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E4A – Test equipment: analog and digital instruments; spectrum and network analyzers, antenna analyzers; oscilloscopes; RF measurements; computer aided measurements
An instrument that amateur radio operators frequently use when experimenting or when debugging equipment is the oscilloscope, or simply just “scope.” Oscilloscopes have become more common in amateur radio shacks as the price has fallen and the technology has moved from analog to digital.
Analog oscilloscopes use amplifiers, filters, and other analog signal processing circuits to display an input signal on a cathode-ray tube, or CRT. Digital oscilloscopes, on the other hand, use an analog-to-digital converter to convert the input signal into a series of numbers, which are then processed by a computer and displayed on an LCD screen. All of these choices are correct when talking about the advantages of a digital vs. analog oscilloscope: (E4A05)
- Automatic amplitude and frequency numerical readout
- Storage of traces for future reference
- Manipulation of time base after trace capture
One of the most important oscilloscope specifications is its bandwidth. The bandwidth of an oscilloscope determines the maximum frequency at which the oscilloscope can accurately measure a signal. While the characteristics of the analog signal processing circuits determine the bandwith of an analog oscilloscope, sampling rate is the parameter that determines the bandwidth of a digital or computer-based oscilloscope. (E4A01) Similarly, the analog-to-digital conversion speed of the soundcard determines the upper frequency limit for a computer soundcard-based oscilloscope program. (E4A04)
Because digital oscilloscopes sample an input signal at discrete time intervals, it is possible to fool them into displaying an incorrect waveform. This phenomenon is called aliasing. The effect of aliasing in a digital or computer-based oscilloscope is that false signals are displayed. (E4A06) When using a computer’s soundcard input to digitize signals, the highest frequency signal that can be digitized without aliasing is one-half the sample rate. (E4A09)
When making measurements at RF frequencies, it’s important to connect the probe’s ground connection as close to the location of the measurement as possible. Keeping the signal ground connection of the probe as short as possible is good practice when using an oscilloscope probe. (E4A11) Keeping this connection as short as possible reduces the noise picked up by the probe and reduces the inductance of the connection, which in turn, makes the measurement more accurate.
Good quality passive oscilloscope probes have an adjustable capacitor in them that needs to be adjusted so that the probe capacitive reactance is at least nine times the scope input capacitive reactance. When this capacitor is adjusted properly, we say that the probe is properly compensated, and the scope will display the waveform with as little distortion as possible.
How is the compensation of an oscilloscope probe typically adjusted? A square wave is displayed and the probe is adjusted until the horizontal portions of the displayed wave are as nearly flat as possible. (E4A13) High-quality oscilloscopes will have a special square-wave output specifically for the purpose of compensating probes.
Spectrum analyzers display the amplitude of signals in the frequency domain Frequency is the parameter a spectrum analyzer would display on the horizontal axis. (E4A02) The drawing below shows typical displays from an oscilloscope and a spectrum analyzer. Spectrum analyzers are very useful for troubleshooting problems. For example, a spectrum analyzer is used to display spurious signals from a radio transmitter. (E4A03)
Because spectrum analyzers are sensitive instruments, you need to be cautious when using them. For example, an important precaution to follow when connecting a spectrum analyzer to a transmitter output is to attenuate the transmitter output going to the spectrum analyzer. (E4A12) Not doing so could damage the spectrum analyzer because its input circuits are not designed to handle high power.
One of the instruments that I think every amateur radio operator should have (or at least have access to) is the antenna analyzer. Antenna analyzers are versatile instruments that allow amateur radio operators to easily make antenna measurements, as well as other impedance measurements. They can even be used as low power RF signal generators.
An antenna analyzer is the instrument that would be best for measuring the SWR of a beam antenna. (E4A08) Actually, it’s the best instrument for measuring the SWR of any kind of antenna. That’s what they’re made for! When measuring antenna resonance and feed point impedance with a portable antenna analyzer, connect the antenna feed line directly to the analyzer’s connector. (E4B11)
An advantage of using an antenna analyzer compared to an SWR bridge to measure antenna SWR is that antenna analyzers do not need an external RF source. (E4A07) What this means is that you don’t need to connect your transmitter to the antenna to tune it. This is because they have an internal RF signal generator.
