9

PLUTO

INITIALISATION

Transmit

On power-up or reset (RESETB) the Tx reconstruction

filters are tuned to give the specified cut-off frequency. This

calibration is internal and requires no external input. The

calibration time is 1ms.

Receive

On power-up or reset (RESETB) an autocalibration algo-

rithm is started which can be used to tune the programmable

filters in Jupiter. (Jupiter is a programmable active filter de-

signed for use in dual mode CDMA/AMPS system -further

details of which can be found in the Jupiter Datasheet). The

autocalibration is also initiated when Pluto is switched into

CDMA mode via FMB control.

When Pluto enters calibration mode the En Test (pin 68)

goes high. A test signal at 364kHz is then generated at the

Vtest output (pin 67). This signal is input to Jupiter which

provides a response which is digitised by the I and Q Rx ADCs.

An output DAC - BAL - (Pin 71) then tunes the Q channel to

match the I and Q channel amplitude via a successive approxi-

mation routine. The test signal is then switched to 728kHz

which is above the required cut off of the filter. DAC outputs,

FC_I and FC_Q are then adjusted to tune the I and Q filters to

the correct amplitude with reference to the in band test signal.

The filter cut off is tuned to 690kHz. Oversampling in the Rx

ADC's ensures sufficient accuracy for the calibration. This

calibration routine takes 26ms and after completion En Test

goes low and the test signal Vtest is disabled. Only the CDMA

filter is tuned, the matching within Jupiter ensures that the FM

(AMPS) filter performance meets specification.

0

-0.8

-4

Relative

Amplitude

(dB)

-5.8

1k

630k

1.25M

10M

Frequency (Hz)

CDMA TX FILTER RESPONSE

0

-0.6

Relative

Amplitude

(dB)

-3.0

10k

29k

100k

Frequency (Hz)

FM TX FILTER RESPONSE

Figure 9 Baseband RX interface circuit