Here is a highly stable Direct FM Exciter with an RF output 500 miliwatts. A phase Locked Loop (PLL) circuit controls the mean frequency of the oscillator. Stability exceeds that of broadcast service standard 0f +2 or -2 KHz for both long and short term drift. Typical drift is less than 200 Hz.
A buffer amplifier follows the direct fm oscillator. This provide isolation for the oscillator as well as increasing the power output. It operates in a class A/B mode rather than class C which greatly reduces any harmonic content. The power output can be adjusted from near zero to ove 500 miliwatts. Reduced power output is obtained by decreasing coupling or by changing some part values.
The Phase Locked Loop may be programmed for any frequency between 87 MHz and109 MHz in 50 KHz increments. A precision crystal oscillator is used for the PLL reference. Capacitor trimming of the crystal frequency allows setting of the carrier frequency to +/- zero deviation.
Two audio input are provided. One includes the US standard 75 microseconds pre-emphasizes which is for mono audio inputs. The second is broadband input for composite stereo and/or SCA type audio. The required input level to achieve 100% modulation is 3 volt rms. The input in a broadcast standard of -10 dBm and 600 ohm input impedance.
Power requirement is 12 volts att 300 miliamperes. The supply should be well regulated for optimum performance. Recommend you read the entire set of plans before attempting assembly or operation in this circuit. Download manual documentation of the PLL FM Exciter
See more : FM Stereo Encoder - FM Transmitter - TV Transmitter
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