On July 1, 2020, this post will become obsolete as the 2020 Extra Class question pool goes into effect. Please refer to the post from the 2020 No Nonsense Extra Class Study Guide instead.
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.
Voltmeters
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.
RF measurements
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 to modulate 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
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)