Handbook for Sound Engineers

512 Chapter 16

16.3.4.3.1 Capacitor Radio-Frequency Microphones

A capacitor microphone of somewhat different design,
manufactured by Sennheiser and also employing a
crystal-controlled oscillator, is shown in Fig. 16-49. In
the conventional capacitor microphone (without an
oscillator) the input impedance of the preamplifier is in
the order of 100 M: so it is necessary to place the
capacitor head and preamplifier in close proximity. In
the Sennheiser microphone, the capacitive element
(head) used with the RF circuitry is a much lower
impedance since the effect of a small change in capaci-
tance at radio frequencies is considerably greater than at
audio frequencies. Instead of the capacitor head being
subjected to a high dc polarizing potential, the head is
subjected to an RF voltage of only a few volts. An
external power supply of 12 Vdc is required.

Referring to Fig. 16-49, the output voltage of the
8 MHz oscillator is periodically switched by diodes D 1
and D 2 to capacitor C. The switching phase is shifted
90° from that of the oscillator by means of loose

coupling and individually aligning the resonance of the

microphone circuit M under a no-sound condition. As a

result, the voltage across capacitor C is zero. When a

sound impinges on the diaphragm, the switching phase

changes proportionally to the sound pressure, and a

corresponding audio voltage appears across capacitor C.

The output of the switching diodes is directly connected

to the transistor amplifier stage, whose gain is limited to

12 dB by the use of negative feedback.

A high Q oscillator circuit is used to eliminate the

effects of RF oscillator noise as noise in an oscillatory

circuit is inversely proportional to the Q of the circuit.

Because of the high Q of the crystal and its stability,

compensating circuits are not required, resulting in low

internal noise.

The output stage is actually an impedance-matching

transformer adjusted for 100: , for a load impedance of

2000 : or greater. RF chokes are connected in the

output circuit to prevent RF interference and also to

prevent external RF fields from being induced into the

microphone circuitry.

16.3.4.3.2 Symmetrical Push-Pull Transducer

Microphone

Investigations on the linearity of condenser micro-

phones customarily used in the recording studios was

carried out by Sennheiser using the difference frequency

method incorporating a twin tone signal, Fig. 16-50. This

is a very reliable test method as the harmonic distortions

of both loudspeakers that generate the test sounds sepa-

rately do not disturb the test result. Thus, difference

frequency signals arising at the microphone output are

arising from nonlinearities of the microphone itself.

Fig. 16-51 shows the distortion characteristics of

eight unidirectional studio condenser microphones

which were stimulated by two sounds of 104 dB SPL

(3 Pa). The frequency difference was fixed to 70 Hz

while the twin tone signal was swept through the upper

audio range. The curves show that unwanted difference

frequency signals of considerable levels were generated

by all examined microphones. Although the curves are

shaped rather individually, there is a general tendency

Figure 16-48. Basic circuit for the Schoeps radio-frequency
capacitor microphone, series CMT.

Figure 16-49. Basic circuit for the Sennheiser model 105,
405, and 805 capacitor microphones. Courtesy Sennheiser
Electronic Corporation.

Oscillator

Capsule

Phase

demodulator

Capacitor

diode

Modulation

output

9 Vdc^4

AFC +9 Vdc 1

8 MHz

Crystal-controlled

oscillator

Feed-

back

D 1

D 2

Q 1 Q 2

+

+

M

Figure 16-50. Difference frequency test. Courtesy

Sennheiser Electronic Corporation.

f^1

f

2 = Const.

f 2

f (^1) ff^1
2
f 1 f 2 f 1 f 2