Oscillators Circuit Diagrams

VHF Test Transmitter Schematic Circuit Diagram

Building an Independent Frequency-Modulated Oscillator for VHF Receivers

To conduct tests on VHF receivers without relying on local radio stations, it’s essential to have a frequency-modulated oscillator that spans from 89.5 to 108 MHz. However, constructing such an oscillator using discrete components presents challenges. Maxim addresses this issue by introducing a range of integrated oscillator building blocks in the MAX260x series. These modules cover frequencies from 45 to 650 MHz, providing a convenient solution for your testing needs.

VHF Test Transmitter Schematic Circuit Diagram

Selecting Suitable External Components

To complement the MAX2606 module, you’ll require a fitting external coil designed for the midrange frequency. The MAX2606 covers the VHF band, allowing a frequency variation of approximately ±3 MHz around the midrange frequency established by the coil L. The inductance values provided in the table serve as initial benchmarks for further experimentation. SMD coils from the Stettner 5503 series, available from Bürklin in Germany, ranging from 12 nH to 1200 nH, are ideal options for these oscillators.

Coil Winding and Power Supply Configuration

Constructing your own coils is feasible by winding 8 to 14 turns of 0.5-mm-diameter silver-plated copper wire around a 5-mm mandrel. Fine-tuning the coil’s inductance is achievable through slight adjustments like spreading or compressing the turns. The circuit operates on a 9-V battery, where the voltage is stabilized at around 4 V using the BC238C. While the MAX2606 accepts a supply voltage between +2.7 V and +5.5 V, employing a stabilized voltage enhances the frequency stability of the oscillator. Proper decoupling is ensured by placing 1-nF capacitors as close as possible to the IC pins for the Vcc (pin 5) and TUNE voltage (pin 3) connections.

Tuning and Output Configuration

The tuning voltage (TUNE) on pin 3 can vary between +0.4 V and +2.4 V. The module provides asymmetric output through the OUT+ and OUT– pins. In its simplest form, the output can be utilized in a single-ended setup. To achieve this, pull-up resistors are linked to each output. You can tap the radio signal from either of these resistors using a capacitor. A signal amplitude of 10 to 20 mV at the audio input is adequate for generating the standard VHF frequency deviation of ±40 kHz.

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