JPH049452B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a gas pressure measuring device that uses a crystal oscillator to measure the pressure of gas around it.

[Prior art] It has recently become clear that the resonance resistance of crystal vibration (particularly the bending mode) varies over a wide range with respect to the pressure of the surrounding gas, and if this is utilized, it can be used ^to It has become clear that it is possible to realize a gas pressure gauge that can continuously measure with one sensor. This is, for example, the monthly magazine "Instrument", 1984
It is disclosed in the section ``Development of ultra-small vacuum sensor using crystal oscillator'', Vol. 27, No. 7, 2013.

Next, the operating principle of a gas pressure gauge that utilizes the pressure dependence of the resonance resistance of a crystal oscillator will be explained with reference to the drawings.

FIG. 1 is a diagram showing the functions of gas (N ₂ ) pressure and resonant resistance, resonant current, and resonant frequency of a crystal resonator in a bending mode of about 32 kHz. The resonance frequency begins to change when the pressure exceeds 10 Torr, but the frequency pressure sensitivity is almost zero below this pressure. On the other hand, the resonant resistance of a crystal resonator has effective sensitivity to pressure from atmospheric pressure to about 10 ^-3 Torr. Here, if this crystal resonator is driven at a constant voltage, a resonant current-gas pressure curve indicated by _i0 in the figure is obtained. It is similar to the resonant resistance and is approximately
Pressure sensitive up to 10 ^-3 Torr. In terms of ease of measurement, it is better to measure the resonance current and use this to indicate the gas pressure.

FIG. 2 is an electronic circuit block diagram of a crystal gas pressure gauge (hereinafter referred to as a crystal gas pressure gauge) that utilizes the pressure dependence of the resonance resistance of a crystal resonator.

It is composed of a PLL circuit section, a display conversion circuit section, and a display circuit section. The PLL circuit section is
A variable frequency oscillator 1 controlled by voltage or current, an amplifier 2 that amplifies the resonant current of the crystal oscillator 5 as a voltage, and a comparison of the phase difference between the output signal of the amplifier 2 and the output signal of the variable frequency oscillator 1. and a phase comparator 3 that outputs a signal proportional to the phase difference, and a low-pass filter 4 that generates a DC voltage proportional to the output signal of the phase comparator 3. The output voltage of the generator 4 controls the oscillation frequency of the variable frequency oscillator 1. The crystal resonator 5, which is a pressure sensor, is driven by the output voltage of the variable frequency oscillator 1.

Since the operating principle of the PLL circuit is already widely known, it is omitted here, but the output signal of the variable frequency oscillator 1 (i.e., the drive voltage of the crystal resonator 5) and the output signal of the amplifier 2 (i.e., the The oscillation frequency of the variable frequency oscillator 1 is controlled so that the phase difference with the current flowing through the crystal resonator 5 becomes zero. That is, the crystal resonator 5 is always driven at its own resonant frequency. This allows sufficient tracking even if the resonant frequency of the crystal resonator changes due to the pressure of the surrounding gas.

Next, the display conversion circuit unit is a circuit that converts a signal obtained by converting the resonant current of the crystal oscillator 5 into a voltage (hereinafter referred to as a resonant voltage) into an electric signal capable of displaying pressure. consists of the following circuit. When the display section is composed of an ammeter, these include a main amplifier 6 that further amplifies the output signal of the amplifier 2, a rectifier circuit 7 that converts the output signal of the main amplifier 6 into direct current, and the rectifier circuit 7. This is a meter drive circuit 8 that converts the output voltage of the meter into a meter drive voltage. In this example, the display section is an ammeter (meter) 9
The pressure of the gas is determined by the deflection angle of the needle of the meter 9.

