CC1120 Output Power Calculation and Smart RF Studio

Jeffrey Wilczewski

Prodigy 50 points

Other Parts Discussed in Thread: CC1120

According to the datasheet for the CC1120

Output_Power = ((PA_POWER_RAMP+1)/2)-18

So for example, for an output power of 10 dBm according to the formula

10 dBm : PA_POWER_RAMP = 55

However, the value generated in SmartRF Studio (1.7.0) shows for 10 dBm TX power PA_POWER_RAMP = 0x34 (52 decimal).

This holds true for almost all TX power values, the SmartRF value and the datasheet algorithm value differ.

Any help describing this difference would be greatly appreciated.

over 13 years ago

0 Martin B over 13 years ago

TI__Genius 12735 points

Hi Jeffrey

The formula is a theroretical formula, hoverever lab measurements of the CC1120 has shown better output than the formula gives.

SmartRF Studio uses a lookup table for the PA_POWER_RAMP values to give a better approximation of the output power you should expect from the radio.

0 Jeffrey Wilczewski over 13 years ago in reply to Martin B

Prodigy 50 points

Thank you for the clarification.

For reference, I modeled PA_CFG2.PA_POWER_RAMP and PA_CF0.ASK_DEPTH values (these are the register values that would change when modifying TX power) from SmartRF and came up with a formula that matches the SmartRF values in case someone else is looking for a method to determine these values. The formula holds true for TX Power from -11 to +14 dBm.

pa_cfg2.pa_power_ramp = round( 2.3173*tx_pwr_dbm + 28.909 )

where tx_pwr_dbm is your desired TX Power (-11..14)

pa_cfg0.ask_depth = floor( pa_cfg2.pa_power_ramp/4 )

0 Kraftfahrzeughaftpflichtversicherung over 8 years ago in reply to Jeffrey Wilczewski

Prodigy 200 points

Is this also the reason why the CC1120 datasheet says +16dBm output power but the maximum value through the formula is 14dBm?
ex.
PA_POWER_RAMP max = 0x3F (63)
Output Power = ((63 + 1) / 2) - 18dBm
= 14 dBm

0 TER over 8 years ago in reply to Kraftfahrzeughaftpflichtversicherung

TI__Guru**** 317180 points

Yes, as Martin points out the equation is a theoretical one, the values in the datasheet and SmartRF Studio are measured.

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CC1120 Output Power Calculation and Smart RF Studio