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There are several instruments which use either cathode ray tubes, or LCD or similar flat screen technology. Some also interface to PCs for display.
The standard operating mode for these is to display amplitude (voltage) relative to time. Real CROs have the input driving an amplifier which applies high voltages to the electrostatic deflection plates on the small cathode ray tube to move the beam vertically, as the timebase drives the beam from left to right. Some, such as Aussie made BWD series used valves in the deflection circuitry, as did early Tektronix units (and certainly, were you offered an old CRO for the price of replacing a valve / tube, might be good value). Seriously expensive, and sometimes the size of a bar fridge, there were various analogue storage CROs.
Mostly off the exam, the basic controls are:
The Vertical (X-input) attenuator rotary switch, which sets the number of volts or millivolts per cm, usually with a small knob in the centre, which if off the "Cal" (Calibrated) detent allows you to adjust the displayed level. This knob is good for working relative levels, so if you had a CRO on the output of a low-pass filter, you might set the display to be 8cm high, then turn the frequency up until it reduces to 4 cm, indicating half the voltage, and a 6 dB reduction. There is a vertical position knob to position the trace.
An important switch is the AC - DC coupling switch for the input signal. Some signals have a DC offset, and we may or may not wish to display this. Importantly, a composite video signal has DC content, such as sync signals. This can be a AC - 0 - DC, switch, the zero displaying a straight line, used to set the position of the trace with 0 volts in. Some have a + and - switch, to invert the display.
The timebase is marked in nS, μS, mS, and Seconds per cm. Again, there is a smaller knob to adjust the timebase, for relative readings, or just to fit the waveform nicely on the screen. There is also a horizontal position knob.
The alternative mode of operation is to turn off the timebase, and to feed a second input, the Y input, with another signal. If the X and Y signals are exactly the same, a circle is displayed. If one frequency is is three times the other, then the resulting Lissajous curve is the shape of the ABC (Australian Broadcasting Corporation) logo.
There are also brightness and focus knobs, and typically slots for astigmatism (dot shape) and slope trimming.
On the rear there may be a Z-input used to blank the display. There are some interesting circuits online, to display numbers, or an entire clock face and hands, using X, Y and Z inputs.
Dedicated devices are used at TV waveform monitors, and vector scopes.
To help reduce the effect of capacitance in the leads and the CRO, 10x probes are often used. These have a small trimmer on them, which is adjusted against a 1V p-p output on the CRO, which is usually at the mains frequency. A well shaped square wave on the screen demonstrates good frequency response, the flat tops low frequency response, and sharp corners, good high frequency response.
Modern devices, called digital storage oscilloscopes use high speed digital sampling, and while there are CRT digital devices, most use LCD screens, some colour. Others use a PC as the display. The sampling rate in part dictates the bandwidth.
A DSO can save waveforms to devices such as USB memory sticks, for later comparison. They also allow you to vary the timebase once a waveform is captured, perhaps to zoom in on a "glitch" in a signal. They can automatically measure parameters such as voltage and frequency, and display them on the screen.
Before DSOs, recording a waveform involved a film or Polaroid camera, often mounted on a hood.
It is possible to use a PC soundcard, or a simple analogue to digital converter in a micro-controller to sample and display audio frequency signals, and so act as a simple digital CRO. The problem with these is that while a sample rate of 44.1 kHz or 48 kHz can represent a 20 kHz audio signal in a way which is acceptable to a human listener, these do not give a good graphical representation of the waveform; and more significantly, if the signal is above half the sample frequency, then "aliasing" will occur, resulting in false signals being displayed. While the question alludes to this problem with DSOs, professional DSOs sample at rates such as 1 or 2 GS/s (giga-samples per second), while only displaying 100 or 200 MHz, so typically at least 10 samples for each cycle, so there is at least some representation of the waveform, and we are no where near having aliasing problems.
Note that when displaying a square wave the vertical lines may be absent, or very dim, resulting in something like: –_–_–_
These display the amplitude of RF signals against a range of frequencies. They can show carriers and sidebands of a transmitted signal. They are useful for looking for spurious emissions and harmonics. When I was a young trainee one task was to use a "Spec-An" to evaluate the high level of harmonics generated by a dodgy FM band re-transmitter for a VHF narrow-casting receiver.
