The digital design of an adaptive digital filter to be employed in the processing of microwave remote sensor data was developed. In particular, a complex demodulation approach was developed to provide narrow band power estimation for a proposed Doppler scatterometer system. This scatterometer was considered for application in the proposed National Oceanographic survey satellite, on an improvement of SEASAT features. A generalized analysis of complex diagrams for the digital architecture component of the proposed system

Stoughton, J. W.

English

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DEPARTMENT OF ELECTRICAL ENGINEERING
SCHOOL OF ENGINEERING
OLD DOMINION UNIVERSITY
NORFOLK, VIRGINIA
INVESTIGATION OF DESIGN CONSIDERATIONS
FOR A COMPLEX DEMODULATION FILTER
By
J. W. Stoughton, Principal Invest igatur
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Final Report
For the period June 6 to September 6, 1979
Prepared for the
National Aeronautics and Space Administration
Langley Research Center
Hampton, Virginia
Under
Research Grant NAS-1-15648
Task Authorization No. 16
Richard N. Couch, Technical Monitor
FED-Spacecraft Instrn. Sys. Section
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1DEPARTMENT OF ELECTRICAL ENGINEERING
SCHOOL OF ENGINEERING	 t
OLD DOMINION UNIVERSITY
NORFOLK, VIRGINIA
INIESTIGATION OF DESIGN CONSIDERATIONS
FOR A COMPLEX DEMODULATION FILTER
By
J. W. Stoughton, Principal Investigator
Final Report
For the period June 6 to September 6, 1979
Prepared for the
National Aeronautics and Space Administration
Langley Research Center
Hampton, Virginia 23665
Unde r
Research Grant NAS-1-15648
Task Authorization No. 16
Richard N. Couch, Technical Monitor
FED-Spacecraft Instrn. Sys. Section
;l
Submitted by the	 j
Old Dominion University Research Foundation
P.O. Box 6369
Norfolk, Virginia 23508
^G^	 January 1984	
l
EI
i
INVESTIGATION OF DESIGN CONSIDERATIONS
FOR COMPLEX DEMODULATION FILTER
By
J. W. Stoughton*
This reports presents results accomplished for NASA grant NAS-1-15648,
task authorization number 16. The major objective of the work was to deve-
lop the digital design of an adaptive digital filter to be employed in the
processing of microwave remote sensor data. In particular, a complex de-
modulation approach (ref. 1), was developed to provide narrow band-power
estimation for a proposed Doppler scatterometer system. This scatterometer
system was considered for application in the proposed National Oceanographic
Survey Satellite (NOSS), as an improvement of SEASAT features (ref. 2).
Preliminary research that leads to the theoretical basis for the complex de- 	 {
modulation approach has been reported by Stoughton (ref. 3). In the refer-
enced report, a generalized analysis of complex demodulation is presented
along with descriptive block diagrams for the digital architecture component
of the proposed system.
This informal report presents a summary of a presentation given to the
technical monitor at the conclusion of the task period. This summary pre-
Bents the digital architecture which could be implemented to realize the
adaptive filter power estimation.
*Associate Professor, Department of Electrical Engineering, Old Dominion
University, Norfolk, Virginia 23508.
i
II. SYSTEM DESIGN
System Architecture
A conceptual overview of the estimation system is displayed in figure
1. Note that the inphase (I) and quadratune (Q) form a complex signal U,
where U  = In + JQn . The subscript denotes the nth sampled value.
The proposed complex demodulation system is shown in figure 2. As
previously described (ref. 3), the complex demodulation section performs a
complex heterodyning down of a particular spectral region center, Wk , to
base-band. Note that W kis the center of the power spectrum band associated
with the center of a particular spacial "cell." Subsequent low pass
filtering establishes the required selective bandwidth and hence the spacial
cell resolution. If the bandwidth of the low pass section is fixed, then
modest error in cell resolution will occur due to latitude and ranging
effect. If necessary, the bandwidth can be modified to accommodate the
latitude effect. This modification can be achieved by varying the coeffic-
ient in the representative digital filter section.
The overall structure of the system suggests a "pipelined" realization.
That is, each block will process the data then a scheduled series of
bus/register transfer coordinated by a dedicated controller. In view of a
desired throughput rate of 400 KHZ, efficient register transfers and
arithmetic procedures must be performed. These aspects canl be met by
highly parallel operations, coupled with read-only memory (ROM) table-look
up strategies for generati 	 various functions. A more comprehensive system
diagram is shown in figure 3. Note that the architecture diagrams are
t	 presented for one estimation section. There is one section for each
resolution cell (or Wp).
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3
Complex Demodulation Section
Refer to figure 4. The heterodyned output U n (k) is derived by a
sequence of controlled bus/register negative transfers. This output is the
realization of
jW nT
U n (k) = e k	 *(I n + A
JIn(k)
	
