SA676

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© NXP B.V. 2012. All rights reserved.

Product data sheet

Rev. 3 — 19 July 2012

4 of 22

NXP Semiconductors

SA676

Low-voltage mixer FM IF system

7.

Functional description

The SA676 is an IF signal processing system suitable for second IF systems with input

frequency as high as 100 MHz. The bandwidth of the IF amplifier and limiter is at least

2 MHz with 90 dB of gain. 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 7.0 dB, conversion gain of 17 dB, and input third-order intercept of

10 dBm. The oscillator will operate in excess of 100 MHz in L/C tank configurations.

Hartley or Colpitts circuits can be used up to 100 MHz for crystal configurations.

The output impedance of the mixer is 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. The IF amplifier has 44 dB of gain and 5.5 MHz bandwidth. The IF limiter has

58 dB of gain and 4.5 MHz bandwidth.

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 interstage network

does not cause 12 dBV insertion loss, a fixed or variable resistor or an L pad for

simultaneous loss and impedance matching can be added between the first IF output

(IF_AMP_OUT) and the interstage network. The overall gain will then be 90 dB with

2 MHz bandwidth.

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.

The demodulated output of the quadrature drives an internal op amp. This op amp can be

configured as a unity gain buffer, or for simultaneous gain, filtering, and second-order

temperature compensation if needed. It can drive an AC load as low as 10 k

 with a

rail-to-rail output.

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

70 dB and is temperature compensated. This signal drives an internal op amp. The

op amp is capable of rail-to-rail output. It can be used for gain, filtering, or second-order

temperature compensation of the RSSI, if needed.