SA604A_15 Datasheet, PDF (17/31 Pages) NXP Semiconductors – High-performance low-power FM IF system

NXP Semiconductors
SA604A
High-performance low-power FM IF system
V_O

2Q
1
--^----
₁
=
2
Q₁


_{-----1----}+_---1-_----_{---}
(6)
For
2
Q₁
--^----
₁
<<
_--
2
which is discriminated FM output. Note that

is the
deviation
frequency from the carrier ₁ ¹. Example: at 455 kHz IF, with 5 kHz FM deviation, the
maximum normalized frequency is (455  5)/455 kHz = 1.010 or 0.990.
Go to the frequency as a function of normalized frequency curves (Figure 5) and draw a
vertical straight line at
--^----
₁
= 1.01.
The curves with Q = 100, Q = 40 are not linear, but Q = 20 and less shows better linearity
for this application. Too small Q decreases the amplitude of the discriminated FM signal.
Equation 6 => Choose a Q = 20.
The internal resistance of the SA604A is 40 k From Q₁ = RC_P + C_S₁ , and then
₁
=
---------------¹----------------- , it results that C_P + C_S = 174 pF and L = 0.7 mH.
LC_P + C_S
A more exact analysis including the source resistance of the previous stage shows a
series and a parallel resonance in the phase detector tank. To make the parallel and
series resonances close, and to get maximum attenuation of higher harmonics at
455 kHz IF, a C_S = 10 pF and C_P = 164 pF provided the best results. For commercial
purposes, values of 150 pF or 180 pF may be practical. A variable inductor which can be
adjusted around 0.7 mH should be chosen and optimized for minimum distortion. (For
10.7 MHz, a value of C_S = 1 pF is recommended.)
13.6 Audio outputs
Two audio outputs are provided. Both are PNP current-to-voltage converters with 55 k
nominal internal loads. The unmuted output is always active to permit the use of signaling
tones in systems such as cellular radio. The other output can be muted with 70 dB typical
attenuation. The two outputs have an internal 180 phase difference.
The nominal frequency response of the audio outputs is 300 kHz. This response can be
increased with the addition of external resistors between the output pins and ground. The
resistors are placed in parallel with the internal 55 k resistors and they lower the output
time constant. The output structure is a current-to-voltage converter where current is
driven into the resistance, creating a voltage drop. By adding external parallel resistance,
it also lowers the output audio amplitude and DC level.
This technique of audio bandwidth expansion can be effective in many applications such
as SCA receivers and data transceivers. Because the two outputs have a 180 phase
relationship, FSK demodulation can be accomplished by applying the two outputs
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 shift in
opposite directions (higher or lower output voltage) as the input frequency shifts. The
1. Ref. Krauss, Raab, Bastian: Solid-State radio Eng.; Wiley, 1980, p.311.
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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