Transition tracking loop eliminates drifts, leakages, and instabilities inherent in analog filters. Major components are the phase detector, loop filter, voltage-controlled oscillator and timing logic

Anderson, T. O.

Hurd, W. J.

Lindsey, W. C.

English

NASA Technical Reports Server

September 1970
	
Brief 70-10009 
S	 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 
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be directed to the Technology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
Digital Data Transition Tracking Loop Improves Data Reception
1 Phase Detector Loop Filter 
'Phase Channel  
I . I Variable Digital-to- 
riIntegrator•
Detector Converter
-' 
__HBuffer	 1 _ .	 __ Decade Gain i	 I __ Loop Band- H Buffer Arnplifierjl j Amplifier Selection Logic Width Control and Fine IL 
Frequency arid Multiplex Logic Frequency 
Control 
•
Logic -
	 I Gain Control• 
.	 .- ___ ___ 
Analog-to-
___ 
Variable Gain
___ 
Digital-to- 
Integrator Digital Up-Down Analog 
Converter Counter Converter 
Channel
I 
J
+	 /
Timing Logic	 I.,
	 I	 VCO 
The problem: 
Single channel communications and telemetry 
systems employ symbolic coding techniques to ensure 
error-free reception of telemetered data when adverse 
signal-to-noise ratios are encountered. For example, 
a convolutional code that uses five symbols for each 
information bit, provides essentially error-free per-
formance with a ratio of symbol energy to noise 
spectral density of —5 db. A transition tracking loop 
witha very narrow bandwidth is required to achieve 
symbol synchronization and filtering which eliminate 
input signal and receiver generated data errors. 
The solution: 
A digital transition tracking loop, motivated by 
the requirement of extreme stability, eliminates the
drifts, leakages, and instabilities inherent in analog 
filters. The major components of the digital data 
transition tracking loop shown in the figure are the 
phase detector, loop filter, voltage-controlled oscilla-
tor and timing logic. 
How it's done: 
The input to the integrators in the phase detector 
is the received signal which consists of a constant 
amplitude, bipolar video data stream plus white 
Gaussian noise. In the in-phase channel, the integra-
tions are performed over the assumed data symbol 
positions, i.e., from (n-l)T to nT, where T is the sym 
bol duration, soihat -the signs of the integrals repre-
sent estimates of the binary data symbols. In the mid-
phase channel, the integration windows are symmetric 
(continued overleaf) 
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about times nT. Whenever a transition occurs, the 
mid-phase channel integral forms an estimate of the 
timing error between the actual transition time and 
the assumed transition time, nt. In particular, when-
ever the actual transition occurs within the mid-phase 
window, the expected value of the integral is propor-
tional to the absolute value of the timing error. The 
final estimate of timing error is made by the phase 
detector logic circuits. For simplicity in design of the 
loop filter, the phase estimate is quantized to only one 
bit, which causes a loop signal-to-noise ratio degra-
dation of less than 2 db compared to the perfect 
digitization case. The timing error estimate is zero if 
no transition is detected, and +1 or —1 whenever a 
transition is detected, depending on the sign of the 
mid-phase integral and the direction of the transition. 
The improved phase detector results in a higher 
signal-to-noise ratio (SNR) at the phase detector out-
put, and hence a higher loop SNR and lower syn-
chronization error. Compared to more conventional 
transition tracking phase detectors, the improvement 
in signal-to-noise ratio at the phase detector output is 
3 to 9 db. The significance of the improved phase 
detector is that better synchronization is attained 
without lowering the loop bandwidth. This is espe-
cially important when acquisition times or Doppler 
rates are important. 
The components of the loop filter output are gen-
erated separately in the digital domain, and then 
converted to analog voltages and summed in an opera-
tional amplifier; the summation is performed by an 
up-down counter which acts as a perfect integrator. 
The loop bandwidth is controlled by varying the 
two gains of the digital filter. Initial acquisition of 
symbol synchronization may be achieved by increasing
the gains, thus widening the loop bandwidth, and 
selecting the VCO center frequency such that the 
frequency error between the symbol rate and the VCO 
frequency is less than the loop bandwidth. When sym-
bol lock is achieved, the loop bandwidth is narrowed 
to give better tracking performance. 
Notes: 
1. The digital transition tracking loop would be use-
ful in digital communications applications, such 
as long distance telephone or microwave-linked 
computer-to-computer or computer-to-input/out-
put data therminal hookups. Information concern-
ing this innovation would, be of interest to the 
designers and manufacturers of computers, periph-
eral data processing equipment, and communica-
tions equipment. 
2. No additional documentation is available. Specific 
questions, however, may be directed to: 
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: B70-10009 
Patent status: 
This invention is owned by NASA, and a patent 
application has been filed. Royalty-free, nonexclusive 
licenses for its commercial use will be granted by 
NASA. Inquiries concerning license rights should be

made to NASA, Code GP, Washington, D.C. 20546.
Source: T. 0. Anderson, W. C. Lindsey 
and W. J. Hurd of

Caltech/JPL
under contract to

NASA Pasadena Office 
(N P0-10844) 
Brief 7010009	 Category 02


Digital data transition tracking loop improves data reception

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