enter the RTC through 
low-pass fllters (LPF) whose purpose is to prevent aliasing in 
the subsequent sample data process. Then an automatic gain 
control (AGC) normalizes the flltered inputs to a fixed power 
level within the range of the 8-bit analog-to-digital converters 
(ADC). This LPF-AGC-ADC path is implemented as a single 
RF printed circuit board for each input channel. 
The ADC, and an 8-bit first-in first-out memory (FIFO) are 
driven by a separate 10-MHz phase programmable input clock 
for each channel. The input and output clocks are phase 
programmable synthesizers operating at 10 MHz. The neces-
sary central processing unit (CPU) controlled variable delay to 
resynchronize the input signals is obtained by cycle (phase) 
slipping the input clocks relative to the common output clock. 
The synthesizer has 0.1 degree programmable phase resolution 
at 10 MHz, which corresponds to a delay line resolution of 
27.8 picoseconds (far better than required). 
At the output of the FIFO delay lines both signals have 
been resynchronized. The analog signals are then reconstructed 
by 8-bit digital-to-analog converters (DAC). Low-pass fllters 
(LPF) remove the DAC clocking noise, and fmally an analog 
summing amplifier produces the RTC combined output. 
The RTC is driven by a microprocessor controller consisting 
of two boards, a central processing unit module and an 
extended memory module (XMEM). The design is an out-
growth of the system described in Ref. 3. 
III. RTC Firmware 
The operating program for the RTC resides as approxi-
mately 40K bytes of resident EPROM on the CPU and XMEM 
modules. The program is written in Intel's PL/M language. The 
function of the program is to provide both normal operational 
and diagnostic-maintenance capabilities for the RTC system. 
A. Normal Operation 
Normal operation consists of two phases; an initialization 
phase, and a tracking phase. 
1. Initialization. Upon initialization, the RTC runs through 
a number of internal self-checks. It verifies the functioning of 
the station clock, checksum verifies the EPROM, initializes 
interfaces, parity checks the random access memory (RAM), 
and tests the delay line functioning and the operation of the 
synthesizers. If any problems are encountered, the RTC enters 
the diagnostic mode; otherwise operator inputs consist of a set 
146 
of pointing predicts (for calculated delay and open-loop 
operation) and signal characteristics (for correlator closed loop 
operation). 
2. Tracking_ During normal tracking the delay lines are 
updated at I-second intervals for either closed-loop or open-
loop operation. These updates are triggered by CPU interrupts 
generated by a programmable real-time clock on the CPU 
module. In addition to the real-time clock interrupt, six other 
levels of interrupt are utilized during normal operation. Three 
interrupts support communications, star ports A and B and the 
RS232C interface; and three interrupts support real-time 
failure diagnosis, RAM parity, and delay line problems for 
FIFOs. 
Because all seven interrupts function asynchronously and at 
their own rates, common routines are re-entrant, and inter-
rupts are disabled and re-enabled around critical calculations. 
B. Diagnostics/Maintenance Operation 
The RTC program provides detailed on-line and off-line 
diagnostic, and maintenance features. 
1. On-line diagnostics. During normal tracking the RTC is 
able to detect and locate hardware problems to the module 
replacement level to permit rapid and effective spares 
substitution. 
2. Off-line maintenance. Although the maintenance depots 
have automatic test equipment (ATE) for board trouble-
shooting it is primarily geared to small- and medium-scale 
integrated circuit technology. In theory the ATE could be 
used on LSI boards such as in the RTC, but in practice, the 
amount of programming effort required could easily exceed 
the development effort of the original machine. Experience 
has shown that resident maintenance software is at present the 
most effective method of providing an on-site repair capability; 
therefore an extensive set of maintenance routines is also part 
of the RTC firmware. These routines allow bit level control of 
the RTC hardware, as well as automated testing of FIFO and 
correlator performance, and permit troubleshooting of the 
hardware in place. 
IV. RTC Testing 
The RTC is designed to perform signal combining with an 
output SNR within 0.2 dB of theoretical as given in Fig. 1. 
Detailed prototype testing was done at the Compatibility Test 
Area (CTA-21) with field verification tests at DSS 14 (Ref. 4). 