As shown in FIG. 3, when the gas pressure decreases, the output voltage V _DC of the rectifier circuit 7 changes to the resonance current i ₀ in FIG.
Consistent with the trend of increasing (the maximum value of V _DC
V _U and the minimum value is V _L ). If the meter 9 is driven in this state, the pressure will decrease and the deflection angle of the meter 9 will increase, giving an indication that is contrary to common sense for a pressure gauge. Therefore, the meter drive circuit reverses the output voltage V _DC of the rectifier circuit 7 as shown by the curve V _M in FIG. At times, the deflection angle of the pointer of the meter 9 is made zero.

[Problem that the invention seeks to solve]

In a conventional gas pressure gauge using an automatic oscillator, a crystal resonator is used in the feedback loop of the self-excited oscillation circuit, so an output voltage corresponding to the resonant resistance of the crystal resonator is obtained. In addition to the resonance resistance value, it is strongly influenced by the loop gain of the self-excited oscillator. In addition, the resonance resistance of the crystal resonator is 3 digits (30kΩ in high vacuum, 300kΩ at atmospheric pressure).
Strong) and above also change depending on the pressure, so in order to prevent the output voltage of the oscillator from saturating in high vacuum, the driving voltage level of the crystal oscillator must be lowered, which makes it possible to reduce the voltage near normal pressure. self-sustained oscillation becomes difficult. In other words, in the self-oscillation method,
In general, the output voltage for reading pressure is strongly influenced by factors other than the resonance resistance of the crystal oscillator that is the pressure sensor, and the pressure measurement range as a measuring device is the pressure range within which the crystal oscillator can maintain effective sensitivity. As a result, the performance of the crystal resonator as a pressure sensor cannot be fully utilized. Also, to maintain stable oscillation,
Normally, the crystal resonator and oscillation circuit must be placed as close as possible (the longer the sensor cable, the more unstable the oscillation will be due to stray capacitance).
This is a major practical limitation.

On the other hand, in the crystal gas pressure gauge according to the present invention, since the output voltage of the variable frequency oscillator 1 is constant regardless of the resonance resistance value of the crystal oscillator 5, the crystal oscillator 5 is always in a resonant state and has a constant state. It is driven by a voltage of amplitude. Therefore, as shown in FIGS. 1 and 3, the output voltage V _DC of the rectifier circuit is equal to the resonance current i _O of the crystal resonator, and the output voltage V _M of the meter drive circuit ( The deflection angle of the needle is V _M
) is the resonant resistance R _O of the crystal resonator
, respectively. That is, since the meter driving voltage V _M is determined only by the resonant resistance R _O of the crystal resonator, such a gas pressure gauge can fully utilize the performance of the crystal resonator. Furthermore, if the drive source impedance of the crystal oscillator and the input impedance of the amplifier that amplifies the resonant current of the crystal oscillator are made sufficiently low, the length of the cable connecting the sensor (crystal oscillator) and the main body of the gas pressure gauge can be reduced. There is no problem even if the length is very long (for example, 30 to 50 meters). These are major practical advantages.

However, this method also has the following problems.
That is, since the resonant current _iO of the crystal resonator 5 differs by more than 10 times between high vacuum and atmospheric pressure, the signal applied from the amplifier 2 to the phase comparator 3 (referred to as crystal resonant voltage Vqr) ) also changes by more than 10 times. On the other hand, the amplification of the other signal applied to the phase comparator 3 (which is the voltage that drives the crystal resonator 5 and is the output signal of the variable frequency oscillator 1 and is referred to as the crystal drive voltage Vqd) is constant. is set to approximately 1.5Vpp. It is essential for stable lock operation of the PLL circuit section that the crystal resonance voltage and the crystal drive voltage have approximately the same oscillation value. However, in high vacuum, Vqr
When the gain of the amplifier 2 is adjusted so that ≒Vqd, at atmospheric pressure, Vqr≪Vqd (Vqr≒0.06Vqd), and the lock of the PLL section becomes unstable, causing sudden pressure changes, changes in ambient temperature, etc. Depending on the situation, the lock may become disengaged or become unlatched. Conversely, if the gain of the amplifier 2 is increased so that Vqr≒Vqd at atmospheric pressure, then at high vacuum
Vqr≪Vqd, and now the phase comparator 3 is
Malfunctions due to excessive input are likely to occur, which can also cause lock loss. This phenomenon is due to the fact that the resonance resistance of the crystal oscillator 5, which is a pressure sensor, changes greatly depending on the surrounding gas pressure, and the crystal gas pressure gauge of the present invention utilizes the above property. Therefore, the above-mentioned problem (unstable operation of the lock) is fundamental to this system.

