Waveform Conversion

Part I – Sine to Square

The system designer is often called upon to convert a sine wave from an oscillator, power splitter, or other RF device into a square wave suitable for driving a logic device. There are numerous acceptable techniques and the best choice will depend upon several factors including the operating frequency, available signal power, available DC power, acceptable edge speeds, and the characteristics of the logic family.

The simplest technique is to directly couple the sine wave into the logic input with a suitable bias circuit. The sine wave should be a few volts p-p for reliable triggering. This technique is most suitable for CMOS devices in the older 4000 series or HC-MOS and only requires a coupling capacitor and two resistors to bias the input to VCC/2. This simple bias scheme should be avoided with AC-MOS devices unless the frequency and amplitude are high and the input signal is always present. Slow-moving or noisy inputs can cause multiple triggers on any gate, however, and Schmitt trigger inputs are recommended when the input sine wave is large enough to overcome the hysteresis. As an example, the 74HC14 Schmitt trigger has a hysteresis voltage of 0.9 volts so an input sine wave should be a couple of volts at a minimum. The simple biased-gate scheme is shown below:

In some instances the input signal is too small to drive the logic devices but sufficient power is available to drive a step-up transformer or matching circuit. One unusual approach uses a series RLC circuit to step up the voltage. At the input frequency, the RLC network simply looks like a resistor since the inductor and capacitor are series resonant. The resistor value is selected to limit the current at resonance (typically 100 ohms) and the inductive reactance is selected to give the desired gate voltage. In the example below, the inductor is selected to give about 5 volts of swing for the signal input current. It is reasonably safe to use the input protection diodes in AC logic to clamp the swing since AC devices are quite immune to latch-up and the input protection circuit is quite robust.

An ordinary RF step-up transformer may also be used to achieve the higher voltage swing and fairly broadband transformers may be constructed on ordinary ferrite beads, pot cores or miniature balun cores.

Although these techniques will give reasonably reliable results, especially when Schmitt trigger inputs are used, more sophisticated approaches may be desired to handle slow-moving signals or wide signal level variation. The following simple two-transistor differential amplifier will give a good square wave for a wide range of sine wave inputs and it has sufficient gain to square up the edges of slower input frequencies. The differential amplifier approach avoids transistor saturation which often limits the speed of single-transistor circuits. Fast diodes may be used to prevent saturation in single-transistor amplifiers but the cost and complexity usually exceeds the addition of another transistor.

Numerous integrated circuit solutions are also available. Line receivers are an excellent choice and are truly designed for the job. The input stages are typically differential amplifiers offering fairly high input impedance and good speed. Some line receivers have built-in voltage dividers allowing the inputs to handle voltages far beyond the power supply rails and some have built-in voltage references for biasing the input stage. They are also available in combination with line drivers and many have tri-state control inputs. The possible choices are numerous – the line driver, receiver data books are among the thicker volumes on the engineer’s bookshelf. Beware that many line receivers have built-in frequency response roll-off making them unsuitable for squaring higher frequency oscillators.

The MC1489 is a typical quad line receiver which operates from a single 5 volt supply and can handle input signals from-30 to +30 volts – but the threshold is internally set. This device features a built-in threshold hysteresis of about 0.3 volts (0.9 for the MC1489A) with the lower threshold near one volt. A “response control” pin is provided to shift the threshold points or to add signal filtering.

Another example is the SN55182 which is a dual differential line receiver that also operates from a single 5 volt supply. It is designed to respond to small differential voltages riding on fairly large common-mode voltages and noise. The inputs may be biased to switch anywhere within the 15 volt bipolar common-mode input voltage range. The input impedance is a few thousand ohms but a built-in 170 ohm line-termination resistor is provided. This line receiver’s frequency response may also be lowered by adding an external capacitor and a strobe input is available to force the output to a high level. Since many line receivers operate from a single supply and can sense below ground, the schematic can be quite simple. The following circuit shows 1/2 of the SN55182 connected to convert a sine wave centered around ground – typical of transformer or power splitter outputs. The only external component indicated is a 0.1 uF power supply capacitor! It may be desirable to add an additional resistor to ground at the input to match a particular source or cable impedance.

High-speed comparators may also be used in a similar manner to line receivers but the resistor biasing must be done externally and some amount of hysteresis should be added in the form of a feedback resistor from the output to the positive input to ensure positive switching. Some comparators have “totem pole” outputs which are specifically designed to drive TTL and CMOS loads and some have open-collector outputs which only need a pull-up resistor to achieve suitable logic levels.

