SA604A_15 Datasheet, PDF (12/31 Pages) NXP Semiconductors – High-performance low-power FM IF system

NXP Semiconductors
SA604A
High-performance low-power FM IF system
13.2 IF amplifiers
The IF amplifier section consists of two log-limiting stages. The first consists of two
differential amplifiers with 39 dB of gain and a small signal bandwidth of 41 MHz (when
driven from a 50  source). The output of the first limiter is a low-impedance emitter
follower with 1 k of equivalent series resistance. The second limiting stage consists of
three differential amplifiers with a gain of 62 dB and a small signal AC bandwidth of
28 MHz. The outputs of the final differential stage are buffered to the internal quadrature
detector. One of the outputs is available at pin LIMITER_OUTPUT to drive an external
quadrature capacitor and L/C quadrature tank.
Both of the limiting amplifier stages are DC biased using feedback. The buffered output of
the final differential amplifier is fed back to the input through 42 kresistors. As shown in
Figure 4, the input impedance is established for each stage by tapping one of the
feedback resistors 1.6 k from the input. It requires one additional decoupling capacitor
from the tap point to ground.
Because of the very high gain, bandwidth and input impedance of the limiters, there is a
very real potential for instability at IF frequencies above 455 kHz. The basic phenomenon
is shown in Figure 10. Distributed feedback (capacitance, inductance and radiated fields)
forms a divider from the output of the limiters back to the inputs (including RF input). If this
feedback divider does not cause attenuation greater than the gain of the forward path,
then oscillation or low-level regeneration is likely. If regeneration occurs, two symptoms
may be present:
1. The RSSI output is high with no signal input (should nominally be 250 mV or lower).
2. The demodulated output demonstrates a threshold. Above a certain input level, the
limited signal begins to dominate the regeneration, and the demodulator begins to
operate in a normal manner.
There are three primary ways to deal with regeneration:
1. Minimize the feedback by gain stage isolation.
2. Lower the stage input impedances, thus increasing the feedback attenuation factor.
3. Reduce the gain. Gain reduction can effectively be accomplished by adding
attenuation between stages, which can also lower the input impedance. Examples of
impedance/gain adjustment are shown in Figure 11. Reduced gain results in reduced
limiting sensitivity.
A feature of the SA604A IF amplifiers, which is not specified, is low phase shift. The
SA604A is fabricated with a 10 GHz process with very small collector capacitance. It is
advantageous in some applications that the phase shift changes only a few degrees over
a wide range of signal input amplitudes.
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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