JPS622720Y2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a pulse average frequency discrimination circuit for a medical telemeter used in a receiver demodulator when biological phenomena such as blood pressure and electrocardiogram are remotely measured using an FM modulation telemeter. It is.

In addition to faithfully reproducing low-frequency waveforms, demodulators for medical telemeters are often required to faithfully reproduce absolute values (DC components) as well.

For this reason, a pulse average type frequency discrimination circuit as shown in FIG. 1 has conventionally been used in this type of demodulator. For example, the carrier or subcarrier (Fig. 3a) which is frequency-modulated and transmitted with a blood pressure signal by the transmitter of an invasive telemeter blood pressure monitor and received by the receiver is converted into a pulse by the pulse forming section 1 (Fig. 3b). figure). This is differentiated by the differentiator 2, and the negative part of the trailing edge of the pulse is clipped (Fig. 3c). The differential pulse at the leading edge of the pulse triggers a monostable multivibrator as the pulse generator 3, and a rectangular wave pulse (FIG. 3d) with a constant time width defined by the CR time constant is generated at the output. The frequency of occurrence of this pulse varies depending on the blood pressure signal, and when it is supplied to the low-pass filter 4 and the high frequency component is removed, a low frequency blood pressure signal superimposed on the DC component is reproduced (Figure 3e). .

In this case, the stability of the pulse time width of the monostable multivibrator with respect to temperature using the CR time constant is:
At most, it is about 2 × 10 ^-3 /10°C, and for invasive telemeter blood pressure monitors, for example, stability of absolute value = stability of pulse width / depth of FM modulation = 2 × 10 ^-3
/10℃/5%/100mmHg = 4mmHg/10℃. This greatly affects the measurement density, especially for small blood pressure values.

In view of this point, the present invention aims to further improve the accuracy of measuring biological phenomena by making the time width of the output pulse of the pulse forming section more stable.

Hereinafter, the present invention will be explained based on the illustrated embodiments.

First, to briefly explain the pulse forming section according to the present invention, the differential pulse from the differential circuit 2 (Figure 4a)
is applied to the counting stage 5, the counting stage 5 is reset and starts counting anew the clock signal from the crystal oscillator clock generation stage 6 (FIG. 4b). When the count value reaches a predefined n,
Counting stage 5 supplies a set pulse to pulse generating stage 7, further counts a specified number of clock signals, and when a count value m is reached, supplies a reset pulse. Therefore, the pulse generation stage 7 generates a rectangular wave pulse (4th c
) is generated and supplied to the low-pass filter 4.

More specifically, in this case, the differential pulse and the clock pulse are not synchronized, so if a rectangular wave is generated based on the nearest count pulse, the pulse time width will be modified by a time corresponding to a maximum of one clock interval. Because of this difference, the counting stage 5 is delayed by at least one clock (on the second or later clock signal after reset) so that it can generate the set pulse. This makes it possible to lower the clock frequency and avoid affecting the pulse time width. However, there is an uncertainty time of one clock in the generation time relationship of the rectangular wave pulse based on the clock signal with respect to the differential pulse of the counting stage 5, and this affects the ripple tolerance of the low-pass filter output. The minimum value of the clock frequency is limited. Furthermore, it is convenient to set the time width of the rectangular wave pulse to approximately 50% of the period of the differential pulse when there is no signal, when the output of the pulse generation stage 7 is averaged and taken out as a direct current or low frequency signal.

Furthermore, the reset pulse m must not be generated so late that it becomes heavier when the period of the differential pulse becomes shorter in accordance with the input signal. Based on these factors, in this example, the clock frequency is set to 3M for the subcarrier of 2KHz (the main carrier is 150MHz) of the blood pressure signal from 0 to 30Hz in the invasive wireless telemeter blood pressure monitor.
Set to Hz. In other words, between the differential pulses when there is no modulation,
There are 1500 clock signals, and the counting stage 5 is a binary counter, so by setting n = 256 and m = 1024, there is no pulse width fluctuation, and the ripple is below the allowable value and the pulse width is about 50% short. Shape waves can always be obtained stably.

As is clear from the above explanation, when the clock frequency of the pulse generator according to the present invention is set in consideration of the above factors with respect to the FM modulation carrier frequency of biological phenomena, which is direct current or low frequency, it naturally depends on the circuit configuration. is also determined in a suitable frequency range.
Furthermore, the counting stage 5 and the pulse generating stage 7 can be integrated as a novel IC counter based on a conventional theoretical circuit.

The frequency stability of the crystal oscillator is 1×10 ^-5 /10℃
can be easily obtained, so for the example of the monostable multivibrator with the CR time constant mentioned at the beginning, according to the present invention, the stability of the absolute value is = 1 × 10 ^-5 / 0.05 / 100 mmHg = 0.02 mmHg
/10°C, and the stability of the frequency discriminator is greatly improved.

[Brief explanation of the drawing]

Figure 1 shows a conventional pulse average type frequency discrimination circuit.
2 shows a pulse average type frequency discriminator circuit according to the present invention, FIGS. 3a to 3e show waveforms of various parts of the discriminator circuit according to FIG. 1, and FIGS. 4a to 4c show waveforms of various parts of the discriminator circuit according to FIG. 2. DESCRIPTION OF SYMBOLS 1... Pulse forming section, 2... Differentiating section, 3... Pulse generating section, 4... Low pass filter, 5... Counting stage, 6... Clock generating stage, 7... Pulse generating stage.

Claims (1)

[Scope of utility model registration request] A carrier or subcarrier pulse forming section whose frequency is modulated by a biological signal, a pulse generating section which is started at the pulse edge of the pulse from the pulse forming section and generates a pulse with a constant time width, and a pulse generating section subsequent to the pulse generating section. In a pulse average type frequency discriminator circuit for a medical telemeter having a low-pass filter, a pulse generating section includes a clock generating stage using a crystal oscillator, and is reset at the pulse edge of a pulse from the pulse forming section, and a counting stage for counting clock signals, and the counting stage is characterized in that the counting stage is capable of generating a pulse that is generated at the second and subsequent prescribed clock signals after a reset, and that disappears when a prescribed number of clock signals are further counted. Pulse average type frequency discrimination circuit for medical telemeter.