System combines and modulates subcarriers and pulse code modulation data entirely with digital logic, and phase modulates an RF carrier with a digital representation of the composite subcarrier signal. It permits programmed control and modification of the modulation indices

Couvillon, L. A.

English

NASA Technical Reports Server

September 1970	 Brief 70-10355 
NASA TECH BRIEF 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage commercial 
application. Tech Briefs are available on a subscription basis from the Clearinghouse for Federal Scientific and Technical 
Information, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
Digital Phase-Modulation/Multiplex System 
Reload From Command or Computer
 
-
	
- -
-
Holding Register	 Holding Register	 Holding Register I I Holding Register 
For Phase 1	 1 1	 For Phase 2	 1 1	 For Phase 3	 1 1	 For Phase 4 
Subcarrier
Gatinge Slects the Output of One of the 4 Registers 
Data	 - 
Unmodulated Input	 .j Digital-To-Phase	 - Modulated Output

Converter 
Figure 1. Programmable Modulator System 
This innovation represents an advance in the art of 
telemetry data modulation /multiplexing, with appli-
cations to terrestrial data communications systems. 
Uses in computer installations and other data links 
using a single channel are also feasible. 
The uniqueness of this system lies in the concepts 
of combining and modulating subcarriers and pulse 
code modulation (PCM) data entirely with digital 
logic, and of phase modulating an rf carrier with a dig-
ital representation of the composite subcarrier signal. 
The advantages in this approach are that (1) the re-
placement of analog circuits by digital logic results in 
greater reliability and cost savings; (2) the interface 
between the data handling system and the rf system 
is made all-digital; and (3) the modulation indices 
or power allocations to various subcarriers are made 
programmable, and therefore more flexible. 
Planetary spacecraft have used subcarrier frequency 
multiplex to separate independent PCM data streams,
transmitting engineering telemetry at a low rate on 
one subcarrier and science data at a higher rate on 
another. The two subcarriers were added in a linear 
summer in a ratio appropriate to the desired modu-
lation indices and applied to a carrier phase modu-
lator. Turnaround ranging modulation was added in 
the phase modulator. 
This new system permits programmed control of the 
modulation indices and, consequently, the modifica-
tion of these indices when required. Another advan-
tage of the new technique is the reliability of its digital 
circuitry. 
The technique currently in use for subcarrier modu-
lation is to have each of several PCM data streams 
phase-shift keyed modulating a subcarrier. The sub-
carriers are then added in ratios appropriate to the 
desired allocation of power between subcarriers, and 
between subcarriers and carrier. The subcarrier mod-
ulators and the summing amplifier are implemented 
(continued overleaf) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19700000343 2020-03-17T00:41:14+00:00Z
with linear circuitry. Provision is usually made for the 
selection of alternate "modes" or weights in the 
weighted summation. 
Under most conditions, the use of square-wave 
signals for subcarriers results in lower intermodu-
lation losses in a phase-modulated carrier system 
than does the use of sinusoidal subcarriers. Because 
the square waves are also easy to generate and 
manipulate, square-wave subcarriers are usually used. 
The essence of the new concept (see Fig. 1) is that 
the discrete carrier phases can be regarded as boolean 
functions of the data, subcarrier and mode control 
signals treated as binary variables. Thus, if one con-
structs a digital carrier phase modulator (digital-to-
phase converter, D/PC), the subcarrier modulators 
and summing amplifier are replaced by code conver-
sion logic which maps the binary data, subcarriers, 
and control into a binary representation of phase. 
Figure 2 shows the data stream DA, DB.....the 
subcarriers SA, SB.....and the mod,e controls C l, C 2 being transformed in "code conversion" logic 
to a set of phase bits which control a D/PC. 
The design of the code conversion logic can be 
accomplished in two ways. One way is to use a pair 
of diode matrices—an encode matrix followed by a 
decode matrix—to convert the parallel k-bit number 
representing the data, subcarrier, and control vari-
ables into an rn-bit number representing the desired 
carrier phase. A less tedious and more flexible method 
is to use a set of holding registers in which the possible 
carrier phase numbers are stored, transferring the 
appropriate register state in parallel to the D/PC 
through gating controlled by the data and subcarrier 
variables. The in-flight programmability of this system 
is apparent— thèhölding registers are ioaded, via the 
command system or the spacecraft computer, with fhe 
desired phase angles.
Note: 
Requests for further information may be directed to: 
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP70-10355 
Patent status: 
Inquiries about obtaining rights for the commer-
cial use of this invention. may be made to NASA, 
Code GP, Washington, D.C. 20546. 
Source: L.A. Couvillon of

Caltech/JPL
under contract to
NASA Pasadena Office
(NPO-I 1338) 
Serial	 DA 
PCM
	
[DB	 Unmodulated 
Data Carrier 
Streams  Input 
Figure 2. Digital Phase Modulation
[1 
ESAJ F-1 I Code Digital-To- 
Square-Wave]
5B Conversion Phase Subcarriers
:
Logic Converter 
Mode, Modulated 
Select, and C 2 Carrier 
Control Output 
Lines
Brief 70-10355	 Category 02 


Digital phase-modulation/multiplex system

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server