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Crystal Controlled Low Frequency Oscillator

IP.com Disclosure Number: IPCOM000078787D
Original Publication Date: 1973-Mar-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 3 page(s) / 59K

Publishing Venue

IBM

Related People

Sabatterie, R: AUTHOR

Abstract

This oscillator obtains a sinusoidal oscillation accurate in frequency (+/- 0.05 % within the 250-2500 Hz range), by programming the period of the oscillator instead of its frequency. It utilizes a servo system to obtain this accurate frequency in a very small number of periods regardless of the programmed period

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Crystal Controlled Low Frequency Oscillator

This oscillator obtains a sinusoidal oscillation accurate in frequency (+/- 0.05 % within the 250-2500 Hz range), by programming the period of the oscillator instead of its frequency. It utilizes a servo system to obtain this accurate frequency in a very small number of periods regardless of the programmed period

The sinusoidal oscillation is obtained by transforming a symmetrical sawtooth signal with a constant amplitude. Since this transformation being carried out by static circuit 10 is conventional, only the device for obtaining the symmetrical sawtooth signal will be described in the following.

The sawtooth signal is obtained by charging and discharging capacitor C1 with capacity c, at constant current.

Voltage Uc at the capacitor terminals is equal to:

Uc = It over c assuming that current I charging the capacitor

is constant.

i.e., if U is the voltage at the terminals of switch S1 and R1 is the resistance of the circuit Uc = U over R1 t over c.

The time required to obtain a constant amplitude Uc corresponding to a side of the sawtooth, i.e., to half a period, is: t = T/2 = R1.c. Uc x 1 over U

i.e., if a 10 volt amplitude signal is desired:

t = T/2 = R1.c. 10 over U.

The time and therefore, the period of the sawtooth signal, is inversely proportional to voltage U.

To obtain the sawtooth signal, switch S1 switches alternatively, thus connecting circuit R1 C1 and gain g1 amplifier A1 either to voltage +U or -U.

This switching of voltage U is ensured by a Schmitt trigger TS which has two accurate switching voltages, +5 volts and -5 volts in this example. When switch S1 is in the position shown in the figure, +U charges the capacitor. Output voltage Us of amplifier A1 decreases linearly, voltage Us being equal to g1 over G1 + 1 . Uc, i.e., Us = Uc if gain g1 of A1 is such that g1>>1.

When voltage Us reaches the low level of the sawtooth (-5 volts in this example), the trigger triggers and switches switch S1 to -U. The sawtooth goes up and, when it reaches the high level (+5 volts), the trigger triggers and switches S1 again. The sawtooth decreases again and a new cycle starts.

Since the period of the sawtooth signal is inversely proportional to voltage U, the period could be programmed by programming voltage U. The programming is performed by using a digital-to-analog converter controlled by a buffer. The number representing the duration of t...