SA605

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© NXP Semiconductors N.V. 2014. All rights reserved.

Product data sheet

Rev. 5 — 14 November 2014

12 of 25

NXP Semiconductors

SA605

High performance low power mixer FM IF system

11.1 Circuit description

The SA605 is an IF signal processing system suitable for second IF or single conversion

systems with input frequency as high as 1 GHz. The bandwidth of the IF amplifier is about

40 MHz, with 39.7 dB of gain from a 50

 source. The bandwidth of the limiter is about

28 MHz with about 62.5 dB of gain from a 50

 source. However, the gain/bandwidth

distribution is optimized for 455 kHz, 1.5 k

 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 5 dB, conversion gain of 13 dB, and input third-order intercept of

10 dBm. The oscillator operates in excess of 1 GHz in L/C tank configurations. Hartley or

Colpitts circuits can be used up to 100 MHz for crystal configurations. Butler oscillators

are recommended for crystal configurations up to 150 MHz.

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

 resistor, permitting direct

connection to a 455 kHz ceramic filter. The input resistance of the limiting IF amplifiers is

also 1.5 k

. With most 455 kHz 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 12 dBV insertion loss between the first and second IF stages. If

the IF filter or inter-stage network does not cause 12 dBV insertion loss, a fixed or variable

resistor can be added between the first IF output (pin 16, IF_AMP_OUT) and the

inter-stage 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

 phase

relationship to the internal signal, drives the other port of the multiplier cell.

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

greater than 455 kHz, special care must be given to layout, termination, and inter-stage

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 60 dB. 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 90 dB and is

temperature compensated. This log signal strength indicator exceeds the criteria for

AMPS or TACS cellular telephone.