JPH0158692B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a phase shift circuit used in a magnetic field control device or the like in a nuclear magnetic resonance apparatus.

The magnetic field used in a nuclear magnetic resonance apparatus is required to have a stability of about 10 ^-8 to ^{10 -9} over a long period of time. To this end, magnetic field control devices have been used that detect magnetic field fluctuations based on magnetic resonance signals and cancel them. In this magnetic field control device, for example, a control sample is mixed into a measurement sample, or a control sample is placed near the measurement sample, and resonance signals (dispersion type) obtained from these control samples are controlled by magnetic field control. The magnetic field is stabilized by using it as a signal and providing negative feedback to the magnet's excitation current control means.
In particular, magnetic field control devices that have been used recently employ a so-called time sharing method in which a control sample is intermittently irradiated with high frequency waves at a constant cycle, and resonance signals are detected during non-irradiation periods.

By the way, this magnetic field control device uses a high frequency phase shift circuit in order to ensure the stability of the magnetic field and always satisfy the resonance condition. FIG. 1 is a circuit diagram showing this phase shift circuit, in which 1 is an input circuit section receiving high frequency input Vin, 2
3 is a first shift circuit section connected to the input circuit ₁ via a capacitor C1, 3 is a second shift circuit section connected to the first shift circuit section ₂ via a capacitor C2, and 4 is a capacitor. This is a third shift circuit section connected to the second shift circuit section 3 via _C3 . These shift circuit sections 2 to 4 have the same circuit configuration. That is, the shift circuit sections 2, 3,
4, each shift circuit section 2, 3, 4 has an LCR parallel resonant circuit connected to its drain and source, respectively.
FETQ ₂₁ , Q ₃₁ , which receives the gate input signal to
The drain signals of the FETs Q ₂₁ , Q ₃₁ , and Q ₄₁ are applied to the capacitors C ₂₁ , C ₃₁ , and C ₄₁ , and the source signals are applied to the FETs _{Q 22 ,} Q ₃₂ , and Q ₄₂ , and the signals are passed through them. The configuration is such that both signals are added and output to the next stage. The amount of phase shift of each shift circuit section 2, 3, 4 is determined by the phase control voltage Vc applied to the gate of each _FETQ22 , _Q32 , _Q42 .
5 connects the output of the third shift circuit section 4 to a capacitor
This is an output circuit section that receives signals via _C4 and outputs an output signal Vout as a phase shift circuit.
Note that the power supply voltage E is applied to each shift circuit section 2, 3, 4 and the output circuit 5 via a filter 6 and the like.

By the way, in the conventional circuit with such a configuration, it is necessary to increase the Q of each shift circuit section 2, 3, and 4, so even a slight change in the frequency of the input Vin causes a large change in the level of the output Vout. There was a problem that the phase variable width changed, and that oscillation occurred when trying to keep the output level constant when changing the phase. Furthermore, since there are a large number of parts, the space occupied on the printed board is large, and the cost is also high.

The present invention was developed in view of these problems, and its purpose is to reduce fluctuations in the output level relative to the amount of phase shift, and to avoid the problem of oscillation.
Another object of the present invention is to provide a phase shift circuit with a small number of parts.

The configuration of the present invention that achieves this objective is to connect a plurality of resonant circuits including variable capacitance diodes in series and whose resonance frequencies are shifted from each other, and to change the control voltage applied to the variable capacitance diodes. This device is characterized in that it is configured to change the amount of phase shift.

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 2 is a circuit diagram showing an embodiment of the present invention. In the figure, _F1 is a first terminal that receives the input Vin applied to the other end of a capacitor _C10 whose one end is grounded via a capacitor _C11 . Resonant circuit, F ₂ is the first resonant circuit
A second resonant circuit receives the output of _F1 via a capacitor _C12 , and _F3 is a third resonant circuit that receives the output of the second resonant circuit _F2 via a capacitor _C13 . Each resonant circuit F ₁ , F ₂ , F ₃ includes a capacitor C ₀₁ , C ₀₂ , C ₀₃ and a variable capacitance diode D ₁ , respectively.
It is composed of an LC parallel resonant circuit consisting of a series circuit of D ₂ and D ₃ and coils L ₁ , L ₂ , and L ₃ connected in parallel to the series circuit. The gain and phase characteristics of the LC parallel resonant circuit can be shown in FIG. From this figure, it can be seen that this circuit outputs an input signal with a frequency shifted from the resonant frequency with a phase shift. However, since it has the function of a bandpass filter, if it deviates from the resonant frequency, the gain will also change significantly. The individual characteristics of the above-mentioned resonant circuits F ₁ , F ₂ , F ₃ are exactly the same as those shown in Fig. 3, but the resonant frequencies ₁ , ₂ , ₃ of these resonant circuits F ₁ , F ₂ , F ₃ are gradually depending on the change in the control voltage Vc applied to the variable capacitance diodes D ₁ , D ₂ , D ₃ via the resistors R ₁ , R ₂ , R ₃ and configured differently (for example ₁ < ₂ < ₃ ). It is configured to increase or decrease depending on the amount of time. Figure 4a
shows the relationship between the individual gain and phase characteristics of these resonant circuits F ₁ , F ₂ , F ₃ , and the cascade connection circuit of resonant circuits F ₁ , F ₂ , F ₃ obtained by combining these characteristics The fourth one shows the overall gain and phase characteristics.
Figure b. In this overall characteristic diagram, approximately ₁ to
The gain is approximately flat in the frequency band ₃ . Furthermore, the phase variable width increases as the number of resonant circuits increases, and the linearity of phase change also improves.

