Ultra accurate filter is used with static type pressure transducers where it is desirable to extract low frequency dynamic signals from combined static and dynamic signal. System can be calibrated at any time with dc voltages

Hansen, I. G.

Peterson, V. S.

English

NASA Technical Reports Server

August 1974	 B74.10083 
• .
	
NASA TECH BRIEF 
Lewis Research Center 
NASA Tech Briefs announce now 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 program may 
be directed to the Technology Utilization Office. NASA. Code KT, Washington. D.C. 20546. 
Electronic High Pass Filter 
C
Static Pressure 
dynamic transducer 
signal source 
Transducer Snal
o.	 Shunt 
per calibrate 
supply
 resistor (R-cal) 
1+	
Nc rm,9 Filter - __ 
—" o	 DC calibrate mode switch Si 
Al
Cal. 
- +
II 
C2 
-
Ilk 
High pass Dl	 D2 
filter cl ccuit
—
R2 
Static
R4	 R5 I +l5vDc p + dynamic
- I
Ill 
signal '> I AMP	 Oui 
Static —i A3	 R3 
dynami
I
signal
4D y^namic
COM D. C. Amp zero
 
C.M.R. •l2Odb trim
-i5VDC 
output
.L.	 circuit common
An ultra accurate electronic high pass filter has been 
designed and developed for use with static type pressure 
transducers where it is desirable to extract low frequency 
dynamic signals from the combined static and dynamic 
signal. The system can be calibrated at any time with DC 
voltages. 
The filter as well as the complete measuring or
adjusted to produce the desired transfer function. 
Accurate DC voltage measurements can be easily obtained 
by a digital voltmeter. The resultant DC transfer function 
is exactly equivalent to the peak amplitude of the AC 
transfer function of the filter just above and beyond its 
roll off frequency. This feature permits the calibration of 
the system just prior to its use. 
In the schematic, the upper section shows the signal 
source and the lower section shows the DC calibrateable 
• recording system can be switched at any time to the 
calibrate mode where DC voltages at the input and output 
of the system can be measured and the amplifier gains 
-	
.	 (continued overleaf) 
This document was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and S pace Administration. Neither the United States 	 information contained in this document, or warrants that Such use 
Government nor any person acting on behalf of the United States	 will be free from privately owned rights. 
https://ntrs.nasa.gov/search.jsp?R=19740000083 2020-03-17T14:02:20+00:00Z
high pass filter circuit elements. The signal source is 
shown as a typical example of one containing combined 
static and dynamic voltages. This signal is connected to 
the input of amplifier Al. Oji'e side of amplifier Al 
output is connected to circuit common, the (+) output 
connects to amplifier A2 input through R4 and R5. 
The 120 db common mode rejection of DC capability 
of A2 will not allow DC signals to pass through A2. When 
switch S  is placed in the calibrate position, amplifier A2 
will convert to a DC transfer function amplifier and allow 
both Al and A2 to be adjusted to a desired gain by 
measuring the DC voltage at the A2 output when the 
signal conditioner "R-Cal" resistor is switched in the 
transducer bridge to produce an exact pressure equivalent 
voltage. The high pass filtering process starts when SI is 
placed in the open or normal position. Operational 
amplifier A3 gain is determined by the values of RI and 
R2. The first and conventional DC summing junction 
occurs at A3 negative input terminal. Capacitor C  blocks 
DC from the amplifier's input and makes A3 responsive 
only to AC signals. A second summing function provided 
by feedback capacitor C2 and resistor R5 for AC signals 
occurs at point (X). The low frequency response of A3 is 
controlled by the value of Cl, where an increase in 
capacity lowers the frequency response. The value of C  is 
thus selected to determine the high pass frequency point 
where no signal attenuation occurs. Zener diodes Dl and 
D2 provide a discharge path for Cl at high input voltage 
conditions that would lock up A3. Amplifier A3 reacts to 
high pass frequencies by creating a summing junction at 
circuit point (X) in a manner that keeps point (X) at zero 
volts AC. This has the effect of placing the negative input 
of A2 at circuit common and allows A2 to transfer AC 
voltages to its output. 
In this circuit, the filtering action begins to attenuate 
frequencies below five Hz and achieves a maximum 
attenuation of 120 db at DC conditions. The attenuation 
frequency break point is primarily determined by the 
capacitance of Cl. The gain of A3 and the value of C2 
affects the zero voltage summing ability of junction (X). 
This sets a limitation on the clamping of the AC 
amplitude of the signal from the output of Al.
Notes: 
1. The filter circuit affords its greatest service in its ability 
to operate as a high pass filter at the very low 
frequency end of the AC spectrum when compared to 
other filter circuits. It further excels in the method of 
filtering wherein the signals that pass through the 
system do not directly route through any frequency 
selective filter element where amplitude and phase 
distortion over a wide frequency range would degrade 
the measurement. 
2. The filter system can be used for any application where 
accurate amplitude measurements of wave forms above 
a selected frequency are desired. Amplifiers Al and A2 
can be integrated circuit operatidnal amplifiers where 
site reduction is important. 
3. No additional documentation is available. Specific 
technical questions, however, may be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B74-10083 
Patent Status: 
Inquiries concerning rights for the commercial use of 
this invention should be addressed to: 
NASA Patent Counsel 
Mail Stop 500-113 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Source: V.S. Peterson and I.G. Hansen

Lewis Research Center

(LEW-1 1600) 
B74-10083
	 Category 02


Electronic high pass filter

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server