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The Mikroport wireless microphone system

As Mikroport is a portable radio system, in many countries special licensing regulations for operating the system have to be observed. In the Bundesrepublik Deutschland the notification to the "Deutsche Bundespost" is required. For this purpose a form of application attached to the instruc-

tion manual has to be filled in and se nt to the competent local Oberpost- direktion.

Therewith all formalities are fulfilled.

A. Description and mode of operation

1. Microphones

The microphones specially developed for the Mikraport equipment, i. e. the hand microphone MD 405 S, as standard type, and the very small and unobservable types "fountain-pen microphone" MM 61/2 and "button-hole microphone" MM 23/2 that are meant for the transmission of speech only, must not be replaced by other microphones in Germany with regard to postal regulations. Other types for special purposes can however be delivered on request.

The dynamic hand microphone MD 405 S with cardioid characteristic is equipped with an on/off sliding switch for the transmitter. The magnetic microphones MM 61/2 and MM 23/2 with omni-directional characteristic, however, have no switch due to their tiny dimensions.

The illustration on page 1 shows the circuit of the different microphones. When the microphone switch shall have the funktion of a muting button, the free core in the microphone switch is to be soldered on to contact 4, and a bridge is to be made between contact 1 and contact 2. The transmitter can then no Ionger be switched on or off by the microphone switch.

2. Miniature transmitter SK 1006

This transmitter is built up with a printed circuit. Being fully transistorized, it requires only two commercial batteries for feeding, which have a life of approx. 6 to 7 hours of operation.

The unit consists of a three-stage modulation amplifier and a three-stage HF transmitter. The function of the individual stages is shown in the block diagram on page 2.

The audio frequency from the micraphone is amplified, then preemphasized and applied to the gain contral (frequency deviation adjustment). The two following amplifier stages are equipped with an automatic gain control (dynamic compressionJ, so that the frequency deviation of the transmitter is limited to a maximum of :t 75 kc/s. The modulation is achieved directly in the transistor oscillator. To ensure good frequency stability of the transmitter, the oscillator frequency is doubled to the output frequency, then amplified

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to the required power level for transmission. The operating voltage of the transmitter is electronically stabilized. The transmitter has two channels of equal performance normally with frequencies of 36.7 and 37.1 megacps alternatively to be selected by a switch.

Note: In the Bundesrepublik Deutschland solely the two transmission frequencies of 36,7 and 37.1 megacps are licensed by the Deutsche Bundes- post. For use abroad and in case of a fixed order however the channels can be adjusted to two other frequencies in the range between 25 and 45 megacps. The frequency interval mayamount from 0.3 megacps minimum to 1 megacps maximum. This readjustment can be performed exclusively by the manufacturer. Subsequently the receiver has to be provided for these frequencies, too, which requires a corresponding modification.

3. Receiver T 201

The receiver contains an input- and mixer-stage, a two-stage intermediate frequency section, a ratio detector, a two-stage audio frequency section, and a two-stage direct-coupled amplifier for electronic noise suppression. The input stage is equipped with the pentode EF 184 with reinforced grid and extraordinary slope. This grants best sensitivity and minimum noise level. The receiver can be switched to two operating frequencies selec- tively. An automatic resetting section equipped with the silicon diode BA 100 serves for exact tuning to the respective transmission frequency.

The intermediate frequency section delivers a constant output voltage to the ratio detector when the radio frequency voltage at the aerial input exceeds a threshold value of 5 microvolts. By that means the audio frequency output voltage remains constant, too. When the adjusted value of the aerial voltage falls below (2 microvolts... 10mvolts), the electronic noise suppression stage cuts off the units connected to the output of the receiver. The audio frequency voltage is obtained from the cathode circuit of an impedance converter stage earthfree and balanced by means of a coupling transformer. The output level of + 6 dB (1.55 volts) corresponds to the value normally used in public adress techniques. Because of the low source impedance of 30 ohms any normal control or amplifier may be connected to the output of the receiver. The audio monitoring of the performance can be carried out by means of the built-in controllable monitoring loudspeaker which remains in operation also when the electronic noise suppression stage has cut off the public adress system.

Occasionally performances in rooms may be disturbed even within the transmission range of the transmitter as a result of minimum values of field intensity caused by unfavourable conditions. To avoid these disturbances it was provided to assemble two or more receivers for diversity operation. By that the Mikroport system is outstanding by its good reliabi-

lity of operation and immunity from interferences even at disadvantageous conditions.

By means of the built-in automatic switching device moreover a tape recorder can be switched on or off via the transmitter.

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