RU1826120C - Capacitor microphone modulator circuit - Google Patents

Abstract

Using; in radio engineering when designing radio microphones. SUMMARY OF THE INVENTION: a condenser microphone modulator circuit comprises a high frequency circuit comprising a varicap, a microphone capsule, an inductor, an additional varicap. Simplification and reduction of distortion and noise is achieved due to the fact that the microphone capsule is made in the form of differential capsules with a movable membrane and is connected in series with the varicap. 1 s.p. f-ly, 4 ill.

Description

The invention relates to radio engineering and can be used in the design of radio microphones.

The purpose of the invention is to simplify the design, reduce distortion and noise.

A differential capsule with a charged movable membrane with a fixed capacitance controls the junction capacitance by inducing a charge on the electrodes. Charge induction occurs during the movement between the electrodes of the monoelectret membrane, and this movement is proportional to the sound pressure acting on the transducer.

In FIG. Figure 1 shows a circuit diagram of a modulator (varicap bias circuits are not shown), where Ci is the capacitance of the closed junction of a semiconductor diode (varicap); C is the capacitance of a monoelectrode electroacoustic transducer containing between 2 covers a polymer film, 5-15 microns thick, (polypropylene, F-4MB2), carrying a charge of the same sign; U - inductor forming with the total capacitance

Ci c2

With r g r - the oscillatory system of a high-frequency master oscillator.

In FIG. Figure 2 shows a circuit diagram of a modulator (with the introduction of a second varicap connected in series with the first one), where C i is the capacitance of the second varicap that varies.

In FIG. Figures 3 and 4 show examples of specific embodiments of parallel and series connection of a control capacitance with a monoelectret, respectively, where L is the inductance of the generator circuit;

47

N

Yo

00

u o

N3 O

Weld is the capacity of the varicap: Cm is the control capacity; Ci is the separation capacity; Ca is the bias filter capacity; RCM is the bias resistance; 1 and 2 are points for connecting the circuit to the generator; Ry is leakage resistance.

The device operates as follows.

In the absence of sound pressure from a sound source 10, the total capacitance of the modulator circuit is determined as

Ci-C2 Ci + C2

where Ci is the value of the capacitance of the closed pn junction of the diode with a fixed locking bias of the diode;

C2 is the static capacitance of the electro-acoustic transducer, determined by the area of the plates and the air gap between them.

The frequency generated by the master oscillator is determined by the well-known formula:

1

A variable charge will be induced on the electrodes of capacitance C2, the magnitude and frequency of which changes in accordance with the sound pressure acting on the monoelectric membrane. In this case, the capacitance of the electro-acoustic transducer C2 remains constant, and the capacitance Ci will change in accordance with the capacitance-voltage characteristic of the closed pn junction, changing the resonance frequency of the circuit f0. and therefore the frequency of the generator f.

The proposed circuit of the modulator allows eliminating the microphone amplifier of the condenser microphone and the dynamic range converters necessary in the usual circuit in the low-frequency channel of the transmitter. This makes it possible to increase the signal-to-noise ratio of the radio microphone.

I will take the following in more detail.

1. The steepness of the modulation characteristic of the modulator is determined by the relation .. 1. where ACi change Co

capacitance Ci from an alternating, blocking Rp junction, voltage induced by an electro-acoustic transducer.

5

With

Ci c2

- total capacity of con0

5

0

5

0 5

5

0

0

5

Ci + C2 round of the modulator,

where Ca is the constant structural capacitance of the electro-acoustic transducer.

In the proposed circuit, the modulator Co can be 2–3 times smaller than the capacitance Ci at the initial operating point. Thus, only by reducing C0, the sensitivity of such a modulator to a modulating signal without increasing noise (since C2 is the reactance) can be increased by 6-10 dB.

2. In the prototype, the noise voltage of the low frequency path reduced to the input of the modulator is about 0.6 mV. The signal to noise ratio is 70 dB.

Thus, in the proposed modulator, a signal-to-noise ratio of at least 76-80 dB should be expected.

Consider the overload capacity of the modulator with the proposed circuit design.

The maximum sound pressure levels expected from real sound sources at working distances from the microphone can be estimated at 120 dB, which corresponds to a sound pressure of 20 Pa.

If the sensitivity of the electro-acoustic transducer is of the order of 100 mV / Pa, the magnitude of the induced voltage on the electrodes of the capacitance Ca can be estimated as 2 V. The same modulating voltage would be required from a conventional (in prototype) low-frequency path, for example, in a KMS-radio transmitter 29 and Bauerdynamics to provide maximum frequency deviation of 12 kHz with a non-linear distortion of 1-1.5%. Moreover, the dynamic range converters used in the transmitters of these radio microphones distort in some cases the spectrum of the transmitted signals, which was repeatedly noted by experts.

The proposed modulator circuit allows the signal spectrum to be transmitted without distortion with an increased dynamic range.

When a monoelectrode converter is turned on between two controlled capacitances (Fig. 2), the total capacitance of the circuit of the modulator Co turns out to be less than in the modulator according to claim 1: the changes in each controlled capacitance D C add up, which provides a larger ratio

ACi

those. sensitivity increases

With modulator.

Such inclusion of a monoelectrode converter also makes it possible to compensate for the nonlinearity of the characteristics of the varicaps and thereby reduce the nonlinear distortion of the modulator.

In addition, the proposed modulator circuit makes it possible to improve profitability and reliability, as well as reduce the overall dimensions of the device as a whole due to the absence of a low frequency path.

Claims (2)

1. The circuit of the modulator of the condenser microphone, containing high-frequency 0 5 a circuit that includes a varicap, a microphone capsule, an offset and isolation circuit that is connected to a varicap, characterized in that, in order to simplify and reduce distortion and noise, the microphone capsule is made in the form of a differential capsule with a movable membrane and connected in series with the varicap while one of the fixed electrodes of the differential capsule with a movable membrane is connected to the varicap. 2. The condenser microphone modulator circuit according to claim 1, characterized in that, in order to increase the sensitivity, an additional varicap is connected in series with the varicap. ZVig - D /, AND