SA604A_15 Datasheet, PDF (15/31 Pages) NXP Semiconductors – High-performance low-power FM IF system

NXP Semiconductors
SA604A
High-performance low-power FM IF system
In some products, it may be impractical to utilize shielding, but this mechanism may be
appropriate to 10.7 MHz and 21.4 MHz IF. One of the benefits of low current is lower
radiated field strength, but lower does not mean non-existent. A spectrum analyzer with
an active probe clearly shows IF energy with the probe held in the proximity of the second
limiter output or quadrature coil. No specific recommendations are provided, but
mechanical shielding should be considered if layout, bypass, and input impedance
reduction do not solve a stubborn instability.
The final stability consideration is phase shift. The phase shift of the limiters is very low,
but there is phase shift contribution from the quadrature tank and the filters. Most filters
demonstrate a large phase shift across their passband (especially at the edges). If the
quadrature detector is tuned to the edge of the filter passband, the combined filter and
quadrature phase shift can aggravate stability. It is not usually a problem, but should be
kept in mind.
13.4 Quadrature detector
Figure 9 shows an equivalent circuit of the SA604A quadrature detector. It is a multiplier
cell similar to a mixer stage. Instead of mixing two different frequencies, it mixes two
signals of common frequency but different phase. Internal to the device, a constant
amplitude (limited) signal is differentially applied to the lower port of the multiplier. The
same signal is applied single-ended to an external capacitor at pin LIMITER_OUTPUT.
There is a 90 phase shift across the plates of this capacitor. The phase shifted signal
applied to the upper port of the multiplier is at pin QUADRATURE_INPUT. A quadrature tank
(parallel L/C network) permits frequency selective phase shifting at the IF frequency. This
quadrature tank must be returned to ground through a DC blocking capacitor.
The loaded Q of the quadrature tank impacts three fundamental aspects of the detector:
distortion, maximum modulated peak deviation, and audio output amplitude. Typical
quadrature curves are illustrated in Figure 5. The phase angle translates to a shift in the
multiplier output voltage.
Thus a small deviation gives a large output with a high Q tank. However, as the deviation
from resonance increases, the non-linearity of the curve increases (distortion). With too
much deviation, the signal is outside the quadrature region (limiting the peak deviation
which can be demodulated). If the same peak deviation is applied to a lower Q tank, the
deviation remains in a region of the curve which is more linear (less distortion). However,
it creates a smaller phase angle (smaller output amplitude). Thus the Q of the quadrature
tank must be tailored to the design. Basic equations and an example for determining Q
are shown in Section 13.5. This explanation includes first-order effects only.
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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