Frequency counters, logic analyzers
To measure the frequency of a signal, you use an instrument called a frequency counter. When selecting a frequency counter, an important specification is the maximum frequency. If you want to measure the frequency of a signal whose frequency is higher than the maximum frequency of your counter, you might use a prescaler. The purpose of a prescaler function on a frequency counter is to divide a higher frequency signal so a low-frequency counter can display the input frequency. (E4A14)
Most frequency counters work by counting the number of cycles of a signal during a given time period. An alternate method of determining frequency used by some counters is period measurement plus mathematical computation. An advantage of a period-measuring frequency counter over a direct-count type is that it provides improved resolution of low-frequency signals within a comparable time period. (E4A15)
The proper operation of a digital circuit depends on the output state of many nodes at specific times in a circuit. To ensure that a circuit is working properly, or to troubleshoot a circuit, you may want to use a logic analyzer. A logic analyzer displays multiple digital signal states simultaneously. (E4A10)
E4B – Measurement techniques: Instrument accuracy and performance limitations; probes; techniques to minimize errors; measurement of Q; instrument calibration
One thing about test instruments is that you need to take the readings with a grain of salt. By that, I mean that chances are that the instrument reading is not exactly the value of the parameter you’re measuring. The reason for this is that no instrument is 100% accurate.
Let’s consider frequency counters. Frequency counters are useful instruments for measuring the output frequency of amateur radio transceivers. While a number of different factors can affect the accuracy of an instrument, time base accuracy is the factor that most affects the accuracy of a frequency counter. (E4B01) The time base accuracy of most inexpensive frequency counters is about 1 part per million, or 1 ppm.
Now, let’s see how that affects the accuracy of a frequency measurement. If a frequency counter with a specified accuracy of +/- 1.0 ppm reads 146,520,000 Hz,146.52 Hz is the most the actual frequency being measured could differ from the reading. (E4B03) Practically, what this means is that while the frequency counter reads 146,520,000 Hz, or 146.52 MHz, the actual frequency of the signal might be as low as 146.519853 Mhz or as high as 146.520147 MHz.
More accurate—and therefore more expensive—frequency counters might have a specified accuracy of .1 ppm. If a frequency counter with a specified accuracy of +/- 0.1 ppm reads 146,520,000 Hz, 14.652 Hz is the most the actual frequency being measured could differ from the reading. (E4B04) This is very accurate for amateur radio work.
Very inexpensive frequency counters might have an accuracy of only 10 ppm. If a frequency counter with a specified accuracy of +/- 10 ppm reads 146,520,000 Hz,1465.20 Hz is the most the actual frequency being measured could differ from the reading. (E4B05) This might be adequate for amateur radio work, but as you can see, the difference between the frequency counter’s reading and the signal’s actual frequency can be up to ten times as much as with the frequency counter with a 1 ppm accuracy.
Probably the most common test instrument in an amateur radio station is a voltmeter. The voltmeter may be part of a digital multimeter (DMM) or volt-ohm meter (VOM). DMMs have the advantage of high input impedance, and high impedance input is a characteristic of a good DC voltmeter. (E4B08) The higher the input impedance, the less effect the meter will have on the measurement.
The quality of a VOM is given by the VOM’s sensitivity expressed in ohms per volt.The full scale reading of the voltmeter multiplied by its ohms per volt rating will provide the input impedance of the voltmeter. (E4B12) A higher ohms per volt rating means that it will have a higher input impedance than a meter with a lower ohms per volt rating.
