Signal Sources

Most of the instruments discussed in this guide measure some sort of electrical parameter, usually voltage or current. This section discusses a class of instruments that don't measure anything—at least not by them selves. Instead, they provide the signals for the circuit or device being tested, so that other instruments may measure the electrical parameters of interest.

The term signal source will apply to an instrument that supplies a known signal (usually a voltage waveform) to a circuit being tested. The characteristics of that signal will vary depending on the particular type of signal source. Some sources produce only a single-frequency sine wave. Other, more complicated instruments provide signals such as triangle waves, square waves, and pulses and can sweep the frequency of the waveform as well as modulate the signal. Although various distinct categories of signal sources will be discussed, many instruments have characteristics which transcend several categories.

Circuit Model

Although the actual circuitry of a signal source may be very complex, its electrical characteristics can be modeled with a simple circuit (Figure 3-1). Vs is the open-circuit voltage of the source, and R is the output impedance (or resistance, if you prefer). V is not limited to a simple sine wave, but can be any waveform that a particular signal source is capable of producing. This includes square and triangle waves, as well as pulse trains and modulated signals. The output impedance is typically one of several common values: 50, 75, or 600 ohms. Most source specifications assume that the source is connected to an impedance that's equal to the source output impedance.

Figure 3-1. The circuit model for a signal source is a voltage source with a series resistance.

EXAMPLE 3-1. A sine wave source with a 50-ohm output impedance is supplying 3 volts RMS across a 50-ohm resistor (load). How much power will be supplied to the load if the 50-ohm resistor is replaced by a 75-ohm resistor?

Figure 3-2A shows the circuit model for the source connected to a 50-ohm load. Since VL = Vs 50 / (50+50) by the voltage divider equation,

V = 2 VL = 6 volts RMS.

Figure 3-2B shows the situation with the 75-ohm load resistor connected. In this case, VL = (6)75/(75 + 50) = 3.6 volts RMS.

The power delivered to the load is P = VL^2 R = 3.6^2 / 75 = 0.173 watt. The power can be expressed in dBm as P = 10 log (0.173/0.001) = 22.4 dBm.

Figure 3-2. Circuit drawings for Example 3-1. (A) A signal source with a 50-ohm load resistor. (B) The same signal source with a 75-ohm load resistor.

Floating and Grounded Outputs

The output of a signal source may be floating (Figure 3-3A) or grounded (Figure 3-3B). If the output is floating, neither of the two output terminals is connected to the instruments chassis ground. If the output is grounded, one of the output terminals is connected to the instrument chassis ground, which, in turn, is connected to the AC power ground (via the ground connection on the three-terminal AC power plug). Some instruments offer a floating output, with a switch or jumper built in to allow convenient grounding of the output if desired.

Floating outputs are more versatile, but are more difficult to design and manufacture, especially at radio frequencies. Many circuits have one side of their input connected to ground anyway, so a grounded output source can be used with no problem. In some cases, however, it's desirable to have neither side of the signal source connected to ground. For example, the source may be driving a circuit at a point which has both input connections at several volts above ground. If a grounded source were connected, one of these input points would then be grounded and the operation of the circuit would very likely be disturbed.

Figure 3-3. Signal source circuit model. (A) A floating output. (B) A grounded output.

Imperfections In Signal Sources

Signal sources don't produce absolutely perfect, undistorted signals, but instead a variety of imperfections may be present. Different applications require different levels of performance in the signal source, so the user must understand these imperfections and how they relate to the measurement.

A pure, undistorted sine wave would appear in the frequency domain as shown in Figure 3-4A. Recall from section 1 that a sine wave has energy at only one precise frequency with no harmonics or other frequency components. This perfect sine wave does not vary in amplitude or in frequency.

Frequency Accuracy

The frequency accuracy of a signal source determines how precisely the actual frequency of the waveform matches the frequency setting on the front panel of the instrument. It is usually specified in percent. A frequency error shows up in the frequency domain as a horizontal shift of the sine wave’s vertical line (Figure 3-4B).

