MKH 20...70
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).
Summary of Contents for MKH 20
Page 16: ...MKH 20 P 48 16 53 04 2005 MKH 20 circuit diagram ...
Page 22: ...MKH 30 P 48 22 53 04 2005 MKH 30 circuit diagram ...
Page 28: ...MKH 40 P 48 28 53 04 2005 MKH 40 circuit diagram ...
Page 31: ...MKH 50 P 48 04 2005 31 53 Frequency response Equalization Polar diagram ...
Page 34: ...MKH 50 P 48 34 53 04 2005 MKH 50 circuit diagram ...
Page 37: ...MKH 60 1 P 48 04 2005 37 53 Frequency response Equalization Polar diagram ...
Page 41: ...MKH 60 1 P 48 04 2005 41 53 MKH 60 circuit diagram ...
Page 44: ...MKH 60 1 P 48 44 53 04 2005 ...
Page 47: ...MKH 70 1 P 48 04 2005 47 53 Frequency response Equalization Polar diagram ...