User Guide

NHM-8NX
System Module & UI PAMS Technical Documentation
Page 34 ãNokia Corporation Issue 1 05/02
Below the earpiece is the PWB, where 4 holes will secure proper leakage to the volume
between the PWB and the internal antenna. However since the PWB doesn't stretch all
the way up to the top of the phone there will also be some natural leakage where the
PWB is missing.
Microphone electrical interface
In NHM-8NX a differential bias circuit, driven directly from the MICB1 bias output with
external RC-filters is chosen. This is a solution that has previously been used with suc-
cess in other phones. The RC filter (220 Ω, 4.7µF) is scaled to provide damping at 217 Hz.
217 Hz audible noise (TDMA) will occur if the bias output MICB1 demodulates in-coming
radio frequencies.
Common DCT4 BB specifies filtering of the reference voltage for the microphone bias
generators. In below figure this filtering is included on the MICBCAP pin.
Besides pure bias purposes also EMC and ESD protection is shown in figure 11. The RC-
filter 2.2 kΩ and 1nF are EMC-component, while the remaining 10 nF and 1 nF capaci-
tors near the bottom connector are for ESD.
The 33nF and 100nF series-capacitors and 12kΩ parallel resistor create a 2'nd order high
pass filter. The input impedance of the gain stage at MIC1P/N is part of the 2'nd stage of
the RC-circuit. The high pass filter is required due to low-frequency noise, which is one
phenomenon identified as a problem when the internal microphone is used as handsfree
microphone (PPH-1/carkit mode).
The microphone bias is controlled in the 8 bit AudioBiasR register.
Figure 9: Internal microphone electrical interface
Ringer
A speaker is used to generate alerting tones and melodies to indicate incoming call, as
well as used to generate game sound, keypress and warning tones for the user
UEM
1k
1k
MIC+
MIC-
4.7uF
MIC1N
MIC1P
MICB1
2*33n
220
2k2
2k2
10n 10n
1n 1n
MICBCAP
1u
1n
12k
2*100n
Placed near
UEM
Placed near
bottom
connector