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6-4
STH-N375
Circuit Description & Circuit Diagrams
1-3. IF INTERFACE CHIP (IFC: U101)
The IFC provides the radio IF and audio interfaces (Figure2-2). Differential Rx l/Q and Tx l/Q signal pairs
interface directly with the RF interface chip (RFIC: U301) device. MOD, AFC, AGC, RSSI, and PA CNT
signals also directly control the radio circuitry. Three audio paths are implemented. The primary path
utilizes the microphone and speaker, located on the main board. Both microphone and speaker utilize the
main differential circuits in the IFC to minimize noise pickup. The speaker output is amplified internally to
enable direct connection to the speaker. The second audio path connects to the headset jack. The headset
microphone and speaker are passed to the IFC’s auxiliary input and output, respectively. The headset jack
terminals indicate via the HEADSET SENSE line when the headset is present as well as when the headset
button is pushed via the SEND END line. The third audio path to the HFK shares the auxiliary output with
the headset for its speaker and uses the second auxiliary input for its microphone. Since the auxiliary
output is not internally amplified, an external audio amplifier is applied before connection to the headset
and HFK.
Figure 2. IFC interface and block diagram