The output _of the final stage resonant circuit F ₃ is applied to a capacitor C 15 via a capacitor C ₁₄ , and the terminal voltage of the capacitor C ₁₅ is input to an amplifier A. _C20
is output as the output signal Vout of the phase shift circuit. In addition, capacitors C ₁₆ , C ₁₇ and coils
_L4 constitutes a filter, and the power supply voltage E is applied to the amplifier A through this filter. or,
C ₃₀ is a capacitor connected between the control voltage Vc line and ground.

According to such a configuration, the variable capacitance diode
By changing the control voltage Vc applied to _D1 to _D3 , the characteristic curve shown in FIG. 4b can be shifted to the lower frequency side or the higher frequency side. Therefore, if the frequency ₀ of the input Vin is ₂ , then
By changing the control voltage Vc, to the left in Fig. 3b
By moving the characteristic curve _{by about 3-2 ,} the phase of the output Vout can be delayed to _φ1 without reducing the gain. Conversely, by moving to the right by about ₂ - ₁ , the phase of the output Vout can be advanced to _φ2 without reducing the gain. That is, by moving the characteristic curve in the range of ₁ to ₃ , the phase of the output Vout can be changed in the range of -φ ₁ to +φ ₂ , and the output Vout can be changed within that range.
can be kept approximately constant. In addition, in the case of this phase shift circuit, the number of resonant circuits (in the above example, 3
Since it is possible to widen the band where the gain is flat by increasing the number of inputs, it is possible to sufficiently cope with fluctuations in the frequency ₀ of the input Vin. Furthermore, by widening the band in this way, the delay time at a phase difference of 0° can be reduced, and
Even when inputting a gated RF pulse, it is possible to prevent the rise and fall characteristics from being impaired. Furthermore, as is clear from the comparison between FIG. 1 and FIG. 2, the circuit configuration is simpler than the conventional one, so a compact and low-cost phase shift circuit can be obtained. Of course, the problem of oscillation unlike the conventional circuit does not occur.

Note that the configuration of the amplifier A portion in the above embodiment is based on the insertion loss of the resonant circuit _F3 in the previous stage.
sion Loss) or ripple is small. Furthermore, it goes without saying that the number of resonant circuits can be arbitrarily selected, and other configurations can also be selected. For example, an LC series resonant circuit may be used instead of an LC parallel resonant circuit. Figure 5 is similar to Figure 2.
LC series resonant circuit F ₄ instead of LC parallel resonant circuit F ₂
The resonant frequency is selected in the same way as in the previous case, and the same amount of phase shift can be obtained.

As explained above, according to the present invention, it is possible to reduce fluctuations in the output level with respect to the amount of phase shift,
It is possible to realize a phase shift circuit that does not cause the problem of oscillation and has a small number of parts.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing a conventional phase shift circuit.
FIG. 2 is a circuit diagram showing the configuration of an embodiment of the present invention;
Fig. 3 is a gain and phase characteristic curve diagram of an LC parallel resonant circuit, Fig. 4 is an individual and overall gain and phase characteristic curve diagram of three cascade-connected LC parallel resonant circuits, and Fig. 5 is a diagram of the gain and phase characteristic curves of the LC parallel resonant circuit. FIG. 2 is a circuit diagram showing the configuration of an embodiment of the present invention. F ₁ ~ F ₃ ... Resonant circuit, L ₁ ~ _{L 3} ... Coil, D ₁ ~ _{D 3}
…variable capacitance diode, C ₀₁ to C ₀₃ … capacitor.

Claims (1)

[Claims] 1 A resonant circuit including variable capacitance diodes, in which a plurality of resonant circuits whose resonance frequencies are shifted from each other are connected in series, and the amount of phase shift is changed by changing the control voltage applied to the variable capacitance diodes. A phase shift circuit characterized by: