Circuit : Ramon Vargas Patron - Lima Peru
The RF engineer sometimes has to look for an instrument that will check a low frequency
quartz crystal unit reliably and rapidly. This is a difficult piece of equipment to find
and the engineer often has to consult an electronic circuits handbook for the schematic
of a circuit that will perform the task.
Unfortunately, there aren't many such circuits in the technical literature currently
available, and when found, they don't always work as expected. A circuit that has been
found to work at full satisfaction in the frequency range from 10 kHz to 500 kHz is
illustrated in Figure 1.
This is a schematic of a low frequency sine wave oscillator featuring low distortion,
wideband operation and crystal control.The circuit, originally developed for laboratory
use, employs low cost AF bipolar transistors for the oscillator and amplifier sections and
a JFET for loop-gain control. Operation of the oscillator in the 10 kHz to 500 kHz frequency
range has been found to be excellent, while measured distortion is kept under 0.1 percent.
Theory of Operation
Q1, Q2 and associated circuitry form a modified astable multivibrator in which the loop gain
is automatically adjusted to the threshold of oscillation by means of field effect transistor
Q3. Q4 linearly amplifies the signal present at the collector of Q2 and isolates the oscillator
section of the circuit from the output. This stage features wideband operation and delivers a
clean 2.5 Volt amplitude sine wave into a resistive load greater than or equal to 20 kohms. The
stage comprising Q5 has a voltage gain of 1 and its sole purpose is to isolate the non-linear
effects of rectifier D1 from the output. Transistor Q4 also amplifies the minor changes in
amplitude of the oscillator's waveform due to temperature effects and/or power supply variations,
so a magnified version of the perturbance is fedback to rectifier D1 producing a corresponding
change in Q3's gate voltage. This action modifies the FET's drain-source resistance and hence
adjusts the loop gain to a new value slightly above unity, just enough to maintain a constant
amplitude in the output.
Figure 2 shows optimum values for capacitor C according to the crystal's resonant frequency.
Extra gain is needed from transistors Q1 and Q2 at frequencies below 40 kHz. This is due to the
fact that low frequency crystals exhibit large values of series resistance, affecting loop gain
(Table 1 compares typical values of series resistance for low frequency units). According to what
has been stated, resistor R is made 10 kohms for frequencies under 40 kHz. Above this value, 1 kohm
Three final comments are:
1) Better amplitude stability can be attained by increasing the voltage gain of the stage
comprising Q5, but at the expense of reduced oscillator output.
2) The oscillator section is energized from a 3.3 Volt supply. This keeps the crystal power
drive level very low, which is in fact desirable.
3) Due to the dynamic action of the JFET the output level is almost insensitive to power supply
variations. The 3.3 Volt zener diode further enhances this result.
Some completed pictures of Ramon's project. Click any image to zoom:
1.The ARRL Handbook, 1986.
2.Bernd Neubig, "Design of Crystal Oscillator Circuits", Kristall - Verarbeitung Neckarbischofsheim,