Frequency Stability

The sine wave from a signal source may vary somewhat in frequency depending on the source’s frequency stability. This results in a frequency error that varies with time. In the frequency domain, one can imagine an unstable signal wandering slightly back and forth along the frequency axis. This wandering is typically rather slow, but in extreme cases a source could be so unstable that the resulting waveform has noticeable frequency modulation sidebands. There is a fine and arbitrary distinction between frequency instability and actual frequency modulation. Usually, a slow frequency variation (less than 1 Hz) is classified as frequency instability, and a fast variation is classified as undesirable or residual frequency modulation. Frequency stability is usually specified in percent or parts per million (ppm) and may be valid only after a specified warm up time.

Frequency stability and frequency accuracy are related but are not the same. If a source had perfect frequency stability but poor frequency accuracy, the frequency would remain constant but at the wrong value. The frequency inaccuracy of such a source could be compensated for by tuning it to the proper frequency while monitoring it with a more accurate instrument. On the other hand, a source with poor frequency stability can't achieve good frequency accuracy, at least not for any length of time. Such a source could be set to the correct frequency for an instant, but it would quickly drift away in frequency.

Amplitude Accuracy

Amplitude accuracy specifies how precisely the actual amplitude of the waveform matches the instrument’s control setting. An amplitude error in the frequency domain will result in a spectral line at the proper frequency, but with an incorrect height (amplitude), as shown in Figure 3-4C.

Some instruments don't have a specified amplitude accuracy, but have an amplitude flatness specification instead. Amplitude flatness is a measure of how much the output amplitude varies over a specific frequency range, usually the maximum range of the instrument. Once the user sets the source’s amplitude (perhaps by monitoring it with another instrument), the amplitude will remain within the limits of the flatness specification even though the frequency is changed.

Distortion

The waveform may have zero frequency error and zero amplitude error, but have considerable distortion. Distortion is any imperfection in the shape of the waveform as compared against the desired signal. In the sine wave case, distortion shows up in the frequency domain as undesirable harmonics, referred to as harmonic distortion. Harmonic distortion is shown in Figure 3-4D, with the original pure sine wave now accompanied by several frequency components at integer multiples of the original frequency. The maximum amplitude of the harmonics may be specified as a percent of the fundamental or in dB relative to the fundamental. All of the harmonics may be lumped into one distortion number called total harmonic distortion (THD) and specified in percent.

Spurious Responses

A signal source may produce low-level frequency components that are not harmonically related to the output frequency. These components are called spurious responses and are often the result of the particular signal-generation technique used inside the instrument. Alternatively, they may result from the AC power line frequency appearing either directly or as modulation on the carrier. Spurious responses may appear at any frequency and may move when the source’s frequency changes. Figure 3-4E shows the effect of spurious responses in the frequency domain. The maximum value of this type of response is usually specified at an absolute level, or in decibels relative to the desired signal (or both).

Figure 3-4. Signal source characteristics in the frequency domain. (A) A pure sine wave appears as a single-frequency component. (B) A frequency error shows up as a shift along the horizontal axis. (C) An amplitude error causes a change in the height of the spectral line. (D) Harmonics are additional frequency components at multiples of the original frequency. (E) Spurious responses are unwanted frequency components at other non-harmonic frequencies. (F) Close-in sidebands can be caused by residual amplitude, or frequency or phase modulation.

Close-In Sidebands

If the sine wave were absolutely pure, then in the frequency domain it would have an infinitely thin spectral line, indicating that all of the signal’s energy is at exactly one frequency. In reality, the sine wave’s frequency response may spread out slightly, often causing a pedestal effect, as shown in Figure 3-4F. These imperfections usually appear as noise sidebands very close to fundamental and are thought of and specified in a variety of ways. Some instruments specify these sidebands as residual modulation (AM or FM), while others refer to the phenomenon as phase noise. These different mechanisms are not exactly equivalent, but they all produce the result of a noise-like response close the output frequency.

Sine Wave Sources

As the name implies, sine wave sources are capable of supplying only sine wave signals. These sources are economical instruments that generate low distortion signals ranging in frequency from a few hertz to about 1 MHz. The technology used is a free-running oscillator operating at the same frequency as the desired output frequency. This technology is inexpensive, but is limited in frequency range. Sine wave oscillators are used mainly for audio frequency work, with a 600-ohm output impedance being very common. Some sine wave sources are designed to have very low distortion, which is important when measuring distortion in audio amplifiers.

