Oscillator for measuring temperature

A measuring oscillator for measuring temperature has an all-pass filter (1) which drives a current source (16) having a controllable amplification factor and a control element (17). The control element is connected to the control input of the current source (16) and controls its amplification factor in a manner such that the amplitude of the alternating voltage produced by the measuring oscillator assumes a constant value. The all-pass filter comprises a phase-determining bipolar circuit arrangement (10) comprising a voltage follower (12), whose input is connected to one terminal (9) of the circuit arrangement (10) and whose output is connected through an ohmic input resistor (13) to the inverting input of a first operational amplifier (14), which is fed back negatively through a measuring resistor (15) having a temperature-dependent ohmic resistance value. The output of the first op-amp also is connected through an ohmic resistor (41) to the one terminal (9) of the bipolar circuit arrangement. The non-inverting input of the first op-amp is connected to the other terminal (11) of the bipolar circuit arrangement. Small temperature variations sensed by the measuring resistor lead to comparatively large adequately measurable variations of the frequency of the measuring oscillator.

BACKGROUND OF THE INVENTION 
This invention relates to a measuring oscillator for temperature sensing, 
which produces a signal having a temperature-dependent frequency, and 
comprises an amplifier fed back positively through an all-pass filter, the 
all-pass filter comprising a bipolar phase-determining circuit element 
having a temperature-dependent ohmic resistance value which varies the 
frequency to be measured of the oscillator signal in dependence upon 
temperature. 
The magazine etz-b, Vol. 25 of 1973, No. 9, p. 220-222 discloses a 
measuring oscillator of the kind described in the opening paragraph having 
an all-pass filter which comprises, as a phase-determining ohmic resistive 
element, a platinum resistance thermometer. At temperatures between 
0.degree. C. and 100.degree. C., oscillator frequencies between 800 Hz and 
1 kHz are obtained. As a result of this comparatively small frequency 
variation of 20% for a temperature variation of 100.degree. C. small 
temperature variations can be measured with insufficient accuracy. The 
small frequency variations then obtained require a frequency measuring 
appartus of high resolution. 
SUMMARY OF THE INVENTION 
An object of the invention is to increase the sensitivity of the measuring 
oscillator to such an extent that even small temperature variations lead 
to comparatively large, adequately measurable variations of the oscillator 
frequency. 
This object is achieved in a measuring oscillator of the kind mentioned in 
the opening paragraph in that the bipolar circuit element is constructed 
as a bipolar circuit arrangement which is connected to a measuring 
resistor having a temperature-dependent ohmic resistance value, and 
wherein the ohmic resistance of the bipolar circuit arrangement, which can 
be sensed by the all-pass filter, varies more strongly with temperature 
than the resistance of the measuring resistor. 
In an embodiment of the measuring oscillator, the bipolar (i.e. two-pole) 
circuit arrangement comprises a voltage follower having an input connected 
to one terminal of the circuit arrangement and an outputconnected through 
an ohmic input resistor to the inverting input of a first operational 
amplifier. This operational amplifier is negatively fed back through the 
measuring resistor and its output is connected through an ohmic resistor 
to one terminal, and its non-inverting input is connected to the other 
terminal of the bipolar circuit arrangement. A temperature-dependent 
variation of the resistance value of the measuring resistor connected in 
the negative feedback circuit of the operational amplifier then produces a 
large variation of the ohmic resistance value of the bipolar circuit 
arrangement and hence a comparatively large variation of the oscillator 
frequency of the harmonic measuring oscillator. 
A simply constructed first order all-pass filter that can be manufactured 
at low cost is obtained if the input and the output of the all-pass filter 
are interconnected through an ohmic voltage divider, whose centre tapping 
is connected to the inverting input of a second operational amplifier, 
which is connected on the output side to the output of the all-pass filter 
and whose non-inverting input is connected on the one hand through a 
capacitor to the input of the all-pass filter and on the other hand to one 
terminal of the bipolar circuit arrangement, whose other terminal is 
connected to ground. 
In an advantageous embodiment of the measuring oscillator, the output of 
the voltage follower is connected to the input of a non-inverting 
amplifier, which is connected on the output side to one terminal of the 
bipolar circuit arrangement. Thus, the first operational amplifier fed 
back negatively through a temperature-dependent resistor has connected 
parallel to it a non-inverting amplifier, which increases the ohmic 
resistance of the bipolar circuit arrangement so that as a result the 
oscillator frequency of the measuring oscillator can be reduced through 
the amplification factor of the non-inverting amplifier by a constant 
amount. 
