Opamp VHF FM Transmitter

The circuit has a few striking characteristics.
Firstly, unlike normal oscillators that contain transistors this one
does not have any inductors. Secondly, there is no need for a varicap
diode to do the FM modulation. The opamp is configured as a Schmitt
trigger with only a small amount of hysteresis. The output is fed back
via an RC circuit. In this way, the trimmer capacitor is continually
being charged and discharged when the voltage reaches the hysteresis
threshold. The output continually toggles as a consequence.

This
results in a square wave output voltage. With a 10-pF trimmer capacitor
the frequency can be adjusted into the VHF FM broadcast band 88-108
MHz). The frequency of the oscillator is stable enough for this. The
output voltage is about 6 Vpp at a power supply voltage of 9 V. The
transmitter power amounts to about 50 mW at a load of 50R. This is about
20 times as much as the average oscillator with a transistor. With a
short antenna of about 10 cm, the range is more than sufficient to use
the circuit in the home as a test transmitter.

Because
the output signal is not free from harmonics the use of an outdoor
antenna is not recommended. This requires an additional filter/adapter
at the output (you could use a pi-filter for this). The FM modulation is
achieved by modulating the hysteresis, which influences the oscillator
frequency. An audio signal of about 20 mVpp is sufficient for a
reasonable output amplitude. The package for the opamp is an 8-pin SOIC
(provided you use the version with he RD8 suffix). The distance between
the pins on this package is 1/20 inch 1.27 mm).

This is still
quite easy to solder with descent tools. If SMD parts are used for the
other components as well then the circuit can be made very small. If
necessary, a single transistor can be added to the circuit to act as
microphone amplifier. The power supply voltage may not be higher than 12
V, because the IC cannot withstand that. The current consumption at 9 V
is only 15 mA. As with all free-running oscillator circuits, the
output frequency of this specimen is also sensitive to variations of
the power supply voltage.

For optimum stability, a power supply
voltage regulator is essential. As an additional design tip for this
circuit, we show an application as VCO for, for example, a PLL circuit.
When the trimmer capacitor is replaced with a varicap diode, the
frequency range can be greater than that of an LC oscillator. That’s
because with an LC-oscillator the range is proportional to the square
root of the capacitance ratio. With an RC oscillator the range is equal
to the entire capacitance ratio. For example: with a capacitance ratio
of 1:9, an LC oscillator can be tuned over a range of 1:3.

With
an RC oscillator this is 1:9. For the second tip, we note that the
circuit can provide sufficient power to drive a diode mixer (such as a
SBL-1) directly. This type of mixer requires a local oscillator signal
with a power from 5 to 10 mW and as already noted, this oscillator can
deliver 50 mW. A simple attenuator with a couple of resistors is
sufficient in this case to adapt the two to each other.