The present invention has been made in view of the above circumstances, and includes adding a new amplifier between the amplifier 2 and the phase comparator 3, the gain of which automatically changes depending on the magnitude of the input voltage. Therefore, regardless of the magnitude of the resonant current of the crystal oscillator 5, the crystal resonant voltage of a substantially constant magnitude is applied to the phase comparator 3, and the PLL circuit is operated even under high vacuum or atmospheric pressure. This provides a means for stabilizing the locking operation of the device.

〔Example〕

Next, the present invention will be explained with reference to the drawings. FIG. 4 is a block diagram showing an embodiment of the present invention. An auxiliary amplifier 10 is provided between the amplifier 2 and the phase comparator 3.
is inserted, and has the function of further amplifying the crystal resonant voltage amplified by the amplifier 2 and feeding it back to the phase comparator 3. Since the auxiliary amplifier 10 is shown out of the display path, its output voltage need not be proportional to the input voltage (non-linear amplification is permitted). FIG. 5 is a circuit diagram explaining the embodiment of the present invention in more detail, and FIG. 6 is a diagram showing output voltage waveforms of the amplifier 2 and the auxiliary amplifier 10 in the circuit diagram shown in FIG.

The amplifier 2 includes an operational amplifier 2a, a resistor 2b,
It is an anti-phase amplifier consisting of a resistor 2c, and the resistor 2c
By making b much lower than the resonant resistance R _O of the crystal resonator, the amplifier 2 can faithfully amplify the resonant current i _O of the crystal resonator 5. The auxiliary amplifier 10 is an operational amplifier 10
a, capacitor 10b, resistor 10c, resistor 10
d, a diode 10e, and a diode 10f. Since diodes 10e and 10f are connected in opposite directions in the feedback loop of the operational amplifier 10a, the operational amplifier 10
When the output voltage of the operational amplifier a exceeds the forward voltage of the diodes 10e and 10f (0.6 to 1 V in the case of silicon), the AC resistance value of the diodes 10e and 10f decreases significantly.
The gain of the anti-phase amplifier due to 0a decreases. The output voltage of the operational amplifier 10a is
If it is lower than the forward voltage of e and 10f,
Since the alternating current resistance values of the diodes 10e and 10f are relatively high, the gain of the anti-phase amplifier maintains approximately the value determined by the ratio of the resistors 10d and 10c. In this way, the auxiliary amplifier 10 has a large gain for a small input signal and a small gain for a large input signal, so the resistor 1
By setting 0d to a value appropriate for the forward AC resistance value of the diodes 10e and 10f (usually determined by experiment), the values shown in FIGS.
As shown in FIG. 2, the output voltage of the auxiliary amplifier 10 can be kept almost constant even when the crystal oscillator 5 is in a high vacuum or under atmospheric pressure.
Therefore, since the amplitude of the signal to be compared by the phase comparator 3 is always kept almost constant, the PLL
The lock operation of the circuit section becomes very stable.

