About radio control cars transmitters and crystals

To learn more about quartz crystals do not stop
reading this document. The oscillators used to transmit in the range of 72MHz
overtone is always an overtone oscillator is a circuit that harnesses the power
of the crystal to oscillate at a frequency harmonic to the underlying carved on
its own merits.

Crystals by design generally range in fundamental
frequency to 25MHz, and then these frequency crystals are used less frequently
in overtone oscillator circuits for its odd harmonics, typically the 3 rd, 5th
and 7th harmonic. In short if we have to make a crystal radio control cars
oscillator give us a frequency of 72,100 MHz should be borne in mind that we
will not find a crystal that naturally oscillate at this frequency, we must
accompany him on a circuit to do it oscillate harmonic or overtone frequency. For
example, for this we often need a glass of 72 100 / 5 = 14.42 MHz (fundamental)
or could also use a range of 10.3 MHz making it in its 7 th harmonic instead of
5 °.

I particularly like overtone oscillators because some
are very easy to walk, those who use a circuit with the crystal in series with
a tuned tank, replacing the glass we fit a wire coil and adjusting the tank
while we observe in a frequency output. When we are in the desired frequency,
frequency counter, connect the glass and we have accordingly adjusted the
oscillator.

In radio circuits, it is very common to find crystal
oscillators, but frequency modulated by a diode that being in parallel with the
crystal, small frequency variations induced by the variation of the capacitive
reactance is made to generate a radio
control cars frequency modulated signal or FM. In the case of AM used is a
constant and the signal oscillator. It is applied to the base of a transistor
switch that makes this transistor acts as a telegraph key by radio control cars
interruptions emission of a signal generated to the following amplifier stages.

The waveform is a sequence of high and low states
exactly the same as those of a radiotelegraph signal, so it is AM, but one
o’clock modulating from 0 to 100% in very short time.