Many prescaler devices are specifically designed to handle small sine wave inputs and all that is required is a DC blocking capacitor. The designer should be aware that some prescalers will “free-run” or oscillate when no input signal is present.

High frequency sine waves can often be directly applied to ECL inputs with the appropriate bias circuitry if the sine wave amplitude falls within the ECL’s voltage specifications. Some ECL devices have a reference voltage output pin which may be used to bias the input. Simply connect a resistor from the reference to the input and capacitor couple the input signal.

Waveform Conversion, Part II – Square to Sine

Oscillators with logic compatible square wave outputs are not suitable for driving many RF devices since the characteristic impedance is usually well below the desired source impedance and the odd harmonics can generate undesired intermodulation products. Converting a square wave to a sine wave is usually accomplished with either a low pass or bandpass filter and a resistor network to achieve the desired impedance. An amplifier at either the input or output of the filter may be necessary to achieve sufficient signal amplitude.

The circuit below uses a single resistor and a pi network to generate a 50 ohm sine wave from ordinary CMOS logic. The series resistor is selected to limit the current and to isolate the logic device from the reactive load presented by the pi network. The resistor should be a minimum of 200 ohms for most inexpensive clock oscillators and low-power logic devices and the output sine wave level will be about 4 dBm. If the logic device can supply higher current, a 100 ohm resistor may be used to achieve an output level of 10 dBm. AC devices can drive even heavier loads and >13 dBm outputs are practical with a 68 ohm series resistor. The pi network is selected to match the series resistance to 50 ohms at the operating frequency. The Q of the pi network may be low since a square wave has little second harmonic and the third harmonic is three times the fundamental frequency. A low-Q pi network allows the use of fixed values with no adjustments. Add a DC blocking capacitor in series with the output if the load has a DC path to ground. This capacitor may be left out if the load can tolerate DC current and the decreased efficiency is acceptable.

The values for the components of the pi network may be found in many references but for reference the reactances for Q = 2 at three resistances are as follows:

R = 200 ohms, C1 = C2 = 100 ohms reactance, L = 100 ohms reactance

R = 100 ohms, C1 = 50 ohms reactance, C2 = 41 ohms reactance, L = 65 ohms reactance

R = 68 ohms, C1 = 34 ohms reactance, C2 = 30 ohms reactance, L = 50 ohms reactance

The nearest values with these reactances should work well and the actual resistor may be a bit lower since the logic device will have some internal resistance. If a DC blocking capacitor is added in series with the output it should be selected to have a very low reactance at the operating frequency – typically a 0.1 uF.

An ordinary NPN transistor makes an excellent power amplifier for achieving output levels above 13 dBm with light loading of the logic device as shown in the figure below:

The above circuit draws about 12 mA from the 15 volt supply while providing 17 dBm at 6 MHz. The pi network is set to a Q of 4 and a step-down ratio of 4:1. Other output networks are acceptable including tuned transformers or other matching networks. A grounded-gate JFET may be used in place of the NPN transistor eliminating the need for the two base bias resistors and the base bypass capacitor. The U310 is an excellent JFET for frequencies up to several hundred MHz. Reduce the series resistor if the output amplitude is too low – the JFET has a source resistance much higher that the NPN emitter resistance.

A “Tee” network may be used in place of the pi network shown above with some advantages. Since the gate is driving a series inductor instead of a grounded capacitor, the harmonic loading is much less and a series resistor can often be avoided entirely. In applications where the source impedance is not particularly critical and the maximum signal level is desired the Tee network can give excellent results. The circuit below shows an inexpensive clock oscillator driving a tee network to provide about 2 volts p-p into 50 ohms. The network reactances are for Q = 2 and a transformation ratio near 2.5. The reactance values for other network Qs and transformation ratios may be found in RF handbooks and application notes – one favorite is AN-267 from Motorola.

A resistor may be added in series with the gate output as before to achieve a good output VSWR and to protect the gate against shorted outputs. A DC blocking capacitor in series with one of the inductors is recommended for most applications to reduce the loading on the gate and to prevent DC from reaching the load. If a DC blocking capacitor is added it should be selected to have a very low reactance at the operating frequency – typically a 0.1 uF.