Test Equipment and It’s Uses
Earlier levels of your license focused on basic electronics test equipment like multimeters. Those measure the performance of different circuit components. As an Amateur Extra you are expected to know more about designing and testing circuits. That necessitates understanding a broad range of test gear.
On a multimeter, voltage testing is a primary function, and that element is called a voltmeter. You might even have a stand-alone voltmeter. Voltmeter sensitivity is expressed in ohms per volt. The full scale reading of the voltmeter multiplied by its ohms per volt rating will indicate the input impedance of the voltmeter.
Voltmeters typically read out in numbers. Those could be on a scale, or a digital display. What if you want to see the voltage on a graphical plot? You would move to an oscilloscope. On an oscilloscope you connect two probes across the circuit to get a reading.
Both your voltmeter and your oscilloscope have a probe to be connected to ground. On an oscilloscope that probe is connected to a signal ground. It’s good practice to keep the signal ground connection of the probe as short as possible. This practice avoids possible interference.
Illustration of oscilloscope connected to a circuit
Let’s go over a couple of oscilloscope settings. It’s important to adjust your probe to help with accurate bandwidth readings of the signal. That’s called probe compensation, and here’s how to do it. On the scope a square wave is displayed and the probe is adjusted until the horizontal portions of the displayed wave are as nearly flat as possible. When you see a near perfect square-wave, you will know the compensation is set correctly.
One of the things you may measure is the ripple of a linear power supply. That’s the fluctuation in the voltage as load increases. The line trigger mode on an oscilloscope is most effective to measure an output ripple.
Your oscilloscope measures changes over time, like every second. This is called the sample rate. A digital oscilloscope has an analog-to-digital signal converter inside. The sampling rate of the analog-to-digital converter limits the highest frequency signal that can be accurately displayed. This is to ensure proper readings. Your sample rate on a digital oscilloscope can be too slow. Then it’s subject to an effect called aliasing. When aliasing happens, a false, jittery low-frequency version of the signal is displayed.
Analog-to-Digital converter
You may want tools to measure the frequency of a signal over time. A frequency counter and a spectrum analyzer both do that in different ways.
Think of a frequency counter as a wide view of the signal across hundreds of megahertz. A frequency counter also measures a frequency over time. This means time base accuracy most affects the accuracy of a frequency counter. Frequency counters have their bandwidth limits as well. Using a tool called a prescaler will reduce the signal frequency to within the counter’s operating range.
Now to the more granular frequency test tool, a spectrum analyzer. Feed a signal in and get a display of signal amplitude and frequency on the vertical and horizontal axes.
Illustration of spectrum analyzer
You can use your spectrum analyzer on an SSB transmitter. It will display the generated spurious signals and/or intermodulation distortion products. How would you measure that intermodulation distortion in an SSB transmitter? Modulate the transmitter using two AF signals having non-harmonically related frequencies and observe the RF output with a spectrum analyzer. We’ll get into intermodulation in more detail in an upcoming lesson.
Moving now to look at tools to analyze antenna networks. We’re calling these “antenna networks” so we cover all elements between the radio and antenna. That the feedline, filters, and any other items in the transmission path. These tools will help you manage the SWR or Standing Wave Ratio of your antenna network. Remember, an optimal SWR reading is 1:1.
To measure SWR your tool options include:
- a directional wattmeter,
- a vector network analyzer or VNA, and
- an antenna analyzer
All these choices are correct when asked about SWR measurement tools.
A directional power meter is also known as a directional wattmeter. They measure the power going through a feed line. You can remember this because power is measured in watts. The meter is connected between a transmitter and a terminating load. How do you know how much power is being absorbed by the load? Let’s say the meter reads 100 watts forward power and 25 watts reflected power. Subtract the 25 from 100 to get the answer. 75 watts is being absorbed by the load.
An SWR Bridge is a similar, lower cost item. It typically has a cross-needle display. It measures forward and reverse power as you operate. It’s nice to have an SWR bridge for instant readings while operating.
You’ll find an advantage to using a dedicated antenna analyzer for long term setups. Antenna analyzers compute SWR and impedance automatically. This is because they have a built-in RF signal generator.
That built in generator helps measure items across the entire antenna network, including:
- the velocity factor of wire,
- cable length, and
- the resonant frequency of a tuned circuit
All these choices are correct when asked about antenna analyzer measurements.
Let’s take an antenna analyzer to the next level by using a vector network analyzer or VNA. You can use a VNA to measure:
- input impedance
- output impedance and
- Reflection coefficient
All of these are correct when asked about a VNA on the exam.
One of the first things you will notice about a VNA is that it has two ports, labeled S11 and S21. The subscripts of S parameters represent the port or ports at which measurements are made. If you want to measure forward gain of your antenna network, connect to the S21 port. The port labeled S11 represents input port return loss or reflection coefficient. That is equivalent to VSWR or Voltage Standing Wave Radio. One of the benefits of having a two-port VNA is the ability to measure filter frequency response.
Most of the smaller VNA’s on the market require regular calibration. When calibrating a VNA, you should use three different test loads: Short circuit, open circuit and 50 ohms.
