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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). Refurbishing a BWD unit, perhaps with the help of a retired electronics technician, could a good learning experience. One tip - at least one switch on the circuit diagram was actually a dot of superglue on the end of the potentiometer track.
Tektronix made a wide range of CROs, with many consisting of a "mainframe" into which input modules are added. These could be analogue or digital, with various frequency and voltage capabilities, or be a spectrum analyser, or a TDR (below), or a curve tracer for transistors or valves.
Seriously expensive (as in the price of a car), and sometimes approaching 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 an 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.
There are often a few triggering options for the horizontal sweep, such as automatic, and ones which only trigger once a certain voltage is reached. They can also be triggered by an external source, perhaps a video sync line in a studio. New-fangled digital units can also trigger from the mains power waveform, useful when working on mains power supplies.
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 or intensity knob, often integrating the power switch; and a focus knob. Screwdriver slots allow 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.
There are units with various dual sweep modes, and fancy-pants ones with will display both the waveform of a video frame, and a specific line from that frame.
Dedicated devices are used as 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: –_–_–_
Some with a numeric display, and often an analogue 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. They cost several hundred dollars, towards 1k. While MFJ's might be the most well known, there are better units available, and perhaps also lower cost ones, but it is likely that you get what you pay for. One brand is Rig Expert. One group in South Australia were selling a kit version for HF at one point. At one point QST had a comparison article on various such units, with the
Ensure that the frequency range of the device matches the bands which interest you, noting that the highest band they typically work at is 70 cm, so for higher bands a VNA or NanoVNA is needed. Some are limited to 60 or 170 MHz, other MFJs to 520MHz, so work on UHF CB and licensed emergency comms frequencies. A few reach 650 or 700 MHz.
Be aware that there is some risk that signals from transmitters on the same site can damage the device. As always, read the fine manual before using test equipment.
These display the amplitude of RF signals across a range of frequencies, increasing left to right across the screen. They can show carriers and sidebands of a transmitted signal. They are useful for looking for spurious emissions and harmonics (multiples of the desired frequency). 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, or a matching network, or is amplified, 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 are very high-end devices, using high precision connector. They can cost $10,000 to $100,000 (!)
A few years ago, the NanoVNA was released. These are a handheld unit which features two or more SMA connectors, and an OLED or similar screen. They sell for around US$100, plus whatever tariffs can be legally imposed* (DOH!), also making them an alternative to the antenna analysers discussed below.
*Many have just been struck down by the Court, as courts must do when a branch of government fails to follow the law.
In both cases the Smith chart is a display option.
Removed from the exam, except as a distractor, but useful if building microcontroller based systems, or repairing the logic section of a modern transceivers: These are devices with a large number of inputs, which allow the timing of signals in digital equipment to be analysed, allowing the cause of glitches to be located. The display has multiple traces, showing the state of multiple lines. They can be stand-alone devices with a CRT or LCD, a box connected to as PC, or a module in a CRO, or function of a mixed signal CRO.
Some can do some degree of protocol analysis, perhaps displaying text in the data stream. A Protocol Analyser is a more advanced device used by telecommunications companies to analyse data streams used to test things such as links between bank data centres and branches. Telecom Australia (now Telstra) used to bear the cost of any theft of their motor vehicles, or damage to them, rather than make insurance company CEOs wealthy. Data staff had great difficulty getting through to fleet management that their vehicles (brand new Holden Commodores*, or Ford Falcons, both as wagons) needed alarms, etc. The cost of the vehicle paled into insignificance compared to the test equipment.
* Think Chevy SS or Vauxhall VX8R, but V6, and white.
Appearing as a distractor, a Bit Error Rate Tester (BERT) is used to test data transmission systems. Digital TVs may have a "quality" bargraph which effectively does this for the signal. They many be integrated into the above.
Appearing as a spoiler, TDRs are used to look for opens and shorts in lengths of cable. A pulse with a very rapid rise time 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...
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 counter to an accurate source. An error of 1 ppm, 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 the inclusion of such an IC is an original design feature.
Various modules are available from China for A$16 to 20; or for US$10 to 18, plus tariffs and fees. These are typically based on the Futitsu MB506, and have SMA in and out. Note that you may need to use a dummy-load with a sample output if the power of the oscillator, exciter or transmitter exceeds a few milliwatts. As is the case with most such ICs division is by a binary value, in this case 64, 128, or 512. The MC12093 IC does division by 2, 4, or 8.
If you have ever used a 4017 to make a LED chaser, a single LED, or the carry out lines both operate at 1/10 of the frequency of the clock, typically a 555, but I have even used a GNSS 1 pulse per second line because I got bored. However, these max out at 2 to 30 MHz depending on version, not 1.1 or 2.4 GHz.
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 (generally) 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.
1 volt over 10,000 ohms is 0.0001 amps, or 100 μA. Some meter movements require just 50 μA to reach full scale deflection, or FSD, meaning the meters using such a movement were 20,000 per volt. Many multimeters provided a 50 μA, meaning direct connection to the coil, so that any connection error could destroy it unless parallel diodes are used. A series resistor is needed to make these display a voltage. For 15 volts FSD in the 50 μA meter, which as a resistance of 2500 ohms, a 297,500 ohm resistance is required. While several precision resistors could be places in series to get this value, a 1% resistor and a single-turn or multi-turn variable resistor can be used, and the reading adjusted against a good multimeter, or a calibrated source. 294k + 2k + 1k5 would work. Only the 294 k needs to be a close tolerance item.
Standard resistor series can be found here: Logwell: Standard EIA Decade Resistor Values Table
Mouser has an extensive range of values. Round, but non-standard values such as 500 ohms are also available.