In an embodiment of the invention, the bipolar circuit arrangement has a 
change-over switch which, in one position, connects the terminal remote 
from the first operational amplifier of the ohmic input resistor to the 
output of the voltage follower, and in the other position of the 
change-over switch to an output of the non-inverting amplifier. By 
actuation of the change-over switch, the non-inverting amplifier can be 
connected in series with the first operational amplifier which is fed back 
negatively through the temperature-dependent resistor. Thus, the input 
voltage of the first operational amplifier is increased so that its 
amplification factor and hence also its phase errors can be reduced.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The measuring oscillator shown in FIG. 1 comprises a first order all-pass 
filter 1 having an input 2 and an output 3 interconnected through an ohmic 
voltage divider 4, 5. The centre tapping 6 of the ohmic voltage divider 4, 
5 is connected to the inverting input of a second operational amplifier 7, 
which is connected at the output side to the output 3 of the all-pass 
filter 1. The non-inverting input of the second operational amplifier 7 is 
connected through a capacitor 8 to the input 2 of the all-pass filter 1 
and, moreover, to one terminal 9 of a bipolar circuit element, whose other 
terminal 11 is grounded. The circuit element comprises a bipolar circuit 
arrangement 10 (e.g. a temperature dependent two pole active network) 
which is connected to a measuring resistor 15. 
The circuit arrangement 10 comprises a voltage follower 12, which may be 
construced for example as an operational amplifier, whose inverting input 
and whose output are interconnected and whose non-inverting input is 
connected to one terminal 9 of the bipolar circuit arrangement 10. The 
output 10 of the voltage follower 12 is connected through an ohmic input 
resistor 13 to the inverting input of a first operational amplifier 14, 
which is fed back negatively through the measuring resistor 15 having a 
temperature-dependent resistance value. The measuring resistor 15 may be 
for example, a Pt 100 resistance thermometer. The operational amplifier 
14, however, may also be fed back negatively through a measuring resistor 
15 whose ohmic resistance value depends upon another physical measuring 
quantity, for example upon a magnetic field. The output of the operational 
amplifier 14 is connected through an ohmic resistor 41 to the terminal 9 
and the non-inverting input of the operational amplifier 14 is connected 
through the other terminal 11 of the bipolar circuit arrangement 10 to 
ground. 
The output 3 of the all-pass filter 1 is connected on the one hand to a 
current source 16 and on the other hand to a control element 17, which has 
an input terminal 18 for supplying an amplitude setpoint and is connected 
on the output side to the control input of the current source 16 for 
controlling the current supplied by the current source. 
The current source 16 is connected through a 90.degree. phase shift element 
19 to the input 2 of the all-pass filter 1. The 90 phase shift element 19 
is constructed, for example, as an inverting operational amplifier 
capacitively fed back negatively. The current source 16 may consist, for 
example, of an ohmic voltage divider 20, 21 which is connected to the 
output 3 of the all-pass filter 1 and whose centre tapping 22 is connected 
to the inverting input of a third operational amplifier 23 with a current 
output, which has a control input 24 for adjusting the amplification 
factor and hence the value of the supplied current. 
The control element 17 comprises a rectifier 25, connected to the output 3 
of the all-pass filter 1. The rectifier is connected, together with an 
input terminal 18 for supplying the amplitude setpoint, to a summation 
point 26. The summation point 26 is connected to the inverting input of a 
smoothing device 27, which determines the negative linear average value of 
the alternating voltage supplied through the summation point 26 and 
supplies it through the control input 24 to the third operational 
amplifier 23. The smoothing device 27 may be constructed, for example, as 
an integration element with an inverting input. 
The measuring oscillator essentially consists of an amplifier 16, which is 
fed back positively through an all-pass filter 1 and consequently 
oscillates at a frequency dependent upon the impedance values of the 
phase-determining capacitor 8 and of the phase-determining circuit element 
10, 15. The amplitude of the alternating voltage produced by the measuring 
oscillator is controlled by the control element 17 in a manner such that 
it assumes a constant value. For this purpose, the alternating voltage 
supplied by the all-pass filter 1 is rectified in the rectifier 25. The 
direct voltage of negative polarity supplied through the input terminal 18 
and corresponding to the amplitude setpoint is added to the direct ripple 
voltage produced by the rectifier 25 at the summation point 26. The 
smoothing device 27 with an inverting input produces from the resulting 
alternating voltage a negative or positive direct voltage, which is 
supplied to the control input 24 of the third operational amplifier 23 and 
which reduces or enlarges its amplification factor if the amplitude of the 
alternating voltage supplied by the all-pass filter 1 is larger or smaller 
than the given amplitude setpoint. 
