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VHF overtone crystals can be difficult to use due to the many frequencies at which the crystal is perfectly happy to oscillate. A typical 100 MHz, 5th-overtone crystal will oscillate at approximately 20 MHz, 60 MHz, 100 MHz, and even 120 MHz, not to mention other, unintended modes that such crystals often have. Most VHF overtone oscillators use some sort of tuned circuit or frequency selective filter to select the desired mode and reject the other, equally active modes. The oscillator circuit shown below uses a simple tuned circuit to select the desired mode and is suitable for VHF crystals, including high overtone types.

A tank circuit in the collector of the exciter transistor is tuned to the desired crystal resonance, either by a trimmer capacitor as shown in the schematic or by a variable inductor in place of the 0.1 uH. The frequency of the tank is determined by the 0.1 uH choke, the parallel 9-35 pF trimmer capacitor, the series combination of the 15 pF and 39 pF capacitor and the reactance of the collector of the transistor and other stray reactance. The 15 pF and 39 pF provide a low voltage "tap" for driving the crystal and the output buffer.

In practice, the two selected components in series with the crystal would be replaced with jumpers and the tank would be peaked for maximum signal on the 39 pF, observed with a very low capacitance probe. Alternately, the trimmer may be set to the middle point between the extremes where oscillation stops by observing the output of the gate. Once the optimum tuning is achieved, selected components are added in series with the crystal to center the frequency. Either jumper position may be used for fixed tuning or a mechanical trimmer could be added on one side and an additional selected component could center the tuning range. Splitting the tuning to both sides of the crystal in this manner reduced the voltage levels due to these reactances. Adding inductance in either position will lower the frequency and adding capacitance will raise the frequency. For applications where a fixed value is selected to center the frequency at design time, the frequency may be adjusted for individual crystals by adjusting the tank tuning, as long as the tuning doesn't get close to the point where oscillation stops.

The two schottky diodes in series with the 180 ohm resistor limit the signal swing at the collector, keeping the transistor in a linear range. The resistor tends to remove the diodes' capacitive contribution to the tank which can cause start-up problems. The smaller signal at the 39 pF should be adequate for directly driving fast CMOS logic as shown, without overdriving the input of some "Tiny Logic" types that don't have the robust limiting diodes that AC has. The schematic shows the output directly from an AC gate, typically a 74AC04, but a 50 ohm resistor in series with a 0.1 uF capacitor could be added in series with the output for driving 50 ohm loads. Check the specification of the chosen logic device to determine its load capabilities.

The MPS6511 is a relatively fast transistor but many types will suffice. A 10 ohm resistor is added in series with the emitter to help stabilize the grounded-base circuit.

A quick prototype was constructed to verify the circuit:

Large, leaded components were used to make it easier to trace the circuit. Parts are soldered to little "islands" of pcb material on a tinned copper-clad ground plane. A pcb SMT carrier board holds the 74AC00 gate (I didn't have a 74AC04 handy, so I tied one input of a NAND gate high to make an inverter). Only one input of one of the gates is used and the other input is tied to the positive supply. All the other gate inputs are grounded. The output of the gate drives the 49.9 ohm resistor and 68 pF capacitor that would connect to 50 ohm coax. The crystal is a 100 MHz, 5th-overtone AT-cut manufactured by Croven Crystals.

The circuit adjusts smoothly with a nice peak in amplitude without jumps or sudden dying except when the amplitude has rolled off significantly. Once the large trimmer was set to the middle of the active range, tuning components were inserted in place of the two jumpers on each side of the crystal with no problems observed. A complex tuning network could be added to the crystal circuit with a varactor or trimmer capacitor in place of one of the jumpers and a selected component in place of the other.

(The 470 ohm at the top of the photo is shorted and not needed.)

Here's an simple experimental circuit using the readily available TL592 differential amplifier. In this circuit, the overtone mode is selected by a tuned circuit between pins 1 and 8. The phase shift due to the roll-off of the amplifier, the 100 ohm resistor and 22 pF capacitor, and the frequency of the tank combine to give the necessary 180 degree phase shift. In order to prevent feedback through the crystal's shunt capacitance, a selected inductor in series with a 100 ohm resistor is placed across the crystal. The inductance should be selected to resonate with the holder capacitance of the crystal, typically 680 nH for a 100 MHz crystal. When the value is selected correctly, the oscillator will only oscillate on a crystal-controlled frequency with the oscillation dying when the tank is tuned too far from the crystal's frequency. The 100 ohm is necessary to kill the Q of the inductor.

In operation, a rather large voltage is applied to pin 1. As a result, the differential amplifier is acting like a bandwidth-limited comparator, switching as quickly as it can. The large signal helps to reduce phase noise due to the amplifier's input noise. The crystal may be tuned over a wide range by the inductor and the range is somewhat a function of the amplifier gain. The prototype would tune 30 ppm with the 1k, 15 ppm with no gain resistor, and as much as 50 ppm with a 470 ohm gain resistor. Higher gain (lower value resistor) can lead to oscillation at one of the lower frequency crystal modes. The circuit was tested with a TL592 but the lower noise TL592B should also work. The output voltage is about 1 volt p-p but an additional buffer is recommended for driving a low impedance load. A single AC logic gate could be added as in the previous circuit. The variable inductor could be replaced with a fixed value and a combination of a trimmer capacitor and fixed value could replace the 39 pF.