RFM119SW datasheet(12/19 Pages) HOPE

12 / 19 page

RFM119W/RFM119SW

Rev 1.0 | Page 12 /19

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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.

SRMax ≤ 0.5 * (

)

tRAMP

In which the PA ramping “rate” is given by (1/tRAMP). In other words, by knowing the maximum symbol rate in the application,

the PA ramping time can be calculated by formula below.

tRAMP ≤ 0.5 * (

)

SRMAX

The user can select one of the values of the tRAMP in the available options that meet the above requirement. If somehow the

tRAMP is set to be longer than “0.5 * (1/SRMax)”, it will possibly bring additional challenges to the OOK demodulation of the Rx

device. For more detail of calculating tRAMP, please refer to “AN122 CMT2113/19A Configuration Guideline”.

0 us

1 us

2 us

4 us

8 us

512 us

1024 us

Time

Logic 1

Logic 0

Time

Figure 13. PA Ramping Time

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