Simple addition to hardware and new mode of operation of transmitter and receiver in coherent, PCM/PSK/PM configuration greatly improves channel efficiency. Procedure reduces amount of power lost to intermodulation products

Butman, S.

Timor, U.

English

NASA Technical Reports Server

November 1974	 B74-10209 
NASA TECH BRIEF 
NASA Pasadena Office	
£ A 
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 National Technical Information 
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be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Interplex Modulation and a Supressed -Carrier Tracking

Loop for Coherent Communications Systems 
The problem: 
In previous two-channel telemetry systems, there 
has been a power loss due to the intermodulation 
between the high-rate and the low-rate channels. Typi-
cally, this intermodulation loss has been much greater 
than the power allocated to the low-rate channel. Also, 
to minimize that loss, the amount of power allocated to 
the phase reference (carrier) has been far in excess of 
the amount needed for tracking. 
The solution: 
A simple addition to the hardware and a new mode 
of operation of the transmitter and the receiver in a 
coherent, PCM/PSK/PM configuration greatly improve 
channel efficiency. The new system is called Interplex 
from the fact that the high-rate and the low-rate channels 
are multiplied together before modulating the carrier. 
This procedure reduces the amount of power lost to 
intermodulation products. The Interplex system saves 
additional power by maintaining the phase-reference 
power at the design level. The power saved can be 
transferred to one or both of the data channels to 
increase the data rate, the reliability, or both. Alterna-
tively, this saved power can be used to decrease the 
total power requirements for a specified level of 
performance. The new system adds a multiplier to the 
transmitter of a standard two-channel telemetry system. 
At the receiver, low-rate data is taken from the 
quadrature channel of the phase-lock loop detector. 
The modulation angles are changed at the transmitter 
for optimum distribution of power among the carrier 
and the data channels. 
Near 100 percent efficiency can be achieved with 
Enterplex operating in the suppressed-carrier mode, in 
which all available power is allocated to the data-
beating sidebands. In this mode, a new detector system 
is used to maintain frequency and phase sync at the 
receiver. The suppressed-carrier detector loop provides
the phase-tracking control signal by first separating the 
correlated signal and its quadrature into their subcarriers, 
using band-pass filters. The corresponding subcarrier 
signals are then multiplied and combined in a linear 
manner such that the variance of the phase error is 
minimized. 
How it's done: 
In the transmitter (see figure), the high-rate data, 
d 1 (t), is combined in the mixer, X, with the low-rate 
data, d 2 (t), before either data phase modulates its 
respective channel subcarrier. The rest of the transmitter 
is conventional. The output of the transmitter may be 
expressed as 
X"(t) \/i sin [wt+0i a 1 (t)+0 2 d 1 (t)a2(t)] 
where a 1 (t) =d1(t)sq(w1t), 
a2 (t) = d 2 (t)sq(t), and 
0 1 and 0 2 = modulation angles of the first and
second data channels. 
This compares with the output of an unmodified 
transmitter 
X(t) =
	
sin [Wct+0lal(0+02a2(0] 
and reflects the multiplication of the high-rate and low-
rate data channels. 
As shown in the figure, the only change in the 
conventional receiver is an added mixer, Y. This mixer 
combines the quadrature channel output with sq(w1t) 
before it is applied to the final mixer. The output of 
mixer Y is: 
.flsq(w1 t)h/Pd2(t)sq(t), 
and the output of the final mixer is 
./sq(w1 t)sq(c t)+/P d2(t). 
This document was prepared under the sponsorship of the National	 Government assumes any liability resulting from the use of the 
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https://ntrs.nasa.gov/search.jsp?R=19740000209 2020-03-17T13:50:34+00:00Z
A 
d1(t) 
A d2(I) 
act I(t)
INTERPLEX 
TRANSMITTER 
:: 14	 02	 Unit	 Modulator Summing 
sqI(t)
INTERPLEX
. 
SUPPRESSED- CARRIER 
TRACKING LOOP 
Bandpass xIt) 
Filter 
x2It  
L_[,j1t 
tIW
Voltage Loo	 W,ted eig 
sit) BL Owillator 
sin(Wt+8PI
Bandpass
Filter IN 
b y(t)
Bapass VI(t) 
F die, 
we	 F
Improved Coherent Communication System
. 
Such a signal is then a function of the subcarrier of the 
high-rate channel. This signal, when received by either 
receiver of the two-channel system, can be detected at 
the quadrature channel output. The Interplex technique 
is applicable for systems of two or more channels. 
Note: 
Requests for further information may be directed to: 
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP74-10209 
B74.10209
Patent status: 
This invention has been patented by NASA (U.S. 
Patent No. 3,710,257). Inquiries concerning nonexclu-
sive or exclusive license for its commercial development 
should be addressed to: 
Patent Counsel 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Source: Stanley Butman and
Uzi Timor of

Caltech/JPL
under contract to

NASA Pasadena Office

(NPO-1 1572) 
Category 01, 02 is 


Interplex modulation and a suppressed-carrier tracking loop for coherent communications systems

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