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.