Quartz crystals have the
property that their amplitude/phase characteristic repeats itself at
frequencies that are an uneven multiple of the fundamental frequency.
There are so-called overtone crystals that are cut in such a manner that
they possess this property to a greater extent. However, in principle,
any crystal may be used on one or more of its harmonic frequencies.
Harmonic generators based on transistors may operate satisfactorily on
the 3rd harmonic, but if the 5th or 7th harmonic are wanted, the circuit
becomes less reliable and requires frequent adjustment.
This circuit is based on a single, fast opamp and oscillates readily
at the 3rd, 5th or 7th harmonic. The opamp is set up as a non-inverting
amplifier with the quartz crystal connected between its output and the
non-inverting input. The circuit amplification, which in principle must
be unity to ensure oscillation, is determined by the network formed by
R4, R5 and trimmer capacitor C3. This network is frequency-dependent
such that the amplification increases as the frequency rises. The
network gain is adjustable with C3.
The setting of the capacitor must be such that the gain is too small
for oscillation at the fundamental frequency, but sufficient for, say,
the 5th or 7th harmonic. The author uses a standard computer crystal of
10 MHz. Depending on the setting of C3, the circuit provides a stable
output at frequencies between 50 and 70 MHz. It should be noted that
these frequencies are multiples of the series fundamental frequency of
the crystal. Tuning is carried out simply with a frequency counter
The output frequency is varied with C3. When the capacitor is
roughly at the correct setting, the frequency ‘locks’ as it were at the
harmonic. The area where locking occurs is not well-defined, however, so
that the setting of C3 is not critical. When tuning is completed, the
output frequency is crystal-stable. In principle, the circuit may be
used for frequencies of up to 100 MHz, when the values of R4 and R5 may
need to be reduced.
When a crystal with a higher fundamental frequency, say, 15 MHz, is
used, the circuit may be tuned to the 3rd harmonic, that is, 45 MHz. The
circuit should be tested with a supply voltage of 5–9 V (the maximum
supply voltage for the IC is 12 V). The peak to peak output voltage has a
value of about that of the supply voltage less a few volts. The output
can provide a current sufficient to drive relatively low-impedance
Author: Gert Baars – Copyright: Elektor Electronics