MC33596_10 Datasheet, PDF (23/70 Pages) Freescale Semiconductor, Inc – PLL Tuned UHF Receiver for Data Transfer Applications

Receive Mode
• 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:
• Analog form on pin RSSIOUT
• 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
register RSSI.
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.
IF Filter Output
LNA AGC Out
RSSI Register
ADC
MSB
LSB
S2
D1 R1
Σ
C1 R2
S1
RSSIOUT
C2
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
16 × T_diglck. As a single convertor is used for the two analog signals, the RSSI register content is updated
on a 32 × T_digclk 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.
MC33596 Data Sheet, Rev. 4
Freescale Semiconductor
23