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14.1 Frequency response shaping: Transmit

Fig. 6: Tx signal path

Transmit frequency response shaping is performed by 3 resistors for gain setting and 3..4 capacitors for frequency

shaping:

14.1.1 Microphone gain setting

The electret handset microphone is supplied from the constant voltage at LI (#27), filtered by R20 and

C14 (see fig 6). R21 and R23 are the bias resistors of the electret microphone. Transmit gain is set by both

selecting the proper microphone type (sensitivity) and by varying R21 & R23. Reasonable values for these resistors

are in the range of 1...2.5k

Ω (each), which results in a gain adjustment range in the order of 6..8dB.

14.1.2 Tx frequency shaping:

Transmit frequency shaping can be done by C11,C13 (high pass) and C12( low pass). R22 can be installed to

attenuate the microphone signal without affecting the frequeny response curve,

M1(#23) and M2 (#24)are differential inputs of the microphone amplifier. Please note the proper connection of the

M1 and M2 inputs in respect to the positive and negative side of the microphone.

To find the correct values, a SLR (sending loudness ratings) measurement with the specific handset used in the

customer’s application must be made.

14 Further Adjustments