Directional power meters and RF ammeters are two instruments that you can use to make antenna measurements. With a directional power meter, you could measure the forward power and reflected power and then figure out how much power is being delivered to the load and calculate the SWR of the antenna system. For example, 75 watts is the 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. (E4B06)
With an RF ammeter, you measure the RF current flowing in the antenna system. 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 it means there is more power going into the antenna. (E4B09)
There are a number of instruments that you can use to measure the impedance of a circuit. An antenna analyzer is one. Some sort of bridge circuit is another. An advantage of using a bridge circuit to measure impedance is that the measurement is based on obtaining a signal null, which can be done very precisely. (E4B02)
That’s the principle behind the dip meter. You adjust the meter’s controls so that the reading “dips” to a minimum value. The controls then indicate the resonant frequency. When using a dip meter, don’t couple it too tightly to the circuit under test. A less accurate reading results if a dip meter is too tightly coupled to a tuned circuit being checked. (E4B14)
For some experiments, you’ll want to know not only the resonant frequency of a circuit but also the quality factor, or Q, of the circuit. The bandwidth of the circuit’s frequency response can be used as a relative measurement of the Q for a series-tuned circuit. (E4B15)
Another type of instrument that you can use to make impedance measurements is the vector network analyzer. As with any instrument, you need to ensure that it is calibrated properly. Three test loads used to calibrate a standard RF vector network analyzer are short circuit, open circuit, and 50 ohms. (E4B17)
Finally, a method to measure intermodulation distortion in an SSB transmitter is tomodulate the transmitter with two non-harmonically related audio frequencies and observe the RF output with a spectrum analyzer. (E4B10) The instrument we use to do this is called, oddly enough, a two-tone generator. Typically, these generators provide tones of 700 Hz and 1,900 Hz simultaneously.
S-parameters, or scattering parameters, are used to describe the behavior of RF devices under linear conditions. Each parameter is typically characterized by magnitude, decibel and phase.
The subscripts of S parameters represent the port or ports at which measurements are made. (E4B07) The S parameter that is equivalent to forward gain is S21. (E4B13) The S parameter that represents return loss or SWR is S11. (E4B16)
E4C – Receiver performance characteristics, phase noise, noise floor, image rejection, MDS, signal-to-noise-ratio; selectivity; effects of SDR receiver non-linearity
In the past, sensitivity was one of the most important receiver performance specifications. Today, instead of sensitivity, we speak of a receiver’s minimum discernible signal, or MDS. The MDS of a receiver is the minimum discernible signal. (E4C07) This is the weakest signal that a receiver will detect. One parameter that affects a receiver’s MDS is the noise figure. The noise figure of a receiver is the ratio in dB of the noise generated by the receiver compared to the theoretical minimum noise. (E4C04)
A related specification is the noise floor. When we say that the noise floor of a receiver has a value of -174 dBm/Hz, it is referring to the theoretical noise at the input of a perfect receiver at room temperature. (E4C05) If a CW receiver with the AGC off has an equivalent input noise power density of -174 dBm/Hz, the level of an unmodulated carrier input to this receiver would have to be -148 dBm to yield an audio output SNR of 0 dB in a 400 Hz noise bandwidth. (E4C06)
Another important receiver specification is selectivity. A receiver’s selectivity is the result of a lot of things, including the filters a receiver has. 300 Hz is a desirable amount of selectivity for an amateur RTTY HF receiver. (E4C10) 2.4 kHz is a desirable amount of selectivity for an amateur SSB phone receiver.(E4C11)
In addition to a 300 Hz filter and a 2.4 kHz filter, high-end receivers also have filters called roofing filters. A narrow-band roofing filter affects receiver performance because it improves dynamic range by attenuating strong signals near the receive frequency. (E4C13)
Back in the day, when superheterodyne receivers had intermediate frequencies, or IFs, in the 400 – 500 kHz range, image rejection was a problem. If there was a strong signal present on a frequency about two times the IF away from the frequency your receiver was tuned to, you might hear that signal. Accordingly,15.210 MHz is a frequency on which a station might be transmitting if it is generating a spurious image signal in a receiver tuned to 14.300 MHz and which uses a 455 kHz IF frequency. (E4C14)
