Motorola TALKABOUT T2688, Service Manual

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Topaz T2688/T200, T2988/T205 L3 Circuit Description
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48. These 2 paths are then summed to give a single IF modulated signal, which is then
amplified.
49. From here the output current used to drive the TXVCO will be generated. The
feedback from the TXVCO (DCS or GSM) is mixed with the RXVCO frequency to
produce a reference frequency of 270Mhz. This is then mixed with the modulated IF
with the phase comparator giving an output proportional to the difference between the
phase vales of the 2 frequencies.
50. This output current is fed out on TX / RX IC Pin 11 ( PLLOUT ). R614 is used to
control the current out from PLLOUT .
51. The VCO driving current is then passed through the loop filter made up of R626 and
R627, C645 and C647 . ( C647 is a polyester capacitor used to ensure low noise). The
purpose of the loop filter is to prevent any overshoots due to channel changes, being
transmitted to the TXVCO. The TXVCO driving voltage is between 0.8V – 2.0V.
52. The drive current is fed into the T XVCO U606 on Pin 6 , with the TX frequency
being fed out on Pin 1 . The voltage TX2V8 , which is switched by the signal
TXVCO_OFF , supports the TXVCO. The signal BS Pin 5 controls whether the
TXVCO produces a DCS or GSM output.
53. As the TXVCO produces both GSM900 or DCS1800 frequencies from the same
output line, the signal must be fed into a
diplexer U605 , this circuit uses inductive
properties to effectively delay 1 frequency compared to the other and therefore split
the paths, between the GSM and DCS frequencies.
54. The output frequency then takes 2 paths:
55. Path 1: The output signal is attenuated through R615-R622 , coupled through C631
and C632 and fed into the TX /RX IC on Pin 8 (DCS) and Pin 9 (GSM). This will
then be used as the feedback for the TXVCO Phase lock loop as discussed in point
49.
56. Path 2: The signal is output as TX_GSM or TX_DCS to the PA.
57. The PA U201 is a Dual package three stage PA that is split into the GSM and DCS
part. The supply for the PA is VBAT 3.6V(depends on battery charge level) on Pin 6
(front stage) and Pin 8 (back stage).
58. Band selection is done with BS (Band Select). DCS high and GSM low level.
Signal inputs on pin 2 and 4 from the TXVCO diplexer U605 are TXDCS and
TXGSM. Amplified outputs are Pins 10 and 12 .