Off the exam, in some cases they are linked to a "tracking oscillator", in order to analyse the characteristics of a filter, including to set up cavity filters or "cans", used in repeaters. This pairing of equipment forms the equivalent of a Scalar Network Analyser, a device which displays only frequency and amplitude.
They can also be connected to an antenna to view signals in a band, and may be able to demodulate them.
When a signal passes through a filter, of is amplified, or matching network, its phase is often altered, and this may depend on its frequency. To analyse this, and other parameters of RF circuits, a Vector Network Analyser is used. These display both gain or loss factors, and the phase of the signal passing through the device under test, across a range of frequencies. VNAs have at least two ports. They include directional couplers to read reflected power on the output port.
The phase information can be useful for understanding the operations of things like complex cavity filters.
VNAs must be calibrated before use, using internal test programs, and three test connectors, a short; an open; and a load, typically 50 ohms, as in the exam. Off the exam, a through connector may also be used.
They often display using "S-parameters", or Scattering parameters. S21, often written using an italic and subscript, S21 is forward gain, and as it contains phase information, it is a complex number. S11 or S11 is return loss.
QEX (ARRL's technical magazine) included a home-built VNA using a DDS (Direct Digital Synthesis) IC, and a USB interface, using a PC as the display, back in 2004. Link.
These are fairly specialised, and you may never need to use one, unless you are building building high RF or microwave matching circuits, or working in RF engineering; or perhaps helping set up a repeater's filtering. Just remember 21 = Forward, 11 = Reflected. Perhaps to aid memory, to me 21 is more "substantial" a figure than 11, and forward power is larger than reflected power.
These cost $10,000 to $100,000!
Shortly after these questions were published, the NanoVNA was released. These are a handheld unit which features two SMA connectors, and an OLED or similar screen. They sell for around US$100, making them an alternative to the antenna analysers discussed below.
In both cases the Smith chart is a display option.
These are devices with a large number of inputs, which allow the timing of signals in digital equipment to be analysed. The display has multiple traces, showing the state of multiple lines, and some can do some degree of protocol analysis.
Appearing as a spoiler, these are used to look for opens and shorts in lengths of cable. A pulse is put into the cable, and any impedance variations, or circuit problems appear as positive or negative bumps on the trace, the result of reflections. They can be used for both coaxial and twisted pair cables, including by telephone company lines staff. They can also be used to locate joints at cable pillars or in pits, shown as smaller bumps. There are also optical versions, for evaluating fibre, or simply working out where the back-hoe induced discontinuity is...
Some with a numeric display and meter, others with graphical displays, these consist of an oscillator and SWR test function, they are directly connected to the antenna feedline, to evaluate the antenna's SWR and impedance.
Appearing as a distractor, a Bit Error Rate Tester (BERT) is used to test data transmission systems.
These are typically a desk-top unit, which displays the frequency of audio and radio signals on a LED, LCD, or fluorescent display. The typical unit now displays up to 3 GHz, with 10 digits. At the period lower frequencies, this means they can display fractions of a Hertz. If it is a direct counting device, long counting periods are necessary, 10s or more of seconds. If it measures period between pulses, high resolution readings can be made is shorter times.
For accuracy, serious counters feature a temperature stabilised (ovened) crystal oscillators, or are disciplined to Rubidium standard (atomic clock) and/or frequency output of a specialised GPS (GNSS) receiver.
At the other end of the scale, there are hand-held units, and frequency sniffers. Some multi-meters include frequency meters or counters, of varying accuracy.
Questions regarding accuracy of counters have been removed, but it is probably worth understanding that if your $99 counter disagrees with your $5,000 radio, you might be better trusting the radio, unless you are able to compare the couter to an accurate source. An error of 1ppm, means a reading of 147.250 MHz means the actual frequency, may be up to 147.25 Hertz away from this.
A prescaler is an IC, or a device, which can be fitted into a large old counter, or connected ahead of it, to allow it to read a (significantly) higher frequency, by dividing the signal down. It is possible that its inclusion is an original design feature.
We return to the topic of meter impedance on this paper. For a DC voltmeter, or a DC voltage range of a multimeter, high impedance is considered a desirable feature.
