The Frequency Domain

Previously we looked at the oscilloscope at a time-domain voltimeter--a voltmeter that displays changes in voltage over time. Of course, nobody calls an oscilloscope a time-domain voltimeter. I use that name in this textbook to help illustrate its function. With that in mind, let's imagine a device that scans across the radio spectrum and stops when it encounters a strong signal. You can then listen to the signal and decide if you want to keep listening. If not, you can tell the device to scan further until it encounters another strong signal. If this sounds vaguely familiar, it should. I have just described a typical digital radio receiver. What you are doing with your radio is scanning the frequency domain. Old-time analog radios give a better visual example. As you rotate the tuning knob, a needle moves to indicate what frequency the radio is tuned to, lower frequencies to the left and higher frequencies to the right. You can hear the radio stations broadcasting on the various frequencies as you scan past them. Some of the stations, those nearer or more powerful, are louder, and others are fainter. This gives you a visual representation of the frequency spectrum showing the station's "positions" on the dial while you hear their relative power.

A manually operated volume-indicating analog frequency-domain analyzer.

As an oscilloscope displays voltage over time, your radio indicates power over the frequency spectrum. Welcome to the frequency domain.

A device that measures power across the frequency domain could be called a frequency-domain powerimeter. However, it's called a spectrum analyzer. One type of spectrum analyzer is a modified oscilloscope. A radio tuner scans across a frequency band as the beam scans from left to right. The power encountered at any particular frequency is displayed on the vertical axis.

A Hewlett Packard HP-8566B spectrum analyzer.

Another type of spectrum analyzer also works over time. These indicate the relative power present at a particular frequency by the display's brightness. The horizontal axis usually indicates frequency while the display scrolls vertically. The vertical axis indicates time. To see and hear this type of spectrum analyzer, you can go to websdr.org. Here you can listen to and operate web-based software amateur radio receivers. Most of these receivers include a spectrum analyzer so that you can see where signals are present. 

Part of a screenshot of a web-based software-defined radio receiver with the spectrum analyzer at the top of the display

The above illustration is a partial screenshot of the web-based software-defined radio receiver at http://websdr.k3fef.com:8901/. Follow the link to try it out. You can tune the radio by dragging the yellow indicator below the frequency scale.

Following this topic, we will use frequency domain graphs to explain how components and circuits work at different frequencies. Look for the frequency scale on the horizontal axis.