Signal-to-noise ratio estimates measures from the Symbol 
Synchronizer Assembly (SSA) were found to be insufficiently 
precise to provide the 0.1 dB necessary for RTC performance 
verification. Instead, telemetry symbol error rates (SER) from 
known data were used, and the SNR was derived by inverting 
the relation SER == 1/2 (I -Erf y'SNR). These measurements 
are sufficiently precise to permit OJ-dB performance verifica-
tion. Pre calibration procedures before arraying, (Fig. 3) use 
two completely redundant telemetry strings. They are pro-
vided a common simulated spacecraft test signal and their 
output SER and SNRs are compared. This serves to calibrate 
the indicated SNR with actual SNR (as determined from 
SER). This is important since SERs are unavailable for actual 
spacecraft tracks. The DSS 14 precal also determines which 
telemetry string is performing better. This difference is usually 
very small « 0.2 dB) The best telemetry string is then used 
for the prime data source. 
The normal tracking configuration appears in Fig. 4. The 
64-meter antenna telemetry string acts as backup for the RTC 
string. The 34-meter string is used to provide SSA SNR data 
only, since no useful telemetry is available at 34-meter signal 
levels. 
V. Voyager 2 Jupiter Encounter RTC 
Performance 
Figures 5 and 6 display the results of measurements made 
during the Voyager 2 Jupiter Encounter (July 1979). Figure 5 
displays SSA SNR lO-rninute averages for DSS 14, DSS 12, 
and the RTC. Note that DSS 12 SNRs are offset by 3 dB from 
DSS 14 and the RTC SNRs for plot compactness. Figure 6 
displays SNR differences (DSS 14 - DSS 12, RTC - DSS 14) 
and RTC theoretical gain. These values derived from the 
measurements in Fig. 5, but are displayed in a format where 
RTC performance is independent of output SNR. RTC 
theoretical gain is what would be given from Fig. 1. It is 
important to emphasize that Figs. 5 and 6 represent actual 
measurements taken under field conditions during a planetary 
flyby. Because this data was taken under encounter condi-
tions, there are numerous factors which were beyond experi-
mental control; an explanation of these factors is necessary to 
understand the data presented. 
A. Symbol Rate Normalization 
Data was taken at several symbol rates. If the telemetry 
chain were perfect, the SNR change between symbol rate 
R SY1 and rate RsY2 would be given by 
i.e., SNR dB 67.2 kbps -2.34 dB == SNR dB 115.2 kbps 
Using this relation, all data was normalized to Voyager high 
rate 9f 230400 symbols per second (115.2 kbps, coded). 
Symbol rate SNR dependence of the telemetry showed that 
only the next lower rate 134400 symbols per second (67.2 
kbps, coded) was usable. Even so, significant jumps (0.5 dB) 
are apparent at bit rate changes (telemetry string performance 
degrades at higher rate). This system problem was identified 
and solved after installation of the prototype RTC. 
B. SNR Measurements 
The basic measurement was a lO-minute average of SSA 
SNR estimates. These values are derived from an analog power 
measurement (Ref. 5). Calibration curves exist, but these 
curves were found to be in error in the SNR range of interest 
for DSS 12. In fact, DSS 12 was running so close to SSA 
threshold that it was very difficult to keep the SSA in lock; 
which accounts for the sparsity of DSS 12 data points. No 
acceptable SSA SNR calibration procedures existed during 
prototype testing at symbol SNR less than -2 dB, since for 
single stations these signal levels are normally too low to be of 
any use. It is a problem unique to measurement performance 
when arraying to require accurate SNR measurements down 
to, and occasionally below, previously accepted lock thresh-
olds. This problem has been partially solved by recalibrating 
the SSA so that a symbol SNR measurement as low as - 5 dB i~ 
considered accurate. 
VI. Conclusion 
The prototype RTC has performed to design expectations, 
resulting in a commitment to design operational systems to 
support the Voyager Saturn Encounter. An advanced research 
effort is underway to address solutions to operational system 
problems associated with real-time arraying, the results of 
which will be reported in future Progress Reports. 
147 
148 
References 
1. Winke1stein, R. A., "Analysis of the Signal Combiner for Multiple Antenna Arraying," 
in The DSN Progress Report 42-25, pp. 102-118, Jet Propulsion Laboratory, Pasadena, 
Calif. 
2. Wilck, H., "A Signal Combiner for Antenna Arraying," in The DSN Progress Report 
42-25, pp. 111-117, Jet Propulsion Laboratory, Pasadena, Calif. 
3. Parham, O. B., "Microcomputer Central Processing Unit Module," in The DSN 
Progress Report 42-41, pp. 172-175, Jet Propulsion Laboratory, Pasadena, Calif. 
