Synthesis technique for active filters gives high degree of accuracy for frequencies as high as 500 kHz and covers frequencies most used for telemetry. Frequency response can be realized within few percent of specified cutoff frequencies without need for tuning

Gussow, S.

Weathers, G.

English

NASA Technical Reports Server

December 1970
	 Brief 70-10238 
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 National Technical Information Service, Springfield, 
Virginia 22151. Requests for individual copies or questions relating to the Tech Brief progam may be directed to the Tech-
nology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
Design Procedure for Improved Active Filters
Ri 
A 
0	 T	 0 
ei
G S2 
.2. =	 High Pass 
ei AS2+BS+l
Ri . B
.) 
AS +BS+l	
Low Pass
 
Cl 
Rg	 R1	 R2	 ' Ro	 I	 Rg Cl	 C2	 I	 Ro 
	
______	 I p o—w-1
	
i	
CA i j C2	 '	 RL eo	 R2 VT ei	 CA	 ei	  
TT	 I	 I __________________	 j - p
eo	 eo 
	
GS2 +DS+E	 GS2+DS 
______________ 
- =
	 Low Pass	 - =
	 High Pass ei	
AS2 + BS +1	 eo AS2 + BS + I
RL eo 
Active Quadratic Filter Networks: Conventional (A) and Realistic (B) 
The problem: 
To design an active filter with accuracy for high 
frequencies. Active filters of conventional design are 
accurate for only low frequencies usually below 1 kHz, 
and thus are seldom used. 
The solution: 
A new synthesis technique for active filters gives 
a high degree of accuracy for frequencies as high as
500 kHz, so that most frequencies used for telemetry 
are covered; a frequency response can be realized 
within a few percent of specified cutoff frequencies 
without need for tuning. 
How it's done: 
Basically the technique consists in taking ideal ac-
tive networks used in conventional low-frequency de-
sign (Figure A) and extending their analysis to in-
(continucd overleaf) 
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dude effects of higher frequencies. The reason for the 
inaccuracy at higher frequencies is that conventional 
models do not provide for all the parameters that 
affect the network at such frequencies; they assume 
zero source and infinite load impedance, and an ideal 
unity-gain amplifier element having infinite input im-
pedance and zero output impedance. 
For accurate synthesis for higher frequencies, addi-
tional circuit parameters must be considered for 
development of an accurate realistic model. They 
proved to be the amplifier's input capacitance 
(CA), output resistance (Ro), and gain (K); the 
source impedance of the input generator (Rg), and out-
put load (RI) (Figure B). These parameters add more 
terms to the numerator of the voltage-transfer func-
tions and alter the coefficients of the existing 
terms. By taking these additional parameters into 
consideration one can realize the frequency response 
of the filter with much greater accuracy. 
The new synthesis procedure for design was based 
on these more realistic network models. Equations 
(I) describing the networks were derived for calcu-
lation of the values of components required for 
realization of the networks for a given response. 
Many tests have shown the design techniques to be 
highly accurate. 
The equations describing the higher-frequency 
models are much more complex than for the ideal 
lower-frequency models and require laborious calcu-
lations. Therefore computer programs were developed 
(2) for performance of all computations required for 
the analysis and for synthesis of the active filters. 
With the programs one calculates the component-
network values required for realization of the net-
works, along with the filter's response characteristics 
including amplitude, phase, time delay, and group 
delay responses. The programs are written for easy 
use by designers inexperienced in design of active 
filters. 
The new procedure can be used for design of special 
filters in low-pass, high-pass, or band-pass configura-
tion for frequencies up to 500 kHz. Thetechniques are 
especially applicable for telemetry or communications 
when accuracy in design is important. 
References: 
1. S. Gussow and G. Weathers. Rept. SP-206-0080 
(Sperry Rand Corp., Huntsville, Ala., 1968). 
2. S. Gussow, "Analysis and synthesis of active 
quadratic filter networks for telemetry applica-
tions," thesis, Univ. of Alabama, Huntsville, 1968. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
Marshall Space Flight Center 
Huntsville, Alabama 35812 
Reference: TSP70-10238 
Patent status 
No patent action is contemplated by NASA. 
Source: S. Gussow and G. Weathers of 
Sperry Rand Corp. 
under contract to 
Marshall Space Flight Center 
(MFS-20445) 
Brief 70-10238	 Category 02


Design procedure for improved active filters

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