One solution to this problem is to select an IF higher in frequency. One good reason for selecting a high frequency for the design of the IF in a conventional HF or VHF communications receiver is that it is easier for front-end circuitry to eliminate image responses. (E4C09) A front-end filter or pre-selector of a receiver can also be effective in eliminating image signal interference. (E4C02)
Another way to get rid of image signals is to use a narrow IF filter. An undesirable effect of using too wide a filter bandwidth in the IF section of a receiver is that undesired signals may be heard. (E4C12)
Because most modern transceivers use digital techniques to generate a local oscillator signal to tune a receiver, synthesizer phase noise might be a problem. An effect of excessive phase noise in the local oscillator section of a receiver is that it can cause strong signals on nearby frequencies to interfere with reception of weak signals. (E4C01)
Software-defined radio (SDR) is becoming more popular in amateur radio. It is, therefore, necessary to know something about SDR receiver characteristics. The SDR receiver’s analog-to-digital converter sample width in bits has the largest effect on an SDR receiver’s linearity. (E4C17) An SDR receiver is overloaded when input signals exceeds the maximum count value of the analog-to-digital converter. (E4C08) Distortion is caused by missing codes in an SDR receiver’s analog-to-digital converter. (E4C16)
Finally, here are two miscellaneous questions on receiver performance characteristics. Atmospheric noise is the primary source of noise that can be heard from an HF receiver with an antenna connected. (E4C15) Capture effect is the term for the blocking of one FM phone signal by another, stronger FM phone signal. (E4C03)
E4D – Receiver performance characteristics: blocking dynamic range; intermodulation and cross-modulation interference; 3rd order intercept; desensitization; preselector
One of the most commonly mentioned HF receiver specifications is blocking dynamic range. The blocking dynamic range of a receiver is the difference in dB between the noise floor and the level of an incoming signal which will cause 1 dB of gain compression. (E4D01) Cross-modulation of the desired signal and desensitization from strong adjacent signals are two problems caused by poor dynamic range in a communications receiver. (E4D02)
Another specification commonly bandied about is third-order intercept level. A third-order intercept level of 40 dBm with respect to receiver performance means a pair of 40 dBm signals will theoretically generate a third-order intermodulation product with the same level as the input signals. (E4D10) Compared to other products, third-order intermodulation products created within a receiver are of particular interest because the third-order product of two signals which are in the band of interest is also likely to be within the band. (E4D11)
The term for the reduction in receiver sensitivity caused by a strong signal near the received frequency is desensitization. (E4D12) Strong adjacent-channel signals can cause receiver desensitization. (E4D13) One way to reduce the likelihood of receiver desensitization is to decrease the RF bandwidth of the receiver. (E4D14)
A preselector might help in some cases. The purpose of the preselector in a communications receiver is to increase rejection of unwanted signals. (E4D09)
When operating a repeater, one thing that can occur is intermodulation interference, or simply intermod. Intermodulation interference is the term for unwanted signals generated by the mixing of two or more signals. (E4D06) Nonlinear circuits or devices cause intermodulation in an electronic circuit. (E4D08)
Intermodulation interference between two repeaters occurs when the repeaters are in close proximity and the signals mix in the final amplifier of one or both transmitters. (E4D03) The transmitter frequencies would cause an intermodulation-product signal in a receiver tuned to 146.70 MHz when a nearby station transmits on 146.52 MHz are 146.34 MHz and 146.61 MHz. (E4D05) We get this in the following way:
2 x 146.52 MHz – 146.34 MHz = 146.70 MHz and
2 x 146.61 MHz – 146.52 MHz = 146.70 MHz
A properly terminated circulator at the output of the transmitter may reduce or eliminate intermodulation interference in a repeater caused by another transmitter operating in close proximity. (E4D04) The circulator reduces intermodulation distortion because it helps to reduce the amount of energy from nearby transmitters that might get into a repeater’s final amplifier.
