Frequency-to-analog converter has a fast response time and a low ripple. The circuit uses a frequency-to-pulse converter which provides two pulse trains, both at the same frequency as that of the input signal, but with a 10 microsecond difference between the trains

Hagihara, F. S.

English

NASA Technical Reports Server

July 1967	 Brief 67-10257 
NASA TECH BRIEF 
--- j--.--	 --	 -	 - -	 - 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Fast-Response Frequency-to-Analog Converter 
R2 	 TDI 
Frequency	 at 
To Pulse	 Id)	 03 Converter Input (a) I 
-I 
I	
R J
jC4 
'[T-3  TR 
Ib)	 C1 
.	 )a)1WJ\]Jf 
IbI	 ft	 A	 A	 A	 A	 I 
E	
HA (c)'1 
Idi t 
E
WAVEFORMS AT DESIGNATED POINTS ON CIRCUIT DIAGRAM
Output 
The problem: 
To design a frequency-to-analog converter having a 
fast response time (equal to the period of the input 
signal) and a low ripple (no higher than 1 percent of 
reading). 
The solution: 
The circuit employs a frequency-to-pulse converter 
which provides two pulse trains, both at the same 
frequency as that of the input signal, but with a 10 
microsecond difference between the trains. The oulses 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
should have a duration of 5 to 10 microseconds and 
an amplitude of approximately 5 volts. 
Pulse train (c) is coupled to transistor Q2 through 
capacitor C2. Capacitor C3 is charged by the constant 
current network consisting of R2, Di, and Qj. Potenti-
ometer R2 adjusts the charging current, and diode Di 
biases the FET (field effect transistor) Qi
.
 As transis-
tor Q2 is turned on by pulse train (c), capacitor C3 
discharges to zero potential. Between pulses, this Ca-
pacitor is charged at constant current, resulting in a 
sawtooth waveform at (d) having a peak voltage that 
(continued overleat) 
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19670000257 2020-03-11T20:33:05+00:00Z
is proportional to the period of the input frequency. 
Pulse train (b) (which leads pulse train (c) by 10 
microseconds) is coupled through C1 to Q, a unipolar 
FET. A low-resistance path (approximately 600 ohms) 
is provided between C3 and C4 when Q3 is turned "on" 
by pulse train (b). During the "off" time of Q, the re-
sistance is approximately 150 megohms. Therefore, the 
potential difference between C3 and C4 will fall to zero 
when Q3 is turned "on". Capacitor C4 has a much 
smaller capacitance than C3 to minimize any loading 
effect of C4 on C3 and also to reduce the time for 
potential equalization to less than I microsecond. 
Transistor Q3 is turned "on" at approximately the 
peak of a sawtooth, thereby charging C4 to the peak 
sawtooth potential. The dc level (e) on C4 is directly 
coupled to Q,, an FET with an input impedance of 
approximately 10,000 megohms. Q4 is placed in a 
source follower configuration for low output imped-
ance. The output of Q4 is in turn coupled into a com-
mon emitter configuration for even a lower output 
impedance (approximately 200 ohms). 
The dc output voltage (maximum 20 volts at I cps) 
of the converter is inversely proportional to the input 
frequency. Its upper frequency limit is 5000 cps, with
an output of approximately 5 millivolts. A dc voltage 
monitor can be coupled to the output and calibrated 
to read in cps. 
Notes: 
I. The frequency-to-pulse converter (shown as a 
dashed block) is of standard design. 
2. This frequency-to-analog converter can be used to 
record small response changes in electrical output 
transducers. It can also be used as a low-frequency 
modulation detector. 
3. Inquiries concerning this invention may be directed 
to:
Technology Utilization Officer 
Marshall Space Flight Center 
Huntsville, Alabama 35812 
Reference: B67-10257 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546. 
Source: Frank S. Hagihara
of North American Aviation, Inc. 
under contract to
Marshall Space Flight Center
(M-FS-709) 
Brief 67-10257	 Category 0!


Fast-response frequency-to-analog converter

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