TRF6901

SINGLECHIP RF TRANSCEIVER

SLWS110G − SEPTEMBER 2001 − REVISED JUNE 2004

10

POST OFFICE BOX 655303

detailed description

low-noise amplifier (LNA)/RF mixer

The LNA has differential inputs. The off-chip input matching network has the dual task of matching a 50-

Ω

connector (or antenna, switch, filter, etc.) to the differential inputs and providing a 180-degree phase shift

between the inputs at terminals 1 and 2. The differential input impedance of the LNA is approximately 500

Ω

in parallel with 0.7 pF. The predicted noise figure of the LNA and input matching circuit is 2.5 dB. The cascaded

noise figure for the LNA/mixer is listed in the specifications.

The mixer offers good linearity (high IP3). An external matching network is required to transform the output

impedance of the mixer (1.4 k

Ω) to the input impedance of the IF filter (typically 330 Ω).

0.5

1

2

5

10

2

1

0.5

0.2

0

−0.5

−1

−2

−5

5

↑1 U

START 850 MHz

STOP 1000 MHz

CH1 S11

CAL

OFS

CPL

FIL

1k

Figure 3. Typical LNA Input Impedance (S11) at Device Terminals LNA_IN1,2

IF amplifier/limiter

The IF amplifier has differential inputs to its first stage. The limiting amplifier provides 68 dB of gain. An external

impedance matching network is required between the IF filter output and the IF amplifier inputs at terminals 43

and 44.

RSSI

The received signal strength indicator (RSSI) voltage at terminal 41 is proportional to the log of the

down-converted RF signal at the IF limiting amplifier input. The RSSI circuit is temperature compensated and

is useful for detecting interfering signals, transceiver handshaking, and RF channel selection. In some

applications it can be used as a demodulator for amplitude-shift\keying (ASK) or on-off keying (OOK)

modulation.