Philips Semiconductors

Product specification

SA615

High performance low power mixer FM IF system

1997 Nov 07

5

CIRCUIT DESCRIPTION

The SA615 is an IF signal processing system suitable for second IF

or single conversion systems with input frequency as high as 1GHz.

The bandwidth of the IF amplifier is about 40MHz, with 39.7dB(v) of

gain from a 50

Ω source. The bandwidth of the limiter is about

28MHz with about 62.5dB(v) of gain from a 50

Ω source. However,

the gain/bandwidth distribution is optimized for 455kHz, 1.5k

Ω

source applications. The overall system is well-suited to battery

operation as well as high performance and high quality products of

all types.

The input stage is a Gilbert cell mixer with oscillator. Typical mixer

characteristics include a noise figure of 5dB, conversion gain of

13dB, and input third-order intercept of –10dBm. The oscillator will

operate in excess of 1GHz in L/C tank configurations. Hartley or

Colpitts circuits can be used up to 100MHz for xtal configurations.

Butler oscillators are recommended for xtal configurations up to

150MHz.

The output of the mixer is internally loaded with a 1.5k

Ω resistor

permitting direct connection to a 455kHz ceramic filter. The input

resistance of the limiting IF amplifiers is also 1.5k

Ω. With most

455kHz ceramic filters and many crystal filters, no impedance

matching network is necessary. To achieve optimum linearity of the

log signal strength indicator, there must be a 12dB(v) insertion loss

between the first and second IF stages. If the IF filter or interstage

network does not cause 12dB(v) insertion loss, a fixed or variable

resistor can be added between the first IF output (Pin 16) and the

interstage network.

The signal from the second limiting amplifier goes to a Gilbert cell

quadrature detector. One port of the Gilbert cell is internally driven

by the IF. The other output of the IF is AC-coupled to a tuned

quadrature network. This signal, which now has a 90

relationship to the internal signal, drives the other port of the

multiplier cell.

Overall, the IF section has a gain of 90dB. For operation at

intermediate frequencies greater than 455kHz, special care must be

given to layout, termination, and interstage loss to avoid instability.

The demodulated output of the quadrature detector is available at

two pins, one continuous and one with a mute switch. Signal

attenuation with the mute activated is greater than 60dB. The mute

input is very high impedance and is compatible with CMOS or TTL

levels.

A log signal strength completes the circuitry. The output range is

greater than 90dB and is temperature compensated. This log signal

strength indicator exceeds the criteria for AMPs or TACs cellular

telephone.