CMT2157AW
Rev 0.8 | Page 16/31
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In which the PA ramping “rate” is given by (1/t
RAMP
). In other words, by knowing the maximum symbol rate in the application,
the PA ramping time can be calculated by:
t
RAMP
≤
0.5 * (
1
SR
MAX
)
The user can select one of the values of the t
RAMP
in the available options that meet the above requirement. If somehow the
t
RAMP
is set to be longer than “0.5 * (1/SR
Max
)”, it will possibly bring additional challenges to the OOK demodulation of the Rx
device. For more detail of calculating t
RAMP
, please refer to “AN142 CMT2157A Configuration Guideline”.
Time
RFO Amplitude
Time
Data
Logic 1
Logic 0
1024 us
512 us
8 us
4 us
2 us
1 us
0 us
Figure 13. PA Ramping Time
5.6 Working States
The CMT2157AW has following 4 different working states: SLEEP, XO-STARTUP, TUNE and TRANSMIT. The device stays in
the SLEEP state when no transmission is performed. Once the button(s) is/are pressed, the device goes through the sequence
of SLEEP
XO-STARTUP
TUNE
TRANSMIT to transmit the data. After the transmission the device goes back to the
SLEEP state. When the device works in the periodic transmission mode, the device periodically wakes up from the SLEEP
state, goes the same sequence, performs the transmission and goes back to the SLEEP state. All the details of push button(s)
function and periodic transmission can be referred to “AN142 CMT2157A Configuration Guideline”.
SLEEP
When the CMT2157AW is in the SLEEP state, all the internal blocks are turned off and the current consumption is minimized to
20 nA typically.
XO-STARTUP
Once the CMT2157AW received the valid control signal, it will go into the XO-STARTUP state, and the internal XO starts to
work. The user has to wait for the t
XTAL
to allow the XO to get stable. The t
XTAL
is to a large degree crystal dependent. A typical
value of t
XTAL
is provided in the Table 11.
TUNE
The frequency synthesizer will tune the CMT2157AW to the desired frequency in the time t
TUNE
. The PA can be turned on to
transmit the data generated by the embedded encoder only after the TUNE state is done.
TRANSMIT
The CMT2157AW starts to modulate and transmit the data. The data packets being transmitted are generated by the
embedded encoder, and they are determined by the encoder selected, the button mode and the button being pressed.
