Philips Semiconductors

Product specification

SA604A

High performance low power FM IF system

1997 Nov 07

11

differentially across the inputs of an op amp or comparator. Once

the threshold of the reference frequency (or “no-signal” condition)

has been established, the two outputs will shift in opposite directions

(higher or lower output voltage) as the input frequency shifts. The

output of the comparator will be logic output. The choice of op amp

or comparator will depend on the data rate. With high IF frequency

(10MHz and above), and wide IF bandwidth (L/C filters) data rates in

excess of 4Mbaud are possible.

RSSI

The “received signal strength indicator”, or RSSI, of the SA604A

demonstrates monotonic logarithmic output over a range of 90dB.

The signal strength output is derived from the summed stage

currents in the limiting amplifiers. It is essentially independent of the

IF frequency. Thus, unfiltered signals at the limiter inputs, spurious

products, or regenerated signals will manifest themselves as RSSI

outputs. An RSSI output of greater than 250mV with no signal (or a

very small signal) applied, is an indication of possible regeneration

or oscillation.

In order to achieve optimum RSSI linearity, there must be a 12dB

insertion loss between the first and second limiting amplifiers. With

a typical 455kHz ceramic filter, there is a nominal 4dB insertion loss

in the filter. An additional 6dB is lost in the interface between the

filter and the input of the second limiter. A small amount of

additional loss must be introduced with a typical ceramic filter. In the

test circuit used for cellular radio applications (Figure 5) the optimum

linearity was achieved with a 5.1k

first limiter (Pin 14) to the input of the interstage filter. With this

resistor from Pin 14 to the filter, sensitivity of 0.25

SINAD was achieved. With the 3.6k

optimized at 0.22

linearity.

Any application which requires optimized RSSI linearity, such as

spectrum analyzers, cellular radio, and certain types of telemetry,

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

91k

the input amplitude.

Additional Circuitry

Internal to the SA604A 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

175

150

125

100

75

50

25

0

0.95

0.975

1.0

1.025

1.05

Φ

Q = 100

Q = 80

Q = 60

Q = 20

Q = 10

SR00322

Figure 12. Phase vs Normalized IF Frequency w

w

1

+ 1 ) Dw

w

1