AN1500A South African Micro-Electronic Systems, AN1500A Datasheet - Page 14

• Manufacturer Part Number: AN1500A
• Description: Single Chip Telephone Demo Board
• Manufacturer: South African Micro-Electronic Systems

14.2 Frequency response shaping: Receive
The receive signal is applied at the Rx amplifier input, RI (#28) and STB (#6) and is available at the differential
outputs RO1 and RO2.
14.2.1 Receive gain setting:
Rx gain can be set by both the sensitivity of the earpiece and the value of R25.
14.2.2 Rx frequency shaping
Receive frequency shaping is done by C15 (low pass) at the Rx amplifier output. C17 impedes the DC path through
RO1, RO2.
To find the correct values, a RLR (receive loudness ratings) measurement with the specific handset used in the
customer’s application must be made.
14.2.3 Metering pulses filtering
If filtering of metering pulses (typ. 12 or 16kHz ) is required (for example in Germany) two blocking filters can be
installed:
LBF1 , CBF1 as notch filter at the line input and
LBF2 ,CBF2 as notch filter at the Rx amplifier input
Note:
since CBF1 is connected at the line side, it must be a .../250V capacitor and LBF1 must be able to drive the
maximum line current ( 100mA) without saturation.
LBF2 and CBF2 however, can be low voltage, low current components .
The application AN1500, shown with two resonators, gives adequate attenuation of the metering pulses signal. The
benefit of this application is, that only one filter (LBF1,CBF1) is used on the high voltage/high current side, while the
second notch filter (LBF2,CBF2) is connected at a low voltage/low current node, enabling use of
components.
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Fig. 7: Rx signal path
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