SA604A_15 Datasheet, PDF (18/31 Pages) NXP Semiconductors – High-performance low-power FM IF system

NXP Semiconductors
SA604A
High-performance low-power FM IF system
output of the comparator is logic output. The choice of op amp or comparator depends on
the data rate. With high IF frequency (10 MHz and above), and wide IF bandwidth (L/C
filters) data rates in excess of 4 MBd are possible.
13.7 RSSI
The Received Signal Strength Indicator (RSSI), of the SA604A demonstrates monotonic
logarithmic output over a range of 90 dB. The signal strength output is derived from the
summed stage currents in the limiting amplifiers. It is independent of the IF frequency.
Thus, unfiltered signals at the limiter inputs, spurious products, or regenerated signals
manifest themselves as RSSI outputs. An RSSI output of greater than 250 mV with no
signal (or a very small signal) applied, is an indication of possible regeneration or
oscillation.
To achieve optimum RSSI linearity, there must be a 12 dB insertion loss between the first
and second limiting amplifiers. With a typical 455 kHz ceramic filter, there is a nominal
4 dB insertion loss in the filter. An additional 6 dB 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 6),
the optimum linearity was achieved with a 5.1 k resistor. The resistor was placed
between the output of the first limiter (pin IF_AMP_OUTPUT) and the input of the
interstage filter. With this resistor from pin IF_AMP_OUTPUT to the filter, sensitivity of
0.25 V for 12 dB SINAD was achieved. With the 3.6 k resistor, sensitivity was
optimized at 0.22 V for 12 dB SINAD with minor change in the RSSI linearity.
Any application requiring optimized RSSI linearity, such as spectrum analyzers, cellular
radio, and certain types of telemetry, requires careful attention to limiter interstage
component selection. This is 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 is sliced. Unlike FSK
demodulation, the maximum data rate is limited. An internal capacitor limits the RSSI
frequency response to approximately 100 kHz. At high data rates, the rise and fall times
are not symmetrical.
The RSSI output is a current-to-voltage converter similar to the audio outputs. However,
an external resistor is required. With a 91 k resistor, the output characteristic is 0.5 V for
a 10 dB change in the input amplitude.
13.8 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.
SA604A
Product data sheet
All information provided in this document is subject to legal disclaimers.
Rev. 4.1 — 30 March 2015
© NXP Semiconductors N.V. 2015. All rights reserved.
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