Method is described for designing crystal oscillator which has frequency and symmetry stability suitable for system-clock-generators used in spacecraft data systems. Simple multivibrator is used in which timing capacitor is replaced by a crystal

Schober, R. C.

English

NASA Technical Reports Server

December 1972
	 B72-10155 
•	 NASA TECH BRIEF	
-4r NASA Pasadena Office 
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. 
Crystal-Controlled Multivibrator 
.
The problem: 
To design a crystal oscillator which has the fre-
quency and symmetry stability demanded of system-
clock-generators used in spacecraft data systems. 
ware 
ve 
itput 
The solution: 
A simple multivibrator in which a crystal replaces 
the timing capacitor; latch-up is prevented by diodes 
and a capacitor. 
How it's done: 
The circuit shown in the diagram is a symmetrical 
free-running multivibrator with one of its timing 
capacitors replaced by a crystal which operates in a 
series-resonant mode. The circuit may be set up by 
constructing a multivibrator which runs at a fre-
quency slower than it is to be used, and replacing a 
capacitor with a series-resonant crystal. The crystal 
is used to drive Q-1 with a 50-percent duty cycle at 
the resonant frequency, where it presents a low im-
pedance to the base of Q-1. Coarse timing capacitor 
C-i is used as in a free-running multivibrator; it must 
be large enough to hold transistor Q-2 nonconducting 
for a half cycle. Its principle function is to start the 
oscillator at the desired harmonic of the crystal fre-
quency (normally, the fundamental). Increasing the 
size of C-i will not affect the frequency or symmetry 
because this function is controlled by the crystal; 
therefore, the circuit is not sensitive to the time con-
stant established by C-i. 
The output taken from the collector of Q-2 has 
a nearly square form because the crystal limits 
di/dt at the base of Q-1 to a sinusoidal zero-crossing. 
Transistor Q-2 provides further amplification to ensure 
a square waveform. Two resistors and two diodes 
could be added to improve the squareness of the 
waveform; both sides of the waveform (or phases) 
may be obtained by this expedient for use in timing 
functions. 
The symmetrical configuration of the oscillator 
minimizes generation of noise in power supply lines 
because both states of the multivibrator drain approx-
imately the same power supply current. Transients on 
the power supply lines during switching are minimized 
as usual by use of high-frequency (0.01 /if) and low-
frequency (1.0 j.f) bypass capacitors installed across 
the supply lines near each oscillator. 
A circuit beta of approximately 10 is used to assure 
saturation over extreme transistor degradation; gold-
doped high-speed transistors are used because of their 
low charge-storage time, and they are run at a high 
collector current of about 9 mA to provide a low pull-
up resistance at the gate input and thus a squarer 
risetime waveform to the gate. 
(continued overleaf) 
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Aeronautics and Space 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=19720000155 2020-03-17T03:31:57+00:00Z
For this type of oscillator, the impedance of the 
series-resonant crystal must be less than about 1000 
ohms; high loop voltage-gain is required for crystals 
of higher impedance. 
Startup of the oscillator is assured by diodes CR-i 
and CR-2, which supply base-drive to either transistor 
from the highest voltage collector and prevent the 
only possible mechanism for latch-up, i.e., both tran-
sistors saturated. The oscillator will not latch-up with 
either transistor off or both off because there is no DC 
supply which could hold the transistors at cutoff. 
When both transistors are active, either one must 
drive to a quasi-stable state or both must proceed 
towards saturation because the open-loop gain is 
much greater than one. When both transistors tend 
to saturate, their collectors go low and their base 
drives are removed; however, this condition cannot 
exist for long because both transistors operate about 
their noise region. As soon as one transistor is on a 
little harder than the other, regeneration will take the 
circuit into oscillation.
The minimum voltage for operation is just enough 
to forward-bias two junctiins. At this voltage, the os-
cillator maintains frequency and symmetry. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP 72-10155 
Patent status: 
NASA has decided not to apply for a patent. 
Source: Robert C. Schober of 
Caltech/JPL 
under contract to 
NASA Pasadena Office 
(NPO-11627)
. 
. 
B72-10155	 Category 01


Crystal-controlled multivibrator

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