Figure 3-5 shows a simplified block diagram of a sine wave source. It consists of a free-running oscillator, followed by an amplifier to boost the signal level and an attenuator to vary the output level. The free- running oscillator frequency is usually controlled by a variable capacitor and /or resistor. Figure 3-6 is an example of sine wave source, this one having an auxiliary square wave function.

Figure 3-5. Simplified block diagram of a sine wave source.

Figure 3-6. A sine wave oscillator that can also output a square wave.

Function Generators

Function generators are the most widely used general purpose signal source. They are capable of supplying non-sinusoidal waveforms such as square waves, triangle waves, and pulse trains in addition to sine waves (Figure 3-7). Depending on the particular instrument, a function generator may also provide modulation and swept-frequency capability. The function generator often uses technology similar to the sine wave oscillator. A simplified block diagram is shown in Figure 3-8. One of the required waveforms is produced by a free-running oscillator, and then conversion circuits derive the other waveforms from the original. In Figure 3-8, a square wave is generated by the oscillator. A triangle wave is derived from the square wave by running the square wave through an integrator. (The integrator may actually be part of the square wave oscillator.) A sine wave is derived from the triangle wave by passing it through a wave-shaping circuit (sine shaper). The desired waveform is selected, amplified, and output through a variable attenuator. As the technology has improved, the function generator has gradually replaced the sine wave only type source since it offers sine wave capability along with the other waveforms to provide much greater flexibility.

The output impedance of a function generator is typically 50 or 600 ohms, with 600 ohms being more popular at the low frequencies and 50 ohms provided on generators that exceed 1 MHz.

Figure 3-7. Standard function generator waveforms.

Figure 3-8. Simplified block diagram of a function generator using a free-running oscillator which generates a square wave.

Figure 3-9. Square wave output of a function generator with varying amounts of DC offset added.

Function generators typically provide a DC offset adjustment that allows the user to add a positive or negative DC level to the generator output. Figure 3-9 shows how adding varying amounts of DC to a square wave can produce different waveforms. The DC offset can also be used to control the DC bias level into a solid-state circuit.

Function generators can be used for audio sine wave testing just as the sine wave only generator can. In addition, the square wave output can be used as a clock for digital circuits. Some function generators include a fixed TTL compatible output for such an application. On others, square wave output and DC offset adjustment can be used to generate valid logic levels. The triangle wave can be used in situations where a ramp-like voltage is needed. Figure 3-10 shows an example of a typical function generator.

Pulse Generators

Pulse generators are specifically designed to produce high quality square waves and pulse trains. They generally operate over a frequency range as low as 1 Hz and as high as 50 MHz. As shown in section 1, a pulse train has a large number of significant harmonics requiring an instrument bandwidth much greater than the fundamental frequency. Above 50 MHz it's difficult to generate clean pulses because the bandwidth required extends into the several hundred megahertz range. Figure 3-11 shows a typical pulse generator. Three outputs: 50 ohm, 600 ohm and TTL logic level are provided. Flexible triggering capability is also included.

Figure 3-11. Pulse generator capable of producing frequencies up to 5 MHz.

For convenience, pulse generators are generally specified in the time domain. Waveform characteristics such as period, duty cycle, or pulse width can be selected from the front panel. Distortion is not specified since it's basically a sine wave type of specification. Instead, pulse related parameters like rise time and fall time are used. Usually very flexible control is included to allow positive going pulses, negative going pulses, and symmetrical waveforms (Figure 3-12).

Figure 3-14. Typical waveforms produced by signal generators.

Figure 3-15. A radio frequency signal generator having an internal audio modulation source.

Figure 3-16 is the conceptual block diagram of a signal generator having an internal audio oscillator for use as a source of modulation. The heart of the signal generator is a voltage- controlled oscillator (VCO). The frequency of the VCO is determined by the voltage present at the control input. If the control voltage is increased or decreased, the frequency of the VCO increases or decreases, respectively. So whatever signal is applied to the control voltage shows up as the frequency of the oscillator. This is exactly what is required for frequency modulation. The audio modulating signal drives the control input of the VCO to produce a frequency modulated carrier. For amplitude modulation, a modulator circuit is added after the oscillator. The modulator circuit varies the amplitude of the VCO’s output without modifying its frequency. Thus, the output is an amplitude modulated signal. The actual block diagram used to implement the signal generator may be much more complicated (especially for high generators), but the conceptual block diagram is still a valid tool for understanding signal generators.