Inserting the auxiliary amplifier 10 according to the present invention between the amplifier 2 and the phase comparator 3 also has the following secondary effects. That is, by appropriately selecting the values of the capacitor 10b and the resistor 10c (because it is a phase advance circuit), the phase delay of the signal from the amplifier 2 and the auxiliary amplifier 10 can be adjusted to the input signal of the amplifier 2, that is, the crystal. The resonant current of the vibrator 5 and the auxiliary amplifier 10
The phase of the output signal of the phase comparator, that is, the phase of one input signal of the phase comparator can be completely matched.
This means that when the PLL circuit section is in the locked state,
This means that the driving voltage of the crystal oscillator 5 and the voltage flowing through the crystal oscillator are completely in phase with each other, that is, the crystal oscillator 5 can be maintained in a truly resonant state.
This has a favorable effect on the voltage drop in the lock operation of the PLL circuit section.

〔Effect of the invention〕

As described above, according to the present invention, an amplifier that amplifies the resonant current of a crystal oscillator, which is a pressure sensor, and the phase comparator are connected in parallel in opposite directions in a feedback loop. By inserting an auxiliary amplifier containing a diode, it is possible to eliminate the problem even when the crystal is in a vacuum (its resonant current is very large) or at atmospheric pressure (its resonant current is very large). Since the phase comparator is applied with a signal corresponding to the resonant current of the crystal resonator having a substantially constant voltage, the locking operation of the PLL circuit becomes very stable. moreover,
By appropriately selecting the values of the coupling capacitor and resistor of the auxiliary amplifier, the phase delay due to the amplifier and the auxiliary amplifier is compensated, and the phases of the drive voltage and current of the crystal resonator are completely equalized. This further contributes to stabilizing the lock operation of the PLL circuit section.

As described above, the present invention has a great effect on stabilizing the locking operation of the PLL circuit in a crystal gas pressure type in which a crystal resonator is separately excited at a constant voltage by a PLL circuit.

Note that in this embodiment, the amplifier 2 and the auxiliary amplifier 10 were originally configured as negative-phase amplifiers, but even if they were configured as positive-phase amplifiers, the same effect would be obtained. . Furthermore, it is clear that the same effect can be obtained by using two Zener diodes connected in series in opposite directions in place of the two parallel-connected diodes shown in this embodiment.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between the resonant resistance, resonant current, resonant frequency, and gas (N ₂ ) pressure of a crystal resonator.
Fig. 2 is an electronic circuit block diagram of a crystal gas pressure gauge, Fig. 3 is a diagram showing the relationship between the rectifier circuit output voltage in the electronic circuit and the meter drive voltage, and the gas pressure. A block diagram showing an example,
FIG. 5 is a circuit diagram of an embodiment of the present invention, and FIG. 6 is a diagram showing output voltage waveforms of various parts in the circuit diagram of an embodiment of the present invention. 1... variable frequency oscillator, 2... amplifier, 3...
... Phase comparator, 4 ... Low-pass filter, 5 ... Crystal oscillator, 6 ... Main amplifier, 7 ... Rectifier circuit, 8 ...
Meter drive circuit, 9...meter, 10...auxiliary amplifier, 2a, 10a... operational amplifier, 2b, 2
c, 10c, 10d...Resistor, 10b...Capacitor, 10e, 10f...Diode.

Claims (1)

[Claims] 1. A phase locked loop (PLL) circuit section comprising at least a variable frequency oscillator, a phase comparator, a reduction filter, and an amplifier, and a phase locked loop (PLL) circuit section connected between the variable frequency oscillator and the amplifier. It has a crystal oscillator and a display section connected to the PLL circuit section, and measures the pressure of the gas surrounding the crystal oscillator from the resonance resistance value, resonance current value, or resonance voltage value of the crystal oscillator. In the crystal gas pressure gauge, two diodes for limiting the input voltage to the phase comparator are connected in parallel with a feedback resistor in opposite directions between the amplifier and the phase comparator. A crystal gas pressure gauge characterized by having an in-phase auxiliary amplifier. 2. A capacitor and a resistor whose values are determined to compensate for a phase delay caused by the amplifier and the auxiliary amplifier are connected between the output terminal of the amplifier and the input terminal of the auxiliary amplifier. The crystal gas pressure gauge described in the previous section.