2

Circuit Description

The LT5502 consists of an IF limiting amplifier with 84dB small signal gain,

quadrature-converting mixers, low pass filters, and an RSSI section. Figure 1 shows the

LT5502 block diagram. In operation, an IF signal is limited by high gain amplifiers, and is

then demodulated into in-phase (I) and quadrature (Q) baseband signals using quadrature

local oscillator (LO) carriers that are generated on-chip from an external 2xLO signal. The

demodulated I/Q baseband signals are passed through fully integrated 5th-order low pass

filters and output drivers. Demo board DC333A has additional output drivers, which provide

50-ohm output impedance.

The LT5502 has a 4dB noise figure and an achievable sensitivity of –86 dBm. Figure

2 shows the baseband I/Q output voltage swing versus IF input power at an IF frequency of

280MHz. The receive signal strength indicator (RSSI) is built into the IF limiter, and offers a

linear IF signal detection range of 90 dB.

I/Q Output Swing, RSSI Ouput

vs IF Input Power

200

400

600

800

1000

-85

-70

-55

-40

-25

-10

5

IF Input Power (dBm)

0.2

0.4

0.6

0.8

1

1.2

Figure 2. Baseband I/Q differential output voltage swing and RSSI output versus IF input

power. (IF Frequency = 280MHz, with a 1:4 IF input transformer, and without IF interstage

filtering.)

The quadrature demodulators are double-balanced mixers. The quadrature LO carriers are

obtained from an on-chip divide-by-two circuit. For this reason, the external 2XLO signal must

be twice the LO frequency.

The lowpass filters on the I and Q channels serve for anti-aliasing and pulse-shaping. The 3-dB

corner frequency is 7.7MHz, and the group delay ripple is less than 17ns. The filter

characteristics are stable over the temperature range from -40°C to +85°C.