Philips Semiconductors

Product specification

SA626

Low voltage high performance mixer FM IF system

with high-speed RSSI

1997 Sept 25

7

50.00

48.00

46.00

44.00

42.00

40.00

38.00

36.00

34.00

32.00

30.00

TEMPERATURE (

°C)

65.00

63.00

61.00

59.00

57.00

55.00

53.00

51.00

49.00

47.00

45.00

TEMPERATURE (

°C)

SA626 IF Amplifier Gain vs Temperature vs Supply Voltage

SA626 Limiting Amplifier Gain vs Temperature

vs Supply Voltage

VCC = 3V

VCC = 2.7V

VCC = 5V

VCC = 3V

VCC = 2.7V

VCC = 5V

SR00474

Figure 5. Performance Characteristics

CIRCUIT DESCRIPTION

The SA626 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 38dB of

power gain from a 50

Ω source. The bandwidth of the limiter is about

28MHz with about 54dB of power gain from a 50

Ω source.

However, the gain/bandwidth distribution is optimized for 10.7MHz,

330

Ω 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 14dB, conversion power

gain of 11dB, and input third-order intercept of -16dBm. 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 330

Ω resistor

permitting direct connection to a 10.7MHz ceramic filter. The input

resistance of the limiting IF amplifiers is also 330

Ω. With most

10.7MHz 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 3dB insertion loss

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

network does not cause 3dB 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 power gain of 92dB. For operation at

intermediate frequency at 10.7MHz. Special care must be given to

layout, termination, and interstage loss to avoid instability.

The demodulated output of the quadrature drives an internal op

amp. This op amp is configured as a unity gain buffer. It can drive

an AC load as low as 5k

Ω with a rail-to-rail output.

A log signal strength indicator 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, 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 2nd-order temperature

compensation of the RSSI, if needed.