MC33596FCESLASHR2 datasheet(23/70 Pages) FREESCALE

23 / 70 page

Receive Mode

MC33596 Data Sheet, Rev. 4

Freescale Semiconductor

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At the LNA output, the LNA AGC control voltage is used to monitor input signals in the range

–50 dBm to –20 dBm.

Therefore, the logarithmic amplifier provides information relative to the in-band signal, whereas the LNA

AGC voltage senses the input signal over a wider band.

The RSSI information given by the logarithmic amplifier is available in:

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Analog form on pin RSSIOUT

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Digital form in the four least significant bits of the status register RSSI

The information from the LNA AGC is available in digital form in the four most significant bits of status

The whole content of status register RSSI provides 2 ¥ 4 bits of RSSI information about the incoming

signal (see Section 16.6, “RSSI Register”).

Figure 15 shows a simplified block diagram of the RSSI function.

The quasi peak detector (D1, R1, C1) has a charge time of about 20

μs to avoid sensitivity to spikes.

R2 controls the decay time constant of about 5 ms to allow efficient smoothing of the OOK modulated

signal at low data rates. This time constant is useful in continuous mode when S2 is permanently closed.

To allow high-speed RSSI updating in peak pulse measurement, a discharge circuit (S1) is required to reset

the measured voltage and to allow new peak detection.

Figure 15. RSSI Simplified Block Diagram

S2 is used to sample the RSSI voltage to allow peak pulse measurement (S2 used as sample and hold), or

to allow continuous transparent measurement (S2 continuously closed).

The 4-bit analog-to-digital convertor (ADC) is based on a flash architecture. The conversion time is

× Tdiglck. As a single convertor is used for the two analog signals, the RSSI register content is updated

on a 32

× Tdigclk timebase.

If RSSIE is reset, the whole RSSI module is switched off, reducing the current consumption. The output

buffer connected to RSSIOUT is set to high impedance.

RSSIOUT

ADC

RSSI Register

MSB

LSB

IF Filter Output

LNA AGC Out

MC33596FCE/R2 Datasheet(PDF) 23 Page - Freescale Semiconductor, Inc