The first operational amplifier 14 included in the bipolar circuit 
arrangement 10 produces from the alternating voltage applied to the 
non-inverting input of the second operational amplifier 7 and hence on the 
one hand to one terminal of the resistor 41 and on the other hand through 
the voltage follower 12 also to the inverting input of the first 
operational amplifier 14 an alternating voltage which is in phase 
opposition thereto and is applied to the other terminal of the resistor 
41. The current flowing through the resistor 41 and hence the overall 
current flowing into the circuit arrangement 10 depends upon the 
difference of the voltages of different polarities applied to the two 
terminals of the resistor 41. The larger the temperature to be recorded 
and hence the resistance value of the resistive element 15, the 
amplification factor of the first operational amplifier 14 and the 
alternating voltage produced by the first operational amplifier 14 and 
applied to the resistor 41, the larger is the current flowing into the 
circuit arrangement 10. 
However, an increase of this current has the same meaning as a decrease of 
the ohmic resistance of the circuit arrangement 10. The frequency of the 
alternating voltage produced by the measuring oscillator is inversely 
proportional to the ohmic resistance of the circuit element 10, 15 and 
hence is proportional to the temperature recorded by the measuring 
resistor 15. Furthermore, the frequency of the alternating voltage may 
also depend upon another physical quantity if the impedance values of the 
capacitor 8 or of the measuring resistor 15 vary with this physical 
quantity. 
The embodiment of the bipolar circuit element 10, 15 shown in FIG. 2 
comprises, besides the voltage follower 12, the first operational 
amplifier 14 with the ohmic input resistor 13 and the measuring resistor 
15 having a temperature-dependent ohmic resistance value in the negative 
feedback circuit and the resistor 41, a non-inverting amplifier 28, whose 
input 38 is connected to the output of the voltage follower 12 and whose 
output 39 is connected through a resistor 32 to the input of the voltage 
follower 12. Furthermore, the circuit element shown in FIG. 2 comprises a 
change-over switch 29, which connects the connection terminal remote from 
the operational amplifier 14 of the ohmic input resistor 13 in one 
position shown in FIG. 2 to the output of the voltage follower 12 and in 
the other position indicated by an arrow to a further output 40 of the 
non-inverting amplifier 28. Moreover, a further non-inverting amplifier 30 
is connected between the first operational amplifier and the resistor 41. 
Since the non-inverting amplifier 28 acts as a voltage follower, the 
voltage follower 12 may be dispensed with if the output 40 of the 
amplifier 28 remains electrically connected to the input resistor 13. 
The amplifier 28 amplifies the alternating voltage supplied by the voltage 
follower 12 and applied to the input 38 in the same phase and supplies it 
through the output 39 to one connection of the resistor 32, to whose other 
connection the alternating voltage supplied through the terminal 9 is 
applied. The current flowing through the resistor 32 depends upon the 
difference of the alternating voltages applied to the two connections and 
is in phase opposition to the current flowing through the resistor 41. 
Thus, the overall current flowing into the bipolar circuit element shown 
in FIG. 2 is reduced. This has the same meaning as an increse of the ohmic 
resistance of the bipolar circuit element, as a result of which the 
oscillator frequency of the measuring oscillator is reduced by a constant 
amount. The amplification factors of the non-inverting amplifiers 28 and 
30 can be adjusted through a corresponding proportioning of the resistors 
33, 34, 36 and 37 and the currents can be adjusted by a variation of the 
resistors 32 and 41 so that the ohmic resistance of the bipolar circuit 
element of FIG. 2, and hence also the frequency of the alternating voltage 
produced by the measuring oscillator, can thus be adjusted. 
If, by means of the change-over switch 29, the ohmic input resistor 13 is 
separated from the output of the voltage follower 12 and is connected to 
the output 40 of the non-inverting amplifier 28, the first operational 
amplifier 14 is no longer fed by the voltage follower 12, but by the 
non-inverting amplifier 28 so that measuring errors due to phase shifts 
are thus avoided. By means of the change-over switch 29, the frequency 
produced by the measuring oscillator can moreover be changed over because 
the voltage supplied by the voltage follower 12 in dependence upon the 
position of the switch 29 is amplified either by two or by three 
amplifiers so that the voltage applied to the resistor 41, the current 
flowing through the resistor 41, the ohmic resistance of the whole circuit 
element shown in FIG. 2 and hence the frequency of the measuring 
oscillator can thus be changed over.