TRF6900
SINGLE-CHIP RF TRANSCEIVER
SLAS213C – SEPTEMBER 1999 – REVISED MAY 2000
received signal strength indicator (RSSI)
The received signal strength indicator provides a voltage at terminal 33, RSSI_OUT, that is proportional to the
RF limiter input level. The slope of the RSSI circuit is typically 19 mV/dB over a frequency range of 10 MHz to
21.4 MHz. Because of its ultrafast response time (typically 1 µs per –20 dBm to off step), the RSSI can easily
beusedasanamplitude-shiftkeying(ASK)oron/offkeying(OOK)demodulatorfordataratesupto100kBit/sec.
FM/FSK demodulator
The demodulator is intended for analog (FM) and digital (FSK) frequency demodulation. It consists of a
quadrature demodulator with an external LC tank circuit. A variable inductor, internal to the TRF6900, operates
in parallel with the external tank circuit (see Figure 13), and is used to adjust the external tank circuit’s resonant
frequency. If the tolerances of the external demodulator tank circuit components can provide a maximum
frequency error of less than 5%, then no additional adjustments are required. As long as the device is in the
learning mode, the internal reactance automatically fine-adjusts the resonant frequency of the external LC tank
circuit. Depending on the supply voltage, the tank circuit tuning range is approximately four times the
discriminator 3-dB bandwidth.
While in the learning mode i.e, during a dc-free learning sequence of 0,1,0,1,0,...., the initial tolerances of the
LC demodulator tank circuit are compensated and an external capacitor (connected to terminal 29, S&H_CAP)
is charged to a dc voltage that is proportional to the average demodulation dc level. This level establishes the
decision threshold voltage and consequently sets the zero reference for the data slicer to generate the logical
levels of the data sequence that follow the learning sequence. Therefore, the user can use a non-dc-free data
signal.
The demodulator will be automatically activated if the limiter (x_LIM) and low-pass filter amplifier (x_LPF) are
activated and the data switch is set to FSK/FM reception (x_SW = 0).
data switch
The TRF6900 incorporates an internal data switch used to select the input signal for the low-pass filter
amplifier/post detection amplifier. Depending on the settings in the Mode0 or Mode1 enable registers (C-word,
D-word), the user can select between OOK/ASK or FSK baseband processing without having to change
external components.
low-pass filter amplifier/post-detection amplifier
The low-pass filter amplifier/post-detection amplifier is configured to operate as a current-to-voltage amplifier
and may be used to realize a low pass filter for post detection. The low-pass amplifier bandwidth may be
adjusted according to noise and signal bandwidth requirements. An internal 10 pF capacitor sets the maximum
–3 dB corner frequency to approximately 0.75 MHz (see Figures 11 and 12).
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POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
TI [ TEXAS INSTRUMENTS ]
