SA604A Datasheet, PDF (10/13 Pages) NXP Semiconductors – High performance low power FM IF system

Philips Semiconductors
High performance low power FM IF system
Product specification
SA604A
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), and, with too much
deviation, the signal will be 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 will remain in a
region of the curve which is more linear (less distortion), but 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 below. This explanation
includes first-order effects only.
Frequency Discriminator Design Equations for
SA604A
V_OUT
Figure 11.
(1a)
V_O
=
C_S
C_P + C_S
1
( ) ^ω1
ω₁ 2
1 + _Q1S + _S
V_IN
where ω₁ =
1
(1b)
L(C_P + C_S)
Q₁ = R (C_P + C_S) ω₁
(1c)
SR00321
From the above equation, the phase shift between nodes 1 and 2, or
the phase across C_S will be:
ω₁
(2)
φ = ∠V_O - ∠V_IN = _tg-1
Q₁ω
1
–
(
ω₁
ω
)²
( ) Figure 12 is the plot of φ vs. ω
ω₁
It is notable that at ω = ω₁, the phase shift is
π _{and the response is close to a straight}
2
line with a slope of
∆φ
∆ω
=
2Q₁
ω₁
The signal V_O would have a phase shift of
π
2
–
2Q₁
ω₁
ω
with respect to the V_IN.
If V_IN = A Sin ωt ⇒ V_O = A
(3)
Sin
ωt +
π
2
–
2Q₁
ω₁
ω
Multiplying the two signals in the mixer, and
low pass filtering yields:
V_IN • V_O = A² Sin ωt
(4)
Sin
ωt +
π
2
–
2Q₁
ω₁
ω
after low pass filtering
⇒ V_OUT =
1
2
A²
Cos
π
2
–
2Q₁
ω₁
ω
(5)
( ) ⁼
1
2
A²
Sin
2Q₁
ω₁
ω
( ) ^VOUT
∝
2Q₁
ω
ω₁
=
2Q₁
ω₁ + ∆ω
ω₁
(6)
For
2Q₁ω
ω₁
<<
π
2
Which is discriminated FM output. (Note that ∆ω is the deviation
frequency from the carrier ω₁.
Ref. Krauss, Raab, Bastian; Solid State Radio Eng.; Wiley, 1980, p.
311. Example: At 455kHz IF, with +5kHz FM deviation. The
maximum normalized frequency will be
455 +5kHz
= 1.010 or 0.990
455
Go to the f vs. normalized frequency curves (Figure 12) and draw a
vertical straight line at
ω
ω₁
=
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. (Eq. 6) ⇒ Choose a
Q = 20
The internal R of the 604A is 40k. From Eq. 1c, and then 1b, it
results that
C_P + C_S = 174pF and L = 0.7mH.
A more exact analysis including the source resistance of the
previous stage shows that there is 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 455kHz IF, we have found that a C_S = 10pF and C_P =
164pF (commercial values of 150pF or 180pF may be practical), will
give the best results. A variable inductor which can be adjusted
around 0.7mH should be chosen and optimized for minimum
distortion. (For 10.7MHz, a value of C_S = 1pF is recommended.)
Audio Outputs
Two audio outputs are provided. Both are PNP current-to-voltage
converters with 55kΩ 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 70dB typical
attenuation. The two outputs have an internal 180° phase
difference.
The nominal frequency response of the audio outputs is 300kHz.
this response can be increased with the addition of external
resistors from the output pins to ground in parallel with the internal
55k resistors, thus lowering the output time constant. Singe the
output structure is a current-to-voltage converter (current is driven
into the resistance, creating a voltage drop), adding external parallel
resistance also has the effect of lowering 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 output
1997 Nov 07
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