The frequency accuracy and stability are very important in a signal generator used to test receivers. The passband of the receiver is usually very narrow, much less than one percent of the frequency. An even better frequency accuracy is required for the test signal. Harmonic distortion is not usually very critical in receiver testing since the harmonics are well away from the frequency of the receiver. (For other high-frequency applications however, the harmonic distortion may be important.) Amplitude accuracy is important for accurately measuring the receiver’s sensitivity.

Higher quality signal generators have extremely good close-in side band performance (usually specified as phase noise). This is important in radio and other systems where even very low levels of noise close to the carrier can degrade the system performance. For instance, the rejection of a signal that's very close to but outside a radio receiver’s bandwidth is an important test of a receiver’s performance. This adjacent channel rejection test can be performed by tuning the signal generator to a frequency very close to the receiver’s frequency, and then increasing the generator’s output level until the receiver can no longer reject the signal. The signal generator must not have any close-in sidebands, otherwise these sidebands will spill over into the receiver pass-band and be detected as if they were a valid signal.

Fig 3-16. A conceptual block diagram of a signal generator with an audio source for modulating the carrier.

Sweep Generators

Sweep generators are sine wave sources that have the ability to change their frequency in a controlled manner. This swept- frequency capability is useful for testing circuits over a wide frequency range in a short amount of time. Of course, the sweep feature can be disabled and the source can be used to produce fixed-frequency sine waves. The sweep generator is often combined with a function generator to provide both types of operation in one instrument. Some function generators go so far as to supply sweep, pulse, and modulation capability in one box. A function generator that also includes sweep capability is shown in Figure 3-17.

Figure 3-17. Function generator capable of producing frequencies up to 13 MHz. This model also includes sweep and modulation capability.

Sweep generators generally sweep in frequency in a linear manner but may also be able to sweep logarithmically. A sweep voltage output which is in proportion to the frequency during a sweep is usually pro vided. This output can be used to drive other instruments, particularly an oscilloscope configured for a frequency-response test (see Section 5).

(The sweep voltage is often called the X-Drive output, since it's used to drive the X-axis of the oscilloscope display.) Figure 3-18 shows a sine wave being swept in frequency along with its corresponding sweep voltage. Figure 3-19 shows a simplified block diagram of a sweep generator. A voltage-controlled oscillator is driven by the sweep voltage to produce a frequency sweep. The output of the VCO is amplified and passes through a variable attenuator. The output impedance of the sweep generator is usually 50 or 600 ohms, depending on the frequency range.

Figure 3-18. The output of a sweep generator is a sine wave that increases in frequency. The sweep voltage is proportional to the output frequency.

Figure 3-19. Simplified block diagram of a sweep generator.

Arbitrary Waveform Generators

Figure 3-20. The simplified block diagram of an arbitrary waveform generator.

A special class of sources has recently emerged, which has the ability to generate arbitrary waveforms. A simplified block diagram is shown in Figure 3-20. Digital data representing the desired waveform is stored in digital memory (waveform memory). An address counter sequences through the addresses of the memory, causing the data in the memory to sequentially appear at the digital port of a digital-to-analog converter (DAC). The DAC produces a voltage proportional to the digital data supplied to it. Thus, as the address counter cycles through memory, the waveform data is transformed into a voltage waveform at the output of the DAC. A VCO (or other oscillator) is used to clock the address counter at a variable rate, allowing the waveform period (frequency) to be varied. The faster the address counter is clocked, the faster the memory is cycled through and the higher the waveform frequency. The output of the DAC is amplified and then passed through a variable attenuator to the output.