4. Simon, N. K., and Hoyes, C., "Preliminary Telemetry Operations Experience with the 
Real-Time Combiner: 1 November 1978 to 1 November 1979," in The DSN Progress 
Report 42-55, pp. 90-96, Jet Propulsion Laboratory, Pasadena, Calif. 
5. Lesh, J. R., "Accuracy of the Signal-to·Noise Ratio Estimator," Technical Report 
32-1526, Vol X, pp. 217-232, Jet Propulsion Laboratory, Pasadena, Calif. 
BASEBAND SIGNAL FROM 64-METER IN LOW-PASS RECEIVER- FILTER EXCITER 
SUBSYSTEM 
I 
I 
-----------, 
I 
1.8 
RECEIVER SNR SPREAD, dB (34 m - 64 m) 
Fig. 1. RTC SNR improvement vs 34·m/64-m spread 
AUTOMATIC ANALOG-TO- FIRST IN 
f- GAIN 
-
DIGITAL 
-
FIRST OUT -~ CONTROL CONVERTER MEMORY 
1 t 
I 
INPUT IN-PHASE AND 
CLOCK QUADRATURE CORRELATOR 
t 
CENTRAL I 
I 
~ STARSWOCH I DSN MEMORY SYSTEM I-- CONTROLLER I STANDARD PROCESSING ROM/RAM INTERFACE UNIT 
I 
L 
TERMINAL I (NOT PART OF RTC) -1 
-----------
BASEBAND SIGNAL FROM 34-METER IN LOW-PASS RECEIVER- FILTER EXCITER 
SUBSYSTEM 
-
, 
INPUT IN-PHASE AND QUADRATURE CLOCK CORRELATOR 
I 
I I 
.AUTOMATIC ANALOG-TO- FIRST IN 
GAIN' 
-
DIGITAL 
-
FIRST OUT 0.--
CONTROL CONVERTER MEMORY 
Fig. 2. RTC assembly block diagram 
DIGITAL-TO-
ANALOG f-
CONVERTER 
OUTPUT 
CLOCK 
DIGITAL-TO 
ANALOG f-
CONVERTER 
LOW-PASS 
FILTER 
DUAL E 
AMPLIFIER 
LOW-PASS 
FILTER 
SIGNAL 
~ TO 
SDA 
150 
~,._D_SS_14~L-____ ~ ____ ~~ ____ ~ ____ ~~ ____ ~ ____ ~~~~ SNR AND 
SER 
Mm ANTENNA 
INTERSITE 
MICROWAVE 
LINK 
(10 COM) 
DSS12 
34m ANTENNA 
) 
BER LINE 
RTC 64m IN 
PH ASING OF RCVR's 
SNR AND 
SER 
AND TELEMETRY STRINGS 
CHOSE BEST STRING FOR RTC 
SIGNAL + NOISE 
GENERATOR RTC 
OUTPUT 
PORT 
CHECK PWR 
'"-----' LEVELS, AGC 
34m IN 
DSSI2 
L 
___ ....JHL __ X_-BA_N_D_......JH MODIFIED ~ SET BASEBAND SCA TEST XLATOR BLK III RCV PHASING OR RCV 
AND PWR LEVELS 
X-BAND 
MASER 
X-BAND 
MASER 
Fig. 3. Precalibration configuration 
BLK IV 
RCV 
10 MHz IF 
10 MHz IF 
BLK III RCV 
TELEMETRY 
DEMOD 
BASEBAND 
RTC 
BASEBAND 
SIGNAL 
COMBINER 
BASEBAND 
BLK 111* 
RCV 
10 MHz IF 
*MODIFIED FOR X-BAND 
BLK IV 
SDA 
BLK IV 
SDA 
16.6 km PATH 
--z.. z.. 
55.4/1s FIXED DELAY 
BLK III 
SDA 
SSA 
SSA 
SSA 
Fig. 4. Normal tracking configuration 
VISUALLY CHECK FOR 
DISTORTION, etc 
SEND TEST SIGNAL OVER 
INTERSITE MICROWAVE LINK 
TPA 
64m OR RTC 
WIDEBAND DATA 
TO JPL NOCC FOR 
IMAGE PROCESSING 
TPA 
TPA 
64m SNR 
STATISTICS 
RTC SNR 
STATISTICS 
34m SNR 
STATISTICS 
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4 1.8 
RTC THEORETICAL GAIN 
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Prototype real-time baseband signal combiner

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