Analogue multimeters were sold with the ohms-per-volt figure as one of the headline specifications. Cheapies might have a 1000 ohms per volt figure, a good one having tens of thousands of ohms per volt. When I were a lad, the top-of-the-line analogue meter at Tricky-Dicky's was a FET input device, with 100,000 ohms per volt. Whatever the figure, this is multiplied by the voltage range, so a 1000 Ω/V jobbie on the 25 volt range means 25 kΩ; the FET one on 300 volts is 100,000 x 300 = 30,000,000 ohms, or 30 MΩ. These differ from the typical digital unit, which in most cases has a 10 MΩ input, regardless of range, although I note that some low cost ones are listed as 1M, and some have a higher impedance. A couple of moving coil meters in a retail catalogue are listed as 20 volts with 20,000 ohms impedance, and 30 volts with 30,000 ohms; so as you can see, they use a 1000 ohm / volt movement.
Off the exam, LED panel meters which power themselves from the supply they are testing have a low effective input impedance, as they are also powering the LEDs. They are however convenient for things like monitoring batteries during operation from a emergency operations van.
These can be placed in series with an antenna feedline. The greater the reading, the more power is going into the antenna. They can also locate peaks and nulls along open feed lines.
These are the actual questions from the Extra licence exam pool, as published by the NCVEC.
Which of the following limits the highest frequency signal that can be accurately displayed on a digital oscilloscope?
A. Sampling rate of the analog-to-digital converter
B. Amount of memory
C. Q of the circuit
D. All these choices are correct
The sampling rate is an important parameter in determining the bandwidth, answer C.
Which of the following parameters would a spectrum analyzer display on the vertical and horizontal axes?
A. RF amplitude and time
B. RF amplitude and frequency
C. SWR and frequency
D. SWR and time
RF amplitude and frequency, answer B.
Which of the following test instrument is used to display spurious signals and/or intermodulation distortion products in an SSB transmitter?
A. A wattmeter
B. A spectrum analyzer
C. A logic analyzer
D. A time-domain reflectometer
A spectrum analyser can display "garbage" produced by a transmitter, answer B.
How is the compensation of an oscilloscope probe typically adjusted?
A. A square wave is displayed and the probe is adjusted until the horizontal portions of the displayed wave are as nearly flat as possible
B. A high frequency sine wave is displayed and the probe is adjusted for maximum amplitude
C. A frequency standard is displayed and the probe is adjusted until the deflection time is accurate
D. A DC voltage standard is displayed and the probe is adjusted until the displayed voltage is accurate
Many oscilliscopes have a square-wave output, presented on a tab with a hole to which the probe can be clipped. The probe is adjusted so the displayed trace is a square as possible, answer A. ⎍
What is the purpose of the prescaler function on a frequency counter?
A. It amplifies low level signals for more accurate counting
B. It multiplies a higher frequency signal so a low-frequency counter can display the operating frequency
C. It prevents oscillation in a low-frequency counter circuit
D. It divides a higher frequency signal so a low-frequency counter can display the input frequency
It enables a high frequency signal to be divided down to a lower frequency, allowing an older or lower cost counter to be used, answer D.
What is the effect of aliasing on a digital oscilloscope caused by setting the time base too slow?
A. A false, jittery low-frequency version of the signal is displayed
B. All signals will have a DC offset
C. Calibration of the vertical scale is no longer valid
D. Excessive blanking occurs, which prevents display of the signal
It will result in false signals being displayed, answer A.
Which of the following is an advantage of using an antenna analyzer compared to an SWR bridge to measure antenna SWR?
A. Antenna analyzers automatically tune your antenna for resonance
B. Antenna analyzers do not need an external RF source
C. Antenna analyzers display a time-varying representation of the modulation envelope
D. All of these choices are correct
These do not need an RF source, such as a transceiver, answer B.
They can also more easily test antennas outside Amateur bands, to help you find the resonant frequency. The information they provide helps you adjust variosu aspects of the antenna, such as lengths of elements, and perhaps the angle of radials on vertical antennas for VHF.
Which of the following measures SWR?
A. A spectrum analyzer
B. A Q meter
C. An ohmmeter
D. An antenna analyzer
This is the antenna analyser, answer D.
Which of the following is good practice when using an oscilloscope probe?
A. Keep the signal ground connection of the probe as short as possible
B. Never use a high impedance probe to measure a low impedance circuit
C. Never use a DC-coupled probe to measure an AC circuit
D. All of these choices are correct
The ground connection should be short, answer A.
Which of the following displays multiple digital signal states simultaneously?
