Tuning Range

Mechanical and/or electrical tuning provisions are provided with most crystal
oscillators to adjust the frequency for phase-locking or modulation or to
compensate for long-term drift. Mechanical tuning is often accomplished with a
single or multi-turn trimmer capacitor or inductor in the crystal circuit which
is accessed through a hole in the oscillator housing.

Most Wenzel Associates oscillators employ a varactor diode for all tuning and
the mechanical tuning is accomplished internally with a precision potentiometer
connected to a precision reference voltage. Our tests have shown that the
mechanical stability and hysterisis of the highest quality trimmer
potentiometers exceeds the stability of the best precision trimmer capacitors.
Electrical tuning is accomplished with the same varactor diode in a
configuration yielding high tuning linearity. The mechanical tuning voltage from
the potentiometer is applied to the cathode of the tuning diode and the
electrical tuning voltage is applied to the anode of the diode through a signal
isolating network. The electrical tuning may be centered around zero volts so
that in the absence of input the oscillator reverts to the mid-point of the
electrical tuning.

As an option, the oscillator's internal reference voltage may be brought out
to a pin for connecting to a multi-turn potentiometer for fine tuning.
Alternately, a clean DC derived from a separate supply may be applied to the
electrical tuning pin. If the available supplies are noisy or unstable, it may
be desirable to add a zener diode or reference device and a low-pass noise
filter. A temperature compensated zener such as the 1N821 is an excellent choice
giving good temperature stability and very low noise - in most instances the
22uF filter capacitor shown below may be left out. Reference devices exhibit
excellent stability but they often have rather high noise voltage and the
additional filtering is recommended.

Here are a few points to consider:

The amount of tuning range that may be provided is a function of the
crystal frequency, cut, overtone, and spot size (or motional capacitance).
Precision low frequency oscillators using third-overtone SC-cut crystals will
have a tuning range of only a couple of PPM whereas large spot size AT-cut
fundamental crystals can achieve over a thousand PPM.

The bandwidth and tuning slope of the electrical tuning input may be
specified for PLL applications.

At the time of shipment oven oscillators may exhibit a daily aging rate
which would quickly consume all of the mechanical tuning range. This rapid
aging will decrase significantly within a few weeks of operation at the oven
temperature.

Oven oscillators may use high precision, "stiff" crystals since they
operate at a single temperature and the tuning only compensates for aging
whereas TCXOs must tune far enough to remove temperature effects as well as
compensate for aging.

When an oven oscillator's aging pattern has been established (usually an
upward drift) the frequency may be offset in the opposite direction during
calibration to nearly double the calibration cycle time.

Linearity is usually specified as a percentage of the total tuning
deviation from a straight line fit to the tuning curve. A 10% linearity
specification would allow a deviation of 100 Hz away from the best straight
line fit for an oscillator with a 1 kHz tuning range. A potential problem is
that this deviation can occur rapidly at one end of the range as shown below.
The slope is significantly lower at the top of the curve despite the fact that
the oscillator meets a fairly tight linearity specification. For PLL systems
where the tuning slope impacts the loop stability, it may be appropriate to
specify the minimum and maximum tuning slope at all points on the curve in
addition to the percent linearity.

Using Precision Oscillators with External References

Many instruments employing precision crystal oscillators as an internal
frequency reference have provisions for accepting an external reference as well.
The selection of internal or external may be made by manually throwing a switch
or it may be automatically triggered by the presence of signal at the external
reference input connector. In the simplest of cases the manual switch simply
selects which signal source to direct to the instrument and terminates the
unused source with an appropriate resistor. When the internal reference is an
oven oscillator, it is often desirable to have a "signal-kill" input to turn the
oscillator off while leaving the oven operating. TCXOs and non-compensated
oscillators may be turned off when the external reference is present to avoid
potential interference.

