Opamp VHF FM Transmitter Design (88-108 MHz)

Circuit diagram:

Description

Integrated circuits that were formerly prohibitively expensive for enthusiasts are now more readily available at affordable prices. A notable example is the AD8099 from Analog Devices. This operational amplifier (opamp) is obtainable for just a few pounds sterling. The AD8099 is a highly rapid opamp (1600 V/ms) characterized by high-impedance inputs and a low input capacitance. Despite its wide bandwidth – maintaining a gain of almost 40 at 100 MHz – it’s perfectly suited for creating RC oscillators. This circuit demonstrates that functionality effectively.

The circuit possesses several noteworthy attributes. Unlike conventional oscillators reliant on transistors, it avoids the use of inductors. Furthermore, no varicap diode is required for FM modulation. The opamp is configured as a Schmitt trigger, incorporating a minimal amount of hysteresis. The output signal undergoes feedback via an RC circuit. As a consequence, the trimmer capacitor is consistently charged and discharged when the voltage reaches the hysteresis threshold, resulting in continuous toggling of the output.

This produces a square wave output voltage. When a 10-pF trimmer capacitor is employed, the frequency can be adjusted to encompass the VHF FM broadcast band (88-108 MHz). The oscillator's frequency stability is sufficient for this purpose. The output voltage measures approximately 6 Vpp, supplied by a 9 V power supply, generating a transmitter power of roughly 50 mW with a load of 50R. This represents a 20-fold increase compared to the average transistor oscillator. Utilizing a short antenna of approximately 10 cm, the range is ample for residential use as a test transmitter.

Because the output signal contains harmonics, an outdoor antenna is not advisable. This necessitates an additional filter/adapter at the output (a pi-filter could be used in this instance). FM modulation is achieved by modulating the hysteresis, which subsequently impacts the oscillator's frequency. An audio signal of approximately 20 mVpp is sufficient to achieve a reasonable output amplitude. The opamp's package is an 8-pin SOIC (using the version with the RD8 suffix results in a pin spacing of 1/20 inch, or 1.27 mm).

Soldering this circuit is still relatively straightforward with standard soldering tools. Employing surface-mount device (SMD) components for the other components allows for a significantly reduced circuit size. If needed, a single transistor can be added to the circuit to function as a microphone amplifier. The power supply voltage should not exceed 12 V, as the IC is not designed to withstand higher voltages. The current consumption at 9 V is only 15 mA. As with all free-running oscillator circuits, the output frequency of this specimen is sensitive to fluctuations in the power supply voltage.

For optimal stability, a power supply voltage regulator is recommended. As a supplementary design tip for this circuit, it’s demonstrated as a voltage-controlled oscillator (VCO) within a phase-locked loop (PLL) circuit. Substituting the trimmer capacitor with a varicap diode expands the frequency range beyond that of an LC oscillator. This is because an LC-oscillator’s range is proportional to the square root of the capacitance ratio, whereas an RC oscillator's range is equal to the entire capacitance ratio. For instance, with a capacitance ratio of 1:9, an LC oscillator can be tuned over a 1:3 range, while an RC oscillator maintains this 1:9 ratio.

Furthermore, the circuit can provide sufficient power to drive a diode mixer (such as an SBL-1) directly. Such a mixer requires a local oscillator signal with a power between 5 and 10 mW, and this oscillator can deliver 50 mW. A simple attenuator consisting of a couple of resistors can be used to adapt the two to each other effectively.