A. Network analyzer
B. Bit error rate tester
C. Modulation monitor
D. Logic analyzer
This is the logic analyser, answer D.
These have a multiple inputs, and are used for analysing data and address buses, etc.
How should an antenna analyzer be connected when measuring antenna resonance and feed point impedance?
A. Loosely couple the analyzer near the antenna base
B. Connect the analyzer via a high-impedance transformer to the antenna
C. Loosely couple the antenna and a dummy load to the analyzer
D. Connect the antenna feed line directly to the analyzer's connector
Connect its feedline directly to the analyser, answer D.
Which of the following factors most affects the accuracy of a frequency counter?
A. Input attenuator accuracy
B. Time base accuracy
C. Decade divider accuracy
D. Temperature coefficient of the logic
It is the accuracy of the time-base, answer B.
What is the significance of voltmeter sensitivity expressed in ohms per volt?
A. The full scale reading of the voltmeter multiplied by its ohms per volt rating will indicate the input impedance of the voltmeter
B. When used as a galvanometer, the reading in volts multiplied by the ohms per volt rating will determine the power drawn by the device under test
C. When used as an ohmmeter, the reading in ohms divided by the ohms per volt rating will determine the voltage applied to the circuit
D. When used as an ammeter, the full scale reading in amps divided by ohms per volt rating will determine the size of shunt needed
If a meter is 20,000 ohms per volt, and the range is 100 volts, the impedance is 2,000,000 ohms. On the 5 volt range it is just 100,000 ohms, and on 1000 volt range, it is 20,000,000 ohms. Thus, multiply the range by the ohms per volt figure to get the impedance, answer A.
Which S parameter is equivalent to forward gain?
S21, answer C.
Which S parameter represents return loss or SWR?
S11, answer A.
What three test loads are used to calibrate a standard RF vector network analyzer?
A. 50 ohms, 75 ohms, and 90 ohms
B. Short circuit, open circuit, and 50 ohms
C. Short circuit, open circuit, and resonant circuit
D. 50 ohms through 1/8 wavelength, 1/4 wavelength, and 1/2 wavelength of coaxial cable
Short circuit; open circuit; and its characteristic impedance, 50 ohms, answer B.
How much power is being absorbed by the load when a directional power meter connected between a transmitter and a terminating load reads 100 watts forward power and 25 watts reflected power?
A. 100 watts
B. 125 watts
C. 25 watts
D. 75 watts
This is the difference between the forward and reflected power, 100 - 25 = 75 watts, answer D.
What do the subscripts of S parameters represent?
A. The port or ports at which measurements are made
B. The relative time between measurements
C. Relative quality of the data
D. Frequency order of the measurements
They represent the port or ports at which measurements are made, answer A.
Which of the following can be used as a relative measurement of the Q for a series-tuned circuit?
A. The inductance to capacitance ratio
B. The frequency shift
C. The bandwidth of the circuit's frequency response
D. The resonant frequency of the circuit
The bandwidth of the circuit's frequency response, answer C.
What is indicated if the current reading on an RF ammeter placed in series with the antenna feed line of a transmitter increases as the transmitter is tuned to resonance?
A. There is possibly a short to ground in the feed line
B. The transmitter is not properly neutralized
C. There is an impedance mismatch between the antenna and feed line
D. There is more power going into the antenna
This indicates more power is going into the antenna, answer D.
Which of the following methods measures intermodulation distortion in an SSB transmitter?
A. Modulate the transmitter using two RF signals having non-harmonically related frequencies and observe the RF output with a spectrum analyzer
B. Modulate the transmitter using two AF signals having non-harmonically related frequencies and observe the RF output with a spectrum analyzer
C. Modulate the transmitter using two AF signals having harmonically related frequencies and observe the RF output with a peak reading wattmeter
D. Modulate the transmitter using two RF signals having harmonically related frequencies and observe the RF output with a logic analyzer
You need to modulate the transmitter with two non-harmonically related audio frequencies, and observe the RF output with a spectrum analyzer, answer B.
Which of the following can be measured with a vector network analyzer?
A. Input impedance
B. Output impedance
C. Reflection coefficient
D. All these choices are correct
All these can be done, answer D.
On to: Practices 2 - Receiver Performance
You can find links to lots more on the Learning Material page.
Written by Julian Sortland, VK2YJS & AG6LE, May 2022.
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