LT5502

7

The receive signal strength indicator (RSSI) is built into

the IF limiter. The input IF signal is detected in a current

output proportional to the IF input power. The current

outputs from two cascaded stages of IF amplifiers/limiters

are summed and converted into the RSSI voltage. The

RSSI output has an excellent linear range of 90dB. The

characteristic of RSSI output voltage versus input IF

power is independent of temperature and process varia-

tion. The nominal output impedance is 3.8k

Ω. An off-chip

capacitor C7 is needed to reduce the RSSI voltage ripple.

Its value can be determined using the following formula:

C

f

F

IF

7

1

760

≥

π •

I/Q Demodulators

The quadrature demodulators are double balanced mix-

ers, down converting the limited IF signals from the IF

Limiter into I/Q baseband signals. The quadrature LO

carriers are obtained from the internal quadrature LO

carrier generator. The nominal output voltage of differen-

nitudes are well matched, and their phases are 90

° apart.

Quadrature LO Carrier Generator

The quadrature LO carrier generator consists of a divide-

by-two circuit and LO buffers. An input signal (2XLO) with

twice the desired LO carrier frequency is used as the clock

for the divide-by-two circuit, producing the quadrature LO

carriers for the demodulators. The outputs are buffered

and then drive the down converting mixers. With a full

differential approach, the quadrature LO carriers are well

matched.

Integrated Low Pass Filters

The 5th order integrated lowpass filters are used for

filtering the down converted baseband outputs for both

the I-channel and the Q-channel. They serve as anti-

aliasing and pulse-shaping filters. The I/Q filters are well

matched in gain response and group delay. The 3dB

corner frequency is 7.7MHz and the group delay ripple is

16.4ns. The I/Q differential outputs have output driving

capability of 1.5k

Ω with maximum capacitive loading of

Figure 2 shows the simplified output circuit schematic of

I-channel or Q-channnel.

APPLICATIO S I FOR ATIO

The I/Q baseband outputs can be directly DC-coupled to

the inputs of a baseband chip. For AC-coupled applica-

tions with large coupling capacitors, the STBY pin can be

used to prebias the outputs to the desired quiescent

voltage at much reduced current. This mode only draws

2.6mA. When the EN pin is then turned on, the chip is

quickly switched to normal operating mode without long

time constants due to charging or discharging the large

coupling capacitors. Table 2 shows the logic of the EN pin

and STBY pin. In both normal operating mode and standby

mode, the maximum discharging current is about 200

µA,

and the maximum charging current is more than 10mA.

Table 2. The logic of different operating modes

EN

STBY

Comments

Low

Low

Shutdown Mode

Low

High

Standby Mode

High

Low or High

Normal Operation Mode

5502 F02

200

µA

200

µA

+

–

I-CHANNEL

(OR Q-CHANNEL):

DIFFERENTIAL

SIGNALS FROM LPF

+

–

+

–

Figure 2. Simplified Circuit Schematic

of I-Channel (or Q-Channel) Outputs