Cross modulation is a form of intermodulation. Cross modulation occurs when a very strong signal combines with a weaker signal and actually modulates the weaker signal. The most significant effect of an off-frequency signal when it is causing cross-modulation interference to a desired signal is that the off-frequency unwanted signal is heard in addition to the desired signal. (E4D07)
E4E – Noise suppression: system noise; electrical appliance noise; line noise; locating noise sources; DSP noise reduction; noise blankers; grounding for signals
Noise is often a real problem for radio amateurs. Fortunately, by understanding how noise is generated and how to reduce or eliminate it, noise can be tamed.
Atmospheric noise is naturally-occurring noise. Thunderstorms are a major cause of atmospheric static. (E4E06) There’s not much you can do to eliminate, but you can often use a receiver’s noise blanker to help you copy signals better. Signals which appear across a wide bandwidth (like atmospheric noise) are the types of signals that a receiver noise blanker might be able to remove from desired signals. (E4E03) Ignition noise is one type of receiver noise that can often be reduced by use of a receiver noise blanker. (E4E01)
One undesirable effect that can occur when using an IF noise blanker is that nearby signals may appear to be excessively wide even if they meet emission standards. (E4E09)
Many modern receivers now use digital signal processing (DSP) filters to eliminate noise. All of these choices are correct when talking about types of receiver noise can often be reduced with a DSP noise filter (E4E02):
- Broadband white noise
- Ignition noise
- Power line noise
One disadvantage of using some types of automatic DSP notch-filters when attempting to copy CW signals is that the DSP filter can remove the desired signal at the same time as it removes interfering signals. (E4E12)
While filters can be very effective at reducing noise, it is often better to figure out what is generating the noise and taking steps to reduce or eliminate the amount of noise generated in the first place. For example, one way you can determine if line noise interference is being generated within your home is by turning off the AC power line main circuit breaker and listening on a battery operated radio. (E4E07) If by doing this you determine that an electric motor is a problem, noise from an electric motor can be suppressed by installing a brute-force AC-line filter in series with the motor leads. (E4E05)
All of these choices are correct when it comes to the cause of a loud roaring or buzzing AC line interference that comes and goes at intervals (E4E13):
- Arcing contacts in a thermostatically controlled device
- A defective doorbell or doorbell transformer inside a nearby residence
- A malfunctioning illuminated advertising display
Sometimes your own equipment may be the cause of received noise. Cables in an amateur radio station, for example, can radiate or pick up interference. Common mode currents are the culprits. Common mode currents on the shield and conductors can cause shielded cables to radiate or receive interference. (E4E15) To eliminate this interference, make sure to ground the shield at one end of the cable. Common-mode current flows equally on all conductors of an unshielded multi-conductor cable. (E4E16)
Electrical wiring may also pick up interference. A common-mode signal at the frequency of the radio transmitter is sometimes picked up by electrical wiring near a radio antenna. (E4E08)
The main source of noise in an automobile is the alternator. Conducted and radiated noise caused by an automobile alternator be suppressed by connecting the radio’s power leads directly to the battery and by installing coaxial capacitors in line with the alternator leads. (E4E04)
Personal computer and other digital devices can also generate noise. One type of electrical interference that might be caused by the operation of a nearby personal computer is the appearance of unstable modulated or unmodulated signals at specific frequencies. (E4E14) All of these choices are correct when talking about common characteristics of interference caused by a touch controlled electrical device: (E4E10)
- The interfering signal sounds like AC hum on an AM receiver or a carrier modulated by 60 Hz hum on a SSB or CW receiver
- The interfering signal may drift slowly across the HF spectrum
- The interfering signal can be several kHz in width and usually repeats at regular intervals across a HF band
Noise can even be generated by the most unlikely things. For example, it is mostly likely that nearby corroded metal joints are mixing and re-radiating the broadcast signals if you are hearing combinations of local AM broadcast signals within one or more of the MF or HF ham bands. (E4E11)
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