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SA624

High performance low power FM IF system with high-speed RSSI

Philips Electronics 

Philips Semiconductors

High performance low power FM IF system with

high-speed RSSI

Product specification

SA624

will require careful attention to limiter interstage component

selection. This will be especially true with high IF frequencies which

require insertion loss or impedance reduction for stability.

At low frequencies the RSSI makes an excellent logarithmic AC

voltmeter.

For data applications the RSSI is effective as an amplitude shift

keyed (ASK) data slicer. If a comparator is applied to the RSSI and

the threshold set slightly above the no signal level, when an in-band

signal is received the comparator will be sliced. Unlike FSK

demodulation, the maximum data rate is somewhat limited. An

internal capacitor limits the RSSI frequency response to about

100kHz. At high data rates the rise and fall times will not be

symmetrical.

The RSSI output is a current-to-voltage converter similar to the

audio outputs. However, an external resistor is required. With a

91kΩ resistor, the output characteristic is 0.5V for a 10dB change in

the input amplitude.

Additional Circuitry

Internal to the SA624 are voltage and current regulators which have

been temperature compensated to maintain the performance of the

device over a wide temperature range. These regulators are not

accessible to the user.

200

Φ

175

150

125

100

75

50

25

0

0.95

Q = 20

Q = 10

Q = 100

Q = 80

Q = 60

0.975

1.0

1.025

ω

∆ω

Figure 12. Phase vs Normalized IF Frequency ω1 = 1 + ω1

1.05

SR00451

1997 Nov 07

11

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