An arbitrary waveform generator can be used in a variety of applications where previously it was difficult or impossible to generate appropriate waveforms. Of course, it can also be used to generate any of the waveforms previously discussed (square, triangle, pulse), but the real utility of the generator is in simulating more complex signals. Figure 3-21 shows some examples of waveforms that can be produced using an arbitrary waveform generator. Imperfections in signals (such as overshoot in a square wave and glitches in a digital signal) can be simulated in a controlled manner to determine how sensitive a circuit is to such problems. Also, transient signals such as a damped sine wave can be produced. Another advantage of the arbitrary waveform generator is the ability to quickly and precisely change frequency (frequency hop).

The application of arbitrary waveforms is limited only by the number of different signals present in the world. Any waveform that can be put in a digital form and loaded into the waveform memory can be simulated. Of course, the arbitrary generator is still limited by the same bandwidth constraints that limit other instruments. Also, the address counter clock rate and the size of the waveform memory limit the frequency range of the generator. The actual implementation of the arbitrary waveform generator varies from compact, stand-alone instruments to generators that are dependent on an external computer to load the wave form data. Many generators supply a “waveform calculator” which allows the user to easily define the arbitrary waveform, without manually entering each waveform point.

Figure 3-21. Examples of waveforms produced by an arbitrary waveform generator.

Synthesized Sources

In a traditional signal source, the waveform is produced by one or more free-running oscillators which are adjusted to operate over some frequency range by varying the values of components, usually combinations of capacitors, inductors and resistors. In a synthesized signal source, one or. more reference oscillators operate at a fixed frequency which is then used by the synthesizer circuitry to produce the desired frequency (Figure 3-22). Because the reference oscillator does not have to change frequency, it can be made extremely stable. The synthesizer consists of digital dividers and other circuits which have no frequency variation associated with them. So the source frequency is as stable (and accurate) as the reference oscillator. The result is a variable frequency source that has the precise frequency control that's normally only achievable with a fixed frequency oscillator.

Synthesized sources don't usually have a single-frequency accuracy specification. Instead, the reference oscillator’s frequency drift is specified (usually in parts per million per month or year). The section 6 section on frequency counters discusses the interpretation of this type of specification.

Figure 3-22. Simplified block diagram of a synthesized source has a reference oscillator that's used by the synthesizer to produce the desired output frequency.

Frequency Locking Multiple Sources

The reference oscillator is usually designed such that it can be electronically locked to other synthesized sources. Each source provides a reference output (usually 1, 5 or 10 MHz) as well as a reference input. The reference output of one source is connected to the reference input of one or more other sources (Figure 3-23). These sources lock their internal reference oscillators to the reference output of the original source. This results in all of the sources being locked precisely in frequency to one reference oscillator. Two synthesized sources that are locked together exhibit essentially zero frequency drift between them. (Two free-running sources will drift apart in frequency quite noticeably.)

Figure 3-23. Multiple synthesized sources can be locked together in frequency using the reference inputs and reference outputs.

Some sources are simply called synthesizers because their primary purpose is precise frequency synthesis. However, as technology has improved frequency synthesis techniques have been used to implement all types of signal sources, especially signal generators, sweep generators, and function generators. These instruments perform the same basic function as previously outlined, but when synthesized, their frequency accuracy and stability are greatly improved. Figure 3-24 shows a synthesized source that combines function generator waveforms, frequency sweep capability, and modulation into one instrument with synthesizer frequency accuracy.

Figure 3-24. A synthesized source that has function generator waveforms as well as frequency sweep and modulation capability.

Table 3-1 summarizes the basic characteristics of the various types of sources. It reflects the specifications of a typical instrument in each category. Because of the overlap often found between the categories of sources, this table is only a general guide as to the specifications and capabilities of each type of source. Actual specifications and capability will vary from instrument to instrument.

EXAMPLE 3-2 Of the types of sources described in this section, which ones would suit the following applications: 1. Generate a 1-MHz digital clock? 2. Provide a 2-kHz sine wave for audio testing? 3. Generate a 29.6-MHz frequency modulated sine wave for testing a radio receiver?

1. Any source capable of outputting a square wave, with perhaps some DC offset: function generator or pulse generator.

2. All of the sources except the pulse generator can output a sine wave, but signal generators usually don’t go that low in frequency sine wave source, function generator, sweep generator.

3. Although modulation is sometimes supplied in other sources, they typically don’t go high enough in frequency: signal generator.

Table 3-1: Signal Generator characteristics

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