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SA604 Datasheet(PDF) 10 Page - NXP Semiconductors |
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SA604 Datasheet(HTML) 10 Page - NXP Semiconductors |
10 / 13 page Philips Semiconductors Product specification SA604A High performance low power FM IF system 1997 Nov 07 10 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 VOUT SR00321 Figure 11. VO = CS CP + CS 1 + ω1 S + Q1S 2 () 1 ω1 VIN (1a) L(CP + CS) where ω1 = 1 (1b) Q1 = R (CP + CS) ω1 (1c) From the above equation, the phase shift between nodes 1 and 2, or the phase across CS will be: φ = ∠V O - ∠VIN = (2) tg-1 ω1 ω – Q1ω 2 () ω1 1 Figure 12 is the plot of φ vs. ω () ω1 It is notable that at ω = ω1, the phase shift is π 2 and the response is close to a straight line with a slope of ∆φ ∆ω = ω 1 2Q1 The signal VO would have a phase shift of – ω 1 2Q1 ω π 2 with respect to the VIN. Sin (3) ω If VIN = A Sin ωt ⇒ VO = A ωt + π 2 – ω 1 2Q1 Multiplying the two signals in the mixer, and low pass filtering yields: Sin (4) ω VIN • VO = A2 Sin ωt ωt + π 2 – ω 1 2Q1 after low pass filtering Cos (5) ω ⇒ V OUT = π 2 – ω 1 2Q1 1 2 A2 = 1 2 A2 Sin ω ω 1 2Q1 () (6) VOUT ∝ 2Q1 2Q1 ω 1 ω 1 + ∆ω = ω ω 1 () For 2Q1ω ω 1 << π 2 Which is discriminated FM output. (Note that ∆ω is the deviation frequency from the carrier ω 1. 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 455 = 1.010 or 0.990 Go to the f vs. normalized frequency curves (Figure 12) and draw a vertical straight line at = 1.01. ω ω 1 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 CP + CS = 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 CS = 10pF and CP = 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 CS = 1pF is recommended.) Audio Outputs Two audio outputs are provided. Both are PNP current-to-voltage converters with 55k Ω 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 |
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