S2-LPS datasheet(27/73 Pages) STMICROELECTRONICS

27 / 73 page

4.4.5.2

Automatic gain control

The automatic gain control (AGC for short) algorithm is designed to keep the signal amplitude at the input of the

IF ADC within a specific range by controlling the gain of the RF chain in 6 dB steps, up to a maximum attenuation

of 48 dB, starting at a received signal power of about -50 dBm.

From an implementation point of view, the (peak) signal amplitude is measured in the digital domain after the

primary decimation filters chain and compared to a low threshold and to a high threshold. If the amplitude is

above the high threshold, the attenuation is increased sequentially until the amplitude goes below the threshold; if

the amplitude is below the low threshold, the attenuation is decreased sequentially until the amplitude goes above

the threshold.

The AGC algorithm is controlled by the following parameters:

High threshold: this value sets the digital signal level above which the RF attenuation is increased (AGCCTRL1

Low thresholds: this allows a better tuning of the low thresholds in case the analog attenuation steps

have a significant spread around the nominal 6 dB attenuation. The threshold actually used for each step

is selected through the bits of the LOW_THRESHOLD_SEL (AGCCTRL3) register. The recommended setting

for Low Threshold 0 is 0x5, recommended setting for Low Threshold 1 is 0x4. The recommended value for

LOW_THRESHOLD_SEL is 0x10.

Measure time: this parameter sets the measurement interval during which the signal level is monitored before

the AGC attenuation is decreased. In particular, if the signal level is below the low threshold for all the duration of

such period, then the attenuation is decreased.

The actual time is

TAGC meas= 12fdig∙2MEAS_TIME,rangingfromabout0.5µstoabout15ms.

For frequency modulation, the measurement time is normally set to a few µs in order to achieve fast settling of the

algorithm.

For amplitude, to avoid an unstable behavior, the measure time must be larger than the duration of the longest

train of ‘0’ symbols expected during the preamble/synchronization word.

The default value for such parameter is 0x2.

Hold time: this parameter sets a wait time for the algorithm to let the signal level to settle after a change in the

attenuation level.

The actual time is

TAGC ℎold= 12fdig∙HOLD_TIME,rangingfromabout0.5µstoabout32µs.

The recommended setting for such parameter is 0x0C.

AGC enable: enables the AGC algorithm.

Freeze on sync: freeze the AGC level after when the sync word has been received.

The AGC algorithm works in the following way:

•

If the amplitude is above the high threshold, the attenuation is increased sequentially until the amplitude

goes below the threshold. In this case the Tmeas is set to 0 and only the hold time is used. Thus, if the signal

is above the high threshold, the AGC word is changed each Thold seconds.

•

If the amplitude is below the low threshold, the attenuation is decreased sequentially until the amplitude

goes above the threshold. In this case the Tmeas is set to the value: Thold = (12*2MEAS_TIME)/fdig. Thus, if

the signal is below the low threshold, the AGC word is changed each Tmeas + Thold seconds.

The two operations are repeated in a loop until the input signal strength is between the high and the low

threshold.

4.4.5.3

Symbol timing recovery

The S2-LPS supports two different algorithms for the timing recovery. The selection of the algorithm is done with

the register CLOCK_REC_ALGO_SEL.

•

If CLOCK_REC_ALGO_SEL = 0, then a simple first order algorithm is used (shortly referred to as DLL).

•

If CLOCK_REC_ALGO_SEL = 1, then a second order algorithm is used (shortly referred to as PLL).

Besides the configuration parameters mentioned above, the setting of the following registers also affects the

behavior of the clock recovery algorithms.

•

Post-filter length: this parameter controls the length of the demodulator post-filter (CLOCKREC register).

Setting this value to 1B may improve demodulation performance but requires a slower recovery. The

recommended value for such parameter is 0B.

S2-LPS

Digital modulator

DS13831 - Rev 1

page 27/73

S2-LPS bảng dữ liệu(PDF) 27 Page - STMicroelectronics

S2-LPS bảng dữ liệu(HTML) 27 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73

bảng dữ liệu tải về

Link URL