US2683810A

US2683810A - Piezoelectric crystal oscillator

Info

Publication number: US2683810A
Application number: US149978A
Authority: US
Inventors: Mortley Wilfrid Sinden
Original assignee: Marconis Wireless Telegraph Co Ltd
Current assignee: Marconis Wireless Telegraph Co Ltd
Priority date: 1949-03-30
Filing date: 1950-03-16
Publication date: 1954-07-13
Anticipated expiration: 1971-07-13

Description

July 13, 1954 w. s. MORTLEY PIEZOELECTRIC CRYSTAL OSCILLATOR Filed March 16, 1950 2 Shee tsSheet l K2 QUQRTER WAVE LENGTH UNE 11'] QUARTER WAVE 1 LEA/6TH LINE 4 Jufiy 13, 1954 w, s MQRTLEY 2,683,810

PIEZOELECTRIC CRYSTAL OSCILLATOR Filed March 16, 1950 2 Sheets-Sheet 2 L1 6/ a1 a 7 w kmwh A 37 9 MW Patented July 13, 1954 2,683,810 PIEZOELECTRIC CRYSTAL OSCILLATOR Wilfrid Sinden Mortley, Great Baddow, Chelmsford, England, assigncr to Marconis Wireless Telegraph Company Limited, London, England,

a British company Application March 16, 1950, Serial No. 149,978

Claims priority, application Great Britain March 30, 1949 6 Claims.

This invention relates to piezo electric crystal oscillators and more particularly to oscillators of the customary kind in which a piezo electric crystal is maintained in oscillation by a valve.

As is well known, where it is required to obtain the highest possible frequency stability from a crystal oscillator, it is best to cause the crystal to vibrate at a frequency as near as possible to that of series resonance. It is also a requirement that a suitable impedance should be presented to the maintaining valve and that the Q value of the crystal should not be appreciably reduced by the circuits coupling it to the said valve.

The present invention seeks to satisfy these requirements in a simple way.

According to this invention a piezo electric crystal is connected as a coupling link between two quarter wave lines the outer end of one of which is coupled between the grid point and cathode point of a maintaining valve and the outer end of the other of which is coupled between the anode point and the cathode point of said maintaining valve.

The quarter wave lines may be of any type, real or artificial, i. e. with disturbed impedances or composed of lumped impedances. In the case of artificial quarter wave lines they may be of the series inductance and shunt capacity type or vice versa or they may be of the mixed type provided that at the working frequency both series arms are capacitative or both are inductive and at this frequency w2L1C1=1 and w2L2C2=1 where w is the frequency in angular measure, L1 and C1 are respectively the inductance and capacity values of one quarter wave line, and C2 and L2 are respectively the inductance and capacity values of the other quarter wave line.

The two quarter wave lines need not be of the same characteristic impedance and indeed there may be some advantage (from the point of View of reducing instabilities due to valve capacity) by making the line adjacent the grid of the valve of lower impedance than the other. On the other hand, for example, in the case of a circuit of medium-high stability without automatic gain control, it may be of advantage to making the line adjacent the grid of the valve of higher impedance than the other, for then the crystal oscillation amplitude, and therefore its self-heating, would be less.

The conditions for oscillation are satisfied by the equation Z1Z2=T/gm where Z1= /L1/C1=the characteristic impedance of one line Z2= /L2/C'2=the characteristic impedance of the other line r=series resistance of the crystal gm=the working value of the mutual conductance of the valve.

The invention is illustrated in the accompanying drawings in which Figure 1 is a general diagram of an embodiment of the invention, while igs. 2 to 5 show various forms of lumped impedance line which may be employed for the lines represented by rectangles in Fig. 1. In all the figures line terminals are numbered, the same numerals being used for the same terminals throughout.

