The product-type phase discriminator circuit formed with MC1496 is shown in the figure.
MC1496 / MC1496B / MC1496P pdf datasheet
Among them, C1 and the parallel resonant circuit C2L together form a linear phase-shift network, which transforms the change of the instantaneous frequency of the FM wave into the change of the instantaneous phase. The analysis shows that the expression of the phase frequency characteristic φ (ω) of the transfer function of the network is:
Inside, the phase changes linearly with the frequency offset, thereby achieving a linear phase shift. The function of MC1496 is to multiply the FM wave and the FM phase wave (as in formula 7-21), its output terminal is connected to a differential amplifier formed by an integrated operational amplifier, and the double-ended output is changed to a single-ended output, and then output through the R0C0 filter network The frequency discrimination operation process for the frequency discrimination circuit shown in Figure 7-8 is as follows: First, measure the static operating point of the frequency discriminator (which is basically the same as the static operating point of the circuit in Figure 7-3), and then tune the parallel resonant circuit to make Its resonance (resonance frequency f0 = 20.945MHz). Then input fc = 20.945MHz from the ux terminal,
ucp-p = 40mV carrier (no phase shift network, uy = 0), adjust the balance potentiometer RP to make the carrier suppression best (U0 = 0). Then connect to the phase-shift network, input the FM wave US, its center frequency f0 = 20.945MHz, ucp-p = 40mV,
The frequency of the modulation signal fΩ = 1kHz, the maximum frequency deviation △ fmax = 75kHz, adjust the resonant circuit C2 so that the waveform distortion of the low-frequency modulation signal U0 (t) obtained at the output terminal is minimum and the amplitude is maximum.
(4) Measurement method of frequency discrimination characteristic curve (S curve) The common methods for measuring frequency characteristic curve are point-by-point tracing method and sweep-frequency measurement method. The operation of point-by-point tracing method is: use high-frequency signal generator as the signal source to add To the input terminal US of the discriminator (see Figure 7-8), first adjust the center frequency f0 = 20.945MHz, the output amplitude ucp-p = 40mV. Frequency discriminator output U0
Connect a digital multimeter (placed in the "DC voltage" file) to measure the output voltage U0 value. (Tuned parallel resonance circuit,
Make it resonant). Then change the output frequency of the high-frequency signal generator (maintain the amplitude unchanged), write down the corresponding output voltage U0 value, and fill in the following table. Finally, draw the S curve according to the measured values ​​in the table.
The operation of the frequency sweep measurement method is to add the output signal of the frequency sweeper (such as BT-3G type) to the input terminal Us of the frequency discriminator, and replace the detection probe cable of the frequency sweeper with a clip cable to the frequency discriminator. Output U0, first adjust BT
ï¼3G's “Frequency Offsetâ€, “Output Attenuation†and “Y-axis Gain†knobs make BT-3G display the frequency discrimination characteristics directly, and use the “frequency scale†to draw the S curve. Adjusting the resonance circuit capacitance C2 and balancing the potentiometer RP can change the slope and symmetry of the S curve.
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