User Guide

PAMS
Technical Documentation
NSE–8/9
System Module
Page 2– 67
Issue 1 07/99
4. Conversion:
The Cobba GJP ASIC converts the analog audio signal to a PCM
bitstream which is supplied to the DSP for further speech proces-
sing. The selection between which inputs to use is handled by the
MCU through control registers inside Cobba
5. EMC protection:
The Low pass filter mad by R231, C257 and R232, C256 improves
the immunity towards high frequency signals picked up by the audio
accessory cable. C209 makes the GSM burst signal level picked up
by the cable to common mode signals on the input, which are very
well suppressed by the differential amplifier in the cobba, if it’s not
attenuated by the LP filter made by R218, C215 and R217, C216 to
suppress the remaining HF signals.
ESD protection is made by V207 and R231, R232.
6. HDC–5 HOOK detect:
The transistor V202 (pins 2,3,4) makes together with R212, R225
and R226 a simple level converter, to condition the Hook signal
(generated in the HDC–5 by shorting the mic terminal) to the valid
input range of the MAD2PR1 HookDet input. In the sw is implement-
ed a delay in order not to generate un intended Hook signals due to
hook switch bouncing or blowing in the HDC–5 microphone. Hook
signal is not implemented in PPH–1.
7. PPH–1 mute:
Since the pull–down in AUXOUT is app. 5Kohm when AUXOUT is
set to 0 volt V202 (pins 1,5,6) is used to pull down the dc–level
when activated by PData(4) from Cobba. This is used to signal to
the PPH–1 unit to mute it’s up– and downlink– signals.
Down link signal path
The external down link audio circuitry consists of basically 3 blocks:
1. The cobba ASIC:
Which converts the PCM bitstream from the speech processor in
the DSP to analog signals.
2. The DC separation:
C201 and C203 separates the DC bias levels needed by the Cobba
output stage from the external audio dc–levels. R227 and R228 is
used to separate the capacitive load of the EMC filter capacitors
from the output stage of the Cobba in order not to cause instability.
3. EMC suppression:
L200 and L201 prevents the RF radiated by the antenna or pickled
up by the accessory cable from getting demodulated by the HDC–5
earpiece in the low impedance output of the Cobba. Together with
C235 and C236 the filter improves the immunity of the accessory
cable towards CE related test signals. Additionally are added 27pF
capacitors to remove GSM signals coupled to the wires in the PCB
from the cobba output to the bottom connector. ESD protection is
made by V207