Sennheiser MKH 40-P48 User Manual download pdf (Page 6)

MKH 20...70

6/53 04/2005

Circuit description MKH20...70

RF bridge circuit (capsule bridge)

In connection with the windings 1-2 and 3-4 of the RF coil L1 the push-pull capsule

forms a bridge circuit. Each displacement of the diaphragm by a sound signal

changes the two capsule capacitances in the respective opposite direction,

resulting in a proportional RF voltage at the bridge output 2/3. L3 compensates the

reactive component of the bridge output impedance, so that the bridge output

becomes a real and low-resistance output.

Using the coil core of L1 the bridge is balanced by changing the winding

inductances in the respective opposite directions, so that the bridge output voltage

becomes zero when the diaphragm is in its neutral position. This is the case when

the direct voltage between the measuring points A and B becomes zero (balance).

The bridge output signal is transformed by the RF coil L2 and decoupled from

ground. Its primary winding 13-14 is roughly tuned to the oscillator frequency by

C1. A fine-tuning with the core of L2 is not required, since the circuit is highly

attenuated by the low bridge impedance. For optimum coupling, the core should be

positioned in the center of the coil.

Together with the tapping 16/17 of the secondary winding 15-18 the switch

makes it possible to achieve a signal attenuation of 10dB (pre-attenuation).

The demodulator and oscillator circuits of the microphones MKH20...50 and

MKH60...70 only differ with regard to different component references. In the

following sections, data that exclusively apply to the MKH60...70 are put in

parentheses.

Demodulator

Via L2 the RF bridge signal is supplied to the demodulator with D1, D2, and C5, C6

(or C3, C5) and the symmetrical windings 5-6 and 7-8 of L1. The demodulator

operates as a synchronous rectifier putting through the RF signal of L2 to C6 (or C5)

during one half-wave, while blocking it during the other half-wave. The rectified

voltage at C5 (or C3) biases both D1 and D2 to such an extent that they will securely

block even signals with a higher intensity. The rectified voltage is stabilized with

D3, D4 (or R6, R9).

When zero balancing is correct, the direct voltage between the measuring points A

and B reaches zero, and half the rectified voltage is supplied to C6 (or C5).

Depending on the direction of diaphragm displacement the bridge output voltage

is either in phase or opposite in phase to the voltages at the diode windings 5-8.

The synchronous rectification results in an increase or decrease of the voltage at C6

(or C5) corresponding to the diaphragm movement.

Oscillator

The frequency of the RF oscillator results from the capsule capacitances and the

inductances of the L1 windings 1-2 and 3-4 and ranges from about 6.5 to 7.5MHz.

The collector winding 9-10 and the feedback winding 11-12 couple the oscillator

transistor T1 (or Q1) to the capsule circuit. R1 prevents generation of parasitic os-

cillations in the VHF range. C2, C3, C4 (or C2, C4, C10) are used for RF blocking, L4

(or L5) acts as an RF choke.

The operating current of the microphone flows through the oscillator and is

stabilized to 2mA by T2 (or Q2), R3, R4, and R5 (constant-current circuit). The NTC

resistor R5 stabilizes against temperature influences. R2 ensures a sufficiently high

collector voltage of T2 (or Q2), since the base-emitter voltage of T1 (or Q1) is

negative (Class C mode).

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Sennheiser MKH 40-P48 User Manual Page 6

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