Signal-kill inputs typically accept ordinary TTL levels but they may be
configured for custom switching levels without difficulty. Most signal-kill
inputs are designed to float to the "on" state when not connected and internal
RC circuitry protects the oscillator from damaging voltages or interfering RF
signals which might be present on the control line. The circuit shown below will
detect the presence of an external reference greater than 1 volt p-p and
generate a logic "low" suitable for driving most signal-kill inputs. This
control signal could also be used to gate the external input to the instrument's
circuitry. The circuit is designed to exhibit a fairly high input impedance so
proper cable termination is assumed elsewhere in the instrument.

If the internal oscillator is expected to have better short-term stability
than the external reference it may be desirable to phase-lock the internal
oscillator to the reference. The locked oscillator will exhibit the good
short-term stability of the internal crystal oscillator and the good long-term
stability of the external reference. A simple first-order loop may be made with
a tri-state bus device or analog switch and a few additional parts. The basic
concept is to drive the tri-state control with the external reference signal and
pass the internal oscillator signal through one of the tri-stated gates so that
the reference signal samples the internal oscillator signal. The output of the
tri-state gate is low-pass filtered to remove the RF frequencies and connected
to the electrical tuning of the internal oscillator. The tri-state control line
is biased such that the gate output "floats" when the external reference is
absent. This high-impedance state allows the tuning line to be pulled by a
high-value resistor connected to a tuning potentiometer when no external
reference is present. The 74HC244 has two sets of four tri-state buffers and is
a good choice since the four gates not being used may be employed to buffer the
signals.

The following circuit uses the same technique using only two CMOS devices, a
4069 hex inverter and a 4016 or 4066 analog switch. The frequency trimmer
potentiometer controls the frequency when no reference is present. The circuit
also includes an LED driver to indicate the presence of the external reference
signal. Component values are not particularly critical and optimum values will
depend upon the application.

The circuit as shown is a first-order loop with most precision oscillators.
For example, a 10 MHz oven oscillator might have a tuning sensitivity of 1Hz per
volt and, with a 15 volt supply, the phase slope is about 3 volts per radian.
The bandwidth of the first-order loop is the product of the tuning sensitivity
and the phase slope (1 Hz/volt x 3 volts/rad. = 3 Hz) which is well below the
roll-off frequency of the 10k and 10nF network. If the oscillator has a much
higher tuning sensitivity, the bandwidth will be proportionally higher and a
second-order response may be desired to reduce external reference noise.
Although the traditional lossy-integrator op-amp type-2 circuit could be added,
additional provisions will be necessary to control the frequency in the absence
of the reference since the integrator will tend to wander to one of the supply
rails. An alternative way to achieve a second-order response while preserving
the automatic tuning switchover is to add an additional passive filter. This
type-1 second-order loop is easily achieved by adding a 100k or larger series
resistor to the output. Another resistor in series with a capacitor is now added
to ground at the output to add an additional roll-off. It may be tempting to
simply increase the 10nF capacitor to achieve a lower bandwidth but the damping
may become unacceptably low. In fact, if the oscillator has a very wide tuning
range, the 10nF may be too large for sufficient damping. (In most reference
applications, the first-order loop as shown is usually fine since phase error is
not important and the lock bandwidth is not critical.)

Notes:

The circuit above is operating at the frequency limits of the devices at
10 MHz but the 15 volt supply gives about 12 volts of tuning voltage swing
without amplification. HCMOS devices will improve the high frequency
performance but the lower 5 volt supply will give a smaller tuning voltage
range (under 5 volts). An amplifier could be added for oscillators requiring
more tuning voltage swing. To calculate the bandwidth of such a system,
multiply the amplifier gain by 1.5 to get the amplified phase slope (the 5
volt squarewave will give a 1.5 volt/radian phase slope) and then multiply by
the tuning sensitivity of the oscillator.

The reference input may require a resistor to ground on the reference side
of the 100nF capacitor to properly terminate the input.

The circuit may be built with tri-state gates but if they are
non-inverting gates then a different bias scheme will be required. Remember to
bias the external reference gates so that they turn off the tri-state control
when no signal is present.

The oscillator tuning input must be a high impedance (megohms). Add a
voltage follower to drive lower impedance inputs.

Also read about high-performance Type-2
loops featuring Ultra-Low Noise Oscillators.

View the Oscillator
Catalog