SA604A Datasheet, PDF (11/13 Pages) NXP Semiconductors – High performance low power FM IF system

Philips Semiconductors
High performance low power FM IF system
Product specification
SA604A
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 5.1kΩ resistor from the output of the
first limiter (Pin 14) to the input of the interstage filter. With this
resistor from Pin 14 to the filter, sensitivity of 0.25µV for 12dB
SINAD was achieved. With the 3.6kΩ resistor, sensitivity was
optimized at 0.22µV for 12dB SINAD with minor change in the RSSI
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
91kΩ resistor, the output characteristic is 0.5V for a 10dB change in
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
Φ
175
150
125
100
75
50
25
0
0.95
Q = 20
Q = 10
0.975
Q = 100
Q = 80
Q = 60
1.0
1.025
1.05
Figure 12.
Phase vs Normalized IF Frequency
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SR00322
1997 Nov 07
11