= e
	
* (In + A
a
_ [I n cos* n(k) - Qn sin#n(k)]
i
+ j [ I n sin ^ n (k) + Qn cos ^n(k)]
= In +JQn
where n (k)	 W k T
The argument 4
n 
(k)is developed iteratively as a mod k sequence of 8
bits. Control of the two most significant bits allows a table-look up
procedure with a 8 x 1024 bit ROM to select sin #
n
 (k)and cos m n (k),
respectively. These values are stored in registers B, and A respectively.
The bus/register transfer may be denoted by the following sequence:
1. An + ROM (cos:In)
	
s
2. Bn + ROM (sin:#n)
	
f
3. Sn + MLPY (I n , An)
3
4. a,+Sn - MLPY (%,Bn)
5. Tn
 + MLPY (Q
n , An )
6. Tn
 + T  + MLPY (I n , Bn)
Note that MLPY is the operation of a dedicated multiplier unit.
Thus, after 6 microoperations, the registers S n , and T contain the
complex components I n and Qn , respectively.
To maintain the 400 kHz throughput, the micro-operations must be perform-
ed at a minimum of 2.4 mHz .
Low Pass Filter
The digital filter is based on a third order Butterworth filter. To
facilitate a maximum throughput rate, the transfer characteristic , H(z),
may be decomposed into three parallel sections where
H (z) = YW4(z)
H ( z) = H1(z) + H2 ( z) + Hz ( z)
and
= 1 + Z-1 + (Y+jd) (1 +Z-1 )
1 - BZ-1	 1-(a+JWk)Z-1
+ (Y-j d ) (1+Z-1
1 - (a-JWo)Z-1
This complex form is desirable for filtering the complex signal U(z).
The term W 	 establishes the filter bandwidth, and the other parameters,
a, B, Y, d, are selected to generate the proper gain and pole zero
location for the filter. The block diagram for this parallel realization is
presented in figure 5.
4
•
Power Estimation Section
The final computational block develops a power estimate, P(kth spacial
cell) derived from N complex samples where
N - 2
p (k) -	 1	 E IYnI
N-1 1
N
	
- 1	 E	 [(ReYn)2 + In Y n)2]
	
N-1	 n - 1
A digital architecture for realizing the power estimation is presented in
figure 6. A ROM is employed to provide a "look up" table for the respec-
tive squared value of the complex component of Y n . The estimate of Pk
(kth cell or spectral component) is determined by iteratively accumulating
N
N pairs of the Y 	 complex component. Subsequent dumping of this value
into a data collection system would be administered by the system
controller.
SUMMARY
The digital architecture presented is representative of special archi-
tecture necessary to accomplish the complex demodulation approach to the
scatterometer estimation process. It would be expected that with the advent
of VLSI and large scale m.--mories, different strategies might be warranted.
Further study of this type of design should incorporate finite register
simulation to evaluate quantization noise and evaluation of the degree-of-
freedom of the power estimate.
M •
5
REFERENCES
1. Godfrey, C., Bengham, M.D., and Tukey, J. W., Modern techniques of
power spectrum estimation. IEEE Trans. Audio Electroacoustics, Vol.
AV-18, pp. 139-142, June 196 .
2. "An operational satellite scatterometer for wind vector measurements
over the ocean," NASA Technical Memorandum, TM X-72672, March 10, 1975.
3. Stoughton, J.W., "Preliminary design of an adoptive digital Doppler
filter," Final Report, NASA Contract NAS1-14193-53, July 1979.
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Figure 1. Microwave scatterometer system.
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Investigation of design considerations for a complex demodulation filter

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