RFM119W Datasheet, PDF (12/19 Pages) List of Unclassifed Manufacturers – Low-Cost Consumer Electronics Applications

W
RFM119W/RFM119SW
5.5 PA Ramping
When the PA is switched on or off quickly, its changing input impedance momentarily disturbs the VCO output frequency. This
process is called VCO pulling, and it manifests as spectral splatter or spurs in the output spectrum around the desired carrier
frequency. By gradually ramping the PA on and off, PA transient spurs are minimized. The RFM119W/S has built-in PA
ramping configurability with options of 0, 1, 2, 4, 8, 16, 32, 64, 128, 256, 512 and 1024 us, as shown in Figure 13. These
options are only available when the modulation type is OOK. When the option is set to “0”, the PA output power will ramp up
to its configured value in the shortest possible time. The ramp down time is identical to the ramp up time in the same
configuration.
HOPERF recommends that the maximum symbol rate should be no higher than 1/2 of the PA ramping “rate”, as shown in the
formula below.
SR_Max ≤ 0.5 * (
1
t_RAMP
)
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 formula below.
1
t_RAMP
≤
0.5
*
(
)
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 “AN122 CMT2113/19A Configuration Guideline”.
Logic 1
0 us
1 us
2 us
4 us
8 us
512 us
1024 us
Logic 0
Figure 13. PA Ramping Time
Time
Time
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