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Vest Pocket VHF FM Test Generator Schematic Circuit Diagram

The Emergence of Affordable FM Transmitter Modules

In numerous countries, the relaxation of licensing restrictions for VHF FM band transmitters operating at 50 NW transmit power led to the proliferation of small, budget-friendly FM transmitter modules in the market. These modules, in the author’s perspective, serve as excellent foundations for creating compact FM-band test generators. All that is needed is a sine-wave modulation signal, which can easily be sourced from an existing audio generator. Alternatively, if an appropriate audio generator is not at hand, constructing the Wien bridge oscillator detailed in this article becomes a viable option.

Vest Pocket VHF FM Test Generator Schematic Circuit Diagram

Optimizing Amplitude Stability with FET T1 and Transmitter Integration

FET T1 plays a pivotal role in maintaining amplitude stability, crucial for minimizing distortion. The output signal is directed to the transmitter module through a 3.5-mm stereo headset socket, which connects seamlessly with the standard 3.5-mm stereo plug of the FM transmitter (note that the left and right terminals of the socket are internally linked). To prevent overdriving the transmitter, finely adjust the output level of the audio oscillator using potentiometer P1.

Navigating the FM Transmitter Setup for Signal Analysis

Within the author’s transmitter module, the HF stage is constructed around a Rohm BH1418FV IC. Locating the datasheet for this IC via a simple Google search can aid in identifying the RF output on the transmitter circuit board. Utilizing a length of coaxial cable, extract the FM signal and channel it to the antenna connector of the receiver under scrutiny. Exercise caution regarding the receiver’s maximum rated input level and ensure proper impedance matching. If necessary, incorporate an attenuator at the receiver input. Employing an oscilloscope, meticulously track the signal within the receiver and analyze the resultant output signal.

Harnessing FM Transmitters: Exploring Frequency Bands and Channels

FM transmitters serve the purpose of enabling phones to broadcast stored music over FM frequencies, allowing nearby FM receivers like car radios to tune in. Although not a standard feature in mobile phones, modern models do incorporate FM transmitters. In the United States, FM broadcasting spans from 88.0 MHz to 108.0 MHz. This spectrum is divided into 100 channels, each 200 kHz (0.2 MHz) wide, providing a structured framework for FM transmission and reception.

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