No-bias L-C High-Frequency Oscillator

            This oscillator requires no biasing components. Only an inductor and various matching or tuning capacitors are required to set the operating frequency. The active element is the 74HCU04 hex inverter. Only one of the six inverters is required for the oscillator – another may be used as a buffer. A few oscillator circuits are presented, including design procedures.

Characterizing the 74HCU04

            Most HCMOS gates are “buffered”. In the case of buffered inverters like the 74HC04, a single inverter is constructed from three cascaded inverters. The HCU04 is much simpler with each inverter composed of one enhancement  NMOSfet and one PMOSfet in totem-pole configuration. The NMOSfet’s gate and PMOSfet’s gate are tied together at the input pin, and their drains are tied together at the output pin. When biased mid-way between supply (Vcc) and ground, this structure is a reasonably linear amplifier. Open-loop gain is measured at 12.5, with transconductance 0.023 amps/volt. Output impedance measures 557 ohms. All are dependent on supply voltage (from a Philips data sheet):

Since an oscillator’s input or output voltage will be swinging over a considerable range of the 5v supply voltage, the non-linear characteristics of the amplifying inverter have significant influence over the AC operating amplitude. Swings above Vcc or below ground are clamped by internal diodes and can cause latch-up of the device. Operation too close to the rails should be avoided. The onset of non-linearity such as reduced transconductance or reduced output impedance serve as the mechanism by which amplitude is stabilized – every oscillator employs similar means of amplitude control.

 

LC Oscillator Circuits.

            Of the basic oscillator types, the non-inverting types such as Hartley or Colpitts are not achievable with a single HCU04 amplifier. Pierce and Vackar are two described here. The Pierce variation using a crystal in place of the inductor, is a popular one used to clock many digital circuits. A resistor is added (between input pin and output pin) to bias the inverter as a linear amplifier. An extra series resistor is sometimes added to limit the power flowing through the crystal. In the simple LC resonator version, it is not possible to match the resonator impedance to amplifier. Loop gain is very high, which makes it very likely that the oscillator will start. It also means that circulating resonator energy is lower than it could be.

 

            A Vackar oscillator is often seen using a depletion FET as the amplifier. It is quite appropriate with an HCU04 substituted. Circulating current in the LC resonator is not limited as it is with a crystal. It is easy to match the HCU04 output impedance to the resistive part of the resonator. A fraction of the resonator voltage is fed back out-of-phase to the HCU04 input. This can be done two ways: with a capacitive voltage divider (as in the conventional Vackar FET circuit), or with a tap on the resonating inductor. The tapped inductor is most attractive, resulting in an oscillator having no resistive biasing components – only reactive resonating or matching components. These Vackar circuits can be easily optimized for nearly any LC components.

 

Vackar Oscillator Design

            Upon power-up, oscillator amplitude will increase until the HCU04 becomes non-linear. Since it will self-bias mid-way between ground and supply voltage (+5v nominal), output voltage will swing over some fraction of 5v p-p. At the mid-bias point, transconductance and output resistance is highest: 22mA/V and 500 ohms respectively. This is the starting point for design.

            Resonator Q is mostly determined by the inductor Q. A 3.5uH inductor was wound onto a T50-2 toroid core (25 turns of #22 magnet wire). Its unloaded Q @ 7MHz. was 170. The output shunting capacitor is chosen to match the HCU04’s 500 ohm output resistance to the inductor’s 0.9 ohm series resistance. Capacitive reactance required is about 1200 pf.

            If we assume that output swing is about 3v p-p, total resonator voltage across the inductor will be close to 25 v p-p. A 170 pF resonating capacitor is required at the “hot” end. The hot end will swing with opposite phase compared to the driven end. At some point along the inductor’s length (closer to the driven end) there will be no oscillating voltage – equivalent to AC ground. The proper point to tap the inductor is on the “hotter” side of this pseudo-ground, where the phase opposes that of the driven end. Since the amplifier loaded gain is about five, the tap point should have more than 0.6 volts p-p. A point too close to the driven end will result in too-low loop gain resulting in no oscillation. A point too close to the hot end will over-drive the HCU04. If over-driven too much, the input clamping diodes will conduct on oscillation peaks, killing resonator Q. Finding the proper tap point may require a few attempts. For this oscillator, the tap ended up six turns up from the driven end. It is possible to tweak the tap point by adjusting the value of the output loading capacitor – an overdriven input requires less loading capacitance. An oscillator that doesn’t start requires more loading capacitance.

            The tap connected to HCU04 input pin is at a low impedance point on the inductor, so that almost all the resonator loading is at the driven end. The resonator ends up reasonably well-matched to the HCU04’s output resistance. However, the output resistance becomes non-linear as the output swings near Vcc or near ground. Both transconductance and output driving resistance decrease at these extremes. This non-linear effect limits oscillator amplitude.

            Unless a sinusoidal wave shape is required, use one of the spare inverters to buffer the oscillator output, to drive a load. These amplifiers are not excellent buffers, but do help to reduce frequency shifts when a load impedance changes.

 

            It is possible to make a more conventional Vackar oscillator. Instead of tapping the inductor, the 170 pf. resonating capacitor is replaced by two (or more) series capacitors, providing a capacitive tap. To maintain the input pin at about half the Vcc supply voltage, a resistor is now required for bias. A large-value resistor (1 Mohm) connects HCU04 output pin to input pin, biasing the amplifier mid-way between Vcc and ground.

            The amplifier loaded voltage gain will be slightly less than half of the open loop gain if the resonator is impedance-matched to the HCU04 output impedance of 500 ohms. Loaded gain in this test circuit is about five. The large capacitor connected from HCU04 input pin to ground should be about five times as large as the loading capacitor connected from HCU04 output pin to ground – AC current in these two capacitors has similar magnitude, and opposing phase. Both these capacitors should be grounded very close together. In this case, input loading capacitor is 5 x 1200 pF. With 5600 pF, the oscillator did not start – reduced to 4700 pF, the oscillator powered up with an acceptable AC amplitude.

            In this case, the large input shunting capacitor is used to adjust oscillator amplitude – too much voltage at the input pin requires more capacitance while an oscillator that doesn’t start requires less.

 

            Tuning the Vackar circuit in a variable-frequency oscillator over a wide frequency range becomes a problem, since the resonator includes capacitors that match impedance to the HCU04. The tapped-coil version of the Vackar has an advantage over the tapped-capacitor version in this case.