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SA58641

High performance mixer FM IF system with high-speed RSSI

NXP Semiconductors. 

NXP Semiconductors

SA58641

High performance mixer FM IF system with high-speed RSSI

7. Functional description

The SA58641 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 38 dB of gain from a 50  source. The bandwidth of the

limiter is about 28 MHz with about 54 dB of gain from a 50  source. However, the

gain/bandwidth distribution is optimized for 10.7 MHz, 330  source applications. The

overall system is well-suited to battery operation as well as high performance and high

quality products of all types such as cordless and cellular hand-held phones.

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

a noise figure of 14 dB, conversion gain of 11 dB, and input third-order intercept of

16 dBm. The oscillator will operate 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 330  resistor permitting direct

connection to a 10.7 MHz ceramic filter for narrowband applications. The input resistance

of the limiting IF amplifiers is also 330 . With most 10.7 MHz ceramic filters and many

crystal filters, no impedance matching network is necessary. For applications requiring

wideband IF filtering, such as DECT, external LC filters are used (see Figure 16). To

achieve optimum linearity of the log signal strength indicator, there must be a 6 dBV

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

does not cause 6 dBV insertion loss, a fixed or variable resistor can be added between

the first IF output (IF_AMP_OUT) 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 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 frequency at

10.7 MHz. Special care must be given to layout, termination, and interstage loss to avoid

instability.

The demodulated output (DATA_OUT) of the quadrature is a voltage output. This output is

designed to handle a minimum bandwidth of 600 kHz. This is designed to demodulate

wideband data, such as in DECT applications.

A Received Signal Strength Indicator (RSSI) 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, DECT and RCR-28 cordless

telephone. 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.

Remark: dBV = 20log VO/VI.

SA58641

Product data sheet

All information provided in this document is subject to legal disclaimers.

Rev. 2 — 9 March 2012

© NXP B.V. 2012. All rights reserved.

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