Referring to Fig. 1, the crystal X and two associated quarter wave lines KI and K2 constitute a four terminal network of which the terminals 'l, 8 of one end pair are connected respectively to the control grid G and cathode C of a maintaining valve V (which may be of any type but which for the sake of simplicity will be assumed to be a triode) while the terminals I, 2 of the other end pair are connected between said cathode C and the anode A of the maintaining valve. One terminal of each pair-the terminals I and il-is common and the cathode C is connected to this common point. The crystal X is in series between the

line terminals

3 and 6 and the

terminals

4 and 5 are connected together. A resistance R may be connected across terminals 5 and B or across terminals 3 and 4 (or resistances may be connected in both these places) to prevent oscillation in unwanted modes.

Figs. 2 and 3 show two of the many forms which may be adopted for the quarter wave lines K1 and K2 of Fig. 1. Referring to Fig. 2 and starting with the anode terminal 2 of the network the series arm between this terminal and the grid terminal 1 consists of an inductance of value L1 in series with the crystal X which is in turn in series with an inductance of value L2. There are four shunt capacities one of value C1 between the anode and cathode terminals I, 2 one of the U same value between the terminals 3, 4 (the remaining end of L1 and the common terminal), one of value C2 between the terminals 6, 5 (the crystal end of L2 and the said common terminal), and one of the same value C2 across the remaining pair of

terminals

1, 8.

In the form shown in Fig. 3 the series inductance of values L1 and L2 of Fig. 2 are replaced by series condensers of values Cl and C2 respectively and the shunt: condensers of values 01 and C2 of Fig. 2 are replaced by shunt inductances of values L1 and L2 respectively.

Figs. 4 and 5 are, respectively, the T section equivalents of the Pi sections. of Figs. 2 and 3. It is thought that, in view of the description already given of Figs. 2 and 3, Figs. 4 and 5 will be largely self-explanatory since throughout Figs. 2 to 5 the various circuit elements are indicated by their values.

The invention is not limited to the use of lines with elements dimensioned precisely as above described and in particular if, as will usually be the case, Z1 and Z2 (the characteristic impedances of K1 and K2 respectively) are both large with respect to r (the resistance of the crystal) the shunt. elements adjacent the crystal are not critical in value and may often be omitted altogether without serious loss of stability. If desired resistances may be connected across these elements (in Fig. 1 such a resistance is across terminals 6, 5) or across the crystal in order to suppress any unwanted mode of oscillation caused by coupling through the shunt capacity of the crystal.

For true series resonant operation the crystal shunt capacity should be balanced by a suitable shunt inductance and such an inductance may be provided if required. In practice, however, the frequency departure from that of true series resonance caused by leaving the shunt capacity of the crystal unbalanced is negligible.

Small adjustments of frequency may be made by providing shunt reactances at either or both ends-of the network, i. e. between grid and cathode and/or between anode and cathode.

We claim:

1. A piezo-electric crystal oscillator arrangement comprising a piezo-electric crystal, a valve having at least a cathode, a control grid and an anode, a four terminal quarter wave line having the two terminals at one end connected between the control grid and the cathode of said valve, a second four terminal quarter wave line having the two terminals at one end connected between the anode and the cathode, of said valve, and a coupling link including said crystal connected between the remaining pairs of terminals at the other end of said quarter wave lines.

2.. A piezo-electric crystal oscillator arrangement as set forth in claim 1 wherein the two quarter wave lines have distributed constants.

3. A piezo-electric crystal oscillator arrangement as set forth in claim 1 wherein the two quarter wave lines are constituted by lumped constants.

4. A piezo-electric crystal oscillator arrangement as set forth in claim 1 wherein the two quarter wave lines are of the same characteristic impedance.

5. A piezo-electric crystal oscillator arrangement. as set forth in. claim 1 wherein the two quarter wave lines are of different characteristic impedances.

6. A piezo-electric crystal oscillator arrangement as set forth in, claim 1 which includes a resistance connected across at least one of the quarter wave lines.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,053,524 Heegner Sept. 8, 1936 2,165,517 Stevenson July 11, 1939 2,259,528 Mason Oct. 21, 1941 2,345,491 Mason Mar. 28, 1944 2,551,809 Mortley May 8, 1951