A shunt, meaning a low value resistor (perhaps even metal rods) placed in parallel can be used with these movements to form a current meter, which is also done within a multimeter. For a 10 amp full scale reading you need 9.99995 amps going through the shunt, leaving 50 μA for the meter. Going off-topic a little, a valve transmitter or amplifier may use a single movement, switched between measuring voltages and currents at various points in the device.
For completeness a standard analogue multimeter only needs a cell or battery for resistance measurement, unless it includes a FET. Vacuum Tube Volt Meters (VTVMs) were often mains powered. An "electrometer" has very high impedance.
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 operations in an emergency operations van. They should be checked against a good multimeter, and adjusted if necessary.
Also deleted from the paper, these can be placed in series with an antenna feedline. The greater the reading, the more power is going into the antenna. They can be connected inline, or can use a current transformer.
Alluded to in passing (as an SWR Bridge) SWR meters require a transmitter and are limited to its transmit frequencies. They indicate SWR, but do not indicate details such as inductive or capacitive components. Remember, a resonant (correctly tuned) Delta or similar full-wave loop will have an SWR of 2:1 , as it has a 102 ohm impedance.
Directional Wattmeters, such as the Bird 43 family need appropriate "slugs" for frequency range and power. SWR is calculated from forwards and reflected power. Likewise, they need a transmitter, and have the same limitations re understanding the impedance as an SWR meter.
These are the actual questions from the Extra licence exam pool, as published by the NCVEC.
E4A01
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.
E4A02
Which of the following parameters does a spectrum analyzer display on the vertical and horizontal axes?
A. Signal amplitude and time
B. Signal amplitude and frequency
C. SWR and frequency
D. SWR and time
RF amplitude and frequency, answer B.
E4A03
Which of the following test instruments is used to display spurious signals and/or intermodulation distortion products generated by an SSB transmitter?
A. Differential resolver
B. Spectrum analyzer
C. Logic analyzer
D. Network analyzer
A spectrum analyser can display this "garbage" produced by a transmitter, answer B.
E4A04
How is compensation of an oscilloscope probe performed?
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 oscilloscopes 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. ⎍
E4A05
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.
E4A06
What is the effect of aliasing on a digital oscilloscope when displaying a waveform?
A. A false, jittery low-frequency version of the waveform is displayed
B. The waveform DC offset will be inaccurate
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.
E4A07
Which of the following is an advantage of using an antenna analyzer compared to an SWR bridge?
A. Antenna analyzers automatically tune your antenna for resonance
B. Antenna analyzers compute SWR and impedance automatically
C. Antenna analyzers display a time-varying representation of the modulation envelope
D. All these choices are correct
These compute SWR and impedance automatically, 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 various aspects of the antenna, such as lengths of elements, and perhaps the angle of radials on vertical antennas for VHF. They do not need an RF source, such as a transceiver.
E4A08
Which of the following is used to measure SWR?
A. Directional wattmeter
B. Vector network analyzer
C. Antenna analyzer
D. All these choices are correct
This is the antenna analyser, answer D.
E4A09
Which of the following is good practice when using an oscilloscope probe?
A. Minimize the length of the probe's ground connection
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 these choices are correct
The ground connection should be short, answer A.
E4A10
Which trigger mode is most effective when using an oscilloscope to measure a linear power supply's output ripple?
A. Single-shot
B. Edge
C. Level
D. Line
Line mode uses the mains supply's rise to trigger, useful for looking at ripple on a power supply output, assuming it is linear, so the ripple is at a multiple of the mains frequency, answer D.
E4A11
Which of the following can be measured with an antenna analyzer?
A. Velocity factor
B. Cable length
C. Resonant frequency of a tuned circuit
D. All these choices are correct
These can determine the unknown of velocity factor or cable length, and the resonant frequency of circuits, including antennas, so all these are correct, answer D.
When using an antenna analyzer to measure antenna resonance and feed point impedance it is directly connected to the feedline.
E4B01
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.
E4B02
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 is the input impedance of the voltmeter
B. The reading in volts multiplied by the ohms per volt rating will determine the power drawn by the device under test
C. The reading in ohms divided by the ohms per volt rating will determine the voltage applied to the circuit
D. 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.
E4B03
Which S parameter is equivalent to forward gain?
A. S11
B. S12
C. S21
D. S22
S21, answer C.
E4B04
Which S parameter represents input port return loss or reflection coefficient (equivalent to VSWR)?
A. S11
B. S12
C. S21
D. S22
S11, answer A.
E4B05
What three test loads are used to calibrate an 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.
E4B06
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.
E4B07
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
These numbers, often written subscripted, represent the port or ports at which measurements are made, answer A.
E4B08
Which of the following can be used to determine the Q of a series-tuned circuit?
A. The ratio of inductive reactance to capacitive reactance
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.
Australia used to have high powered pager transmitters from 148.0125 MHz and up. A filter able to significantly attenuate these while passing repeater outputs at 147.375 MHz requires filter, such as a cavity filter, with narrow bandwith, meaning a high Q.
E4B09
Which of the following can be measured by a two-port vector network analyzer?
A. Phase noise
B. Filter frequency response
C. Pulse rise time
D. Forward power
Filters typically have one input and one output, so these are excellent candidates for testing with a VNA, answer B.
And yes, diplexers and duplexers have more connections, but can still be tested using a VNA.
E4B10
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.
Remember that we want a graphical representation of intermodulation products, or hopefully, the lack of them, hence the screen-based spec-an.
E4B11
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, August 2025.
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