A negative feedback phase shift oscillator block diagram is shown to the right T

Question

A negative feedback phase shift oscillator block diagram is shown to the right The Gain and Phase transfer function of blocks 1, 2, 3 and 4 are identical and are shown below What frequency (i.e. f_1, f_2, f_3, or f_4) would the oscillator operate at _____ and why (1/1) b. If R_1 + R_lb = 100k Ohm what would the value of R_1 _____ and R_lb be _____ (1/1) An FM signal source (50volts rms) is sinusoidally modulated by a 30KHz source, Use the Bessel table and chart as an aid: a. If Delta F_C PK = 150 kHz, determine the modulation index, m_f = _____ b. Determine the carrier signal voltage level. A_C = _____ V_RMS c. Determine the voltage level of the second lower sideband signal. A_2LSB = _____ V d. What is the information or 1% bandwidth of the signal? BW_1% = _____ e. If the k_0 of the VCO is 50kHz/Volt what would be the peak of the modulation voltage V_mpk = _____

Explanation / Answer

9a. we know that a negative feedback phase shift oscillator operate at a frequency where the slope is -3dB/decade. From observing the graph we can say that the slope at f1 is 0dB/decade, at f2 its -3dB/decade, at f3 its -20dB/decade. So the given negative feedback phase shift oscillator will operate at frequency f2.

9b. we know that for negative feedback phase shift oscillator has unity loop gain i.e. (-Rfb / R1), negative sign beause its an inverting oscillator and the phase of output is 180 degree out of phase from input signal. Since the loop gain is 1, so Rfb = R1. Its given that R1 + Rfb = 100k.

Now substituting we get,

2R1 = 100k

So, R1 = Rfb = 50k

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