FM transmitter on the MAX2606 chip. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transmitters

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This design considers an integrated voltage controlled intermediate frequency oscillator (VCO) that can relay audio from a cell phone to the FM broadcast band. By placing the phone's speaker near the microphone, the user can use the phone as a speakerphone while driving.

The circuit in Figure 1 can relay the audio signal from a cell phone to the FM broadcast band. When the user places the phone's speaker near the microphone, he or she can listen to cell phone audio through the car's FM receiver. This can be used as a built-in speakerphone in a car.

Rice. 1. Diagram of an FM transmitter with a microphone for use as a hands-free cell phone

IC1 is an integrated voltage controlled intermediate frequency oscillator (VCO) with a differential output that supports a frequency range from 70MHz to 150MHz, covering the entire FM band. Choke L1 sets the VCO's mid-range tuning range. Select the value of L1 such that the effective inductance from the IND pin to GND results in an output frequency of 100MHz. This frequency corresponds to the middle of the FM band. The inductance value of 270nH covers the tuning range from 97MHz to 128MHz. Next, adjust the resistor R_ADJ to tune the VCO to the desired FM station frequency. Resistors R2 and R_ADJ displace the internal varactor. FM stations are located at odd intervals in 0.2 MHz steps. The VCOs provide differential output power down to -8dBm. Refer to Chapter 47 of the Code of Federal Regulations Part 15 of the FCC for current regulations governing the harmful effects of radio signals on the environment. The transmission must be below 250µV, measured at a distance of 3m.

The electret microphone is biased through resistor R1. The audio signal of an AC-coupled microphone modulates the VCO frequency by varying V_TUNE. The output frequency of the VCO will correspond to the volume or amplitude from the microphone. Audio signal about 20mV_RMS (rms operating voltage in volts) is adequate to maintain functionality. Do not overmodulate the VCO or the FM receiver will be distorted as a result. VCO overmodulation also attenuates the carrier signal by radiating energy into an unused receiver band. In Figure 2, the microphone and resistor R1 are replaced by a direct connection to the cell phone's audio out port. Cell phone volume should be adjusted for optimal performance. A direct connection to a cell phone will eliminate background noise from the microphone.

Rice. 2. FM transmitter circuitry with direct audio input eliminates background noise from the microphone

MAX2606 - frequency from 45MHz to 650MHz, integrated voltage controlled intermediate frequency oscillator (VCO) with differential output.

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