The spectrum analyzer-a primer
Over the past several years, the spectrum analyzer has become indispensable to the land mobile radio technician. Although functionally similar to the oscilloscope, the spectrum analyzer conveys different information. The display on an oscilloscope is a plot of amplitude in the time domain, or an amplitude vs. time plot. The spectrum analyzer produces a display of amplitude in the frequency domain, or an amplitude vs. frequency plot.
The spectrum analyzer is useful in studying interference and in troubleshooting radio equipment. This month we will take a closer look at some of the more important specifications and features of the spectrum analyzer and how it aids land mobile radio work.
The instrument Basically, the spectrum analyzer is a superheterodyne receiver with special filters, attenuators, amplifiers and display. High-end models feature many other special enhancements to extend and expand the usefulness of the instrument. Figure 1 below shows a simplified block diagram of a scaled-down version of a spectrum analyzer. At the far left is the input attenuator, followed by the input filter. The RF signal is fed into the mixer along with the swept local oscillator signal. The sweep generator also controls the display so that the horizontal sweep of the display is synchronized to the sweep of the local oscillator.
The bandwidth filter determines the basic resolution of the spectrum analyzer. After the bandwidth filter, the signal is fed to the logarithmic amplifier. This allows a greater range of signal amplitude to be displayed on the screen of the spectrum analyzer. The signal is then detected, cleaned up by the video filter and applied to the display circuitry.
Most spectrum analyzers are more sophisticated than the one depicted in this block diagram. Modern spectrum analyzers are digitized and controlled by microprocessors, which provides for programmability and greater versatility of the instrument.
The user does not always need all of the special features and enhancements provided by such a sophisticated instrument when performing general shop-type work; however, many times such an instrument will reveal problems that cannot be detected by the lower-quality “service-type” instruments. However, a service-type spectrum analyzer is better than none at all. Don’t mistake the spectrum display provided by many service monitors as a spectrum analyzer. Although some service montiors are better than others, none can equal a stand-alone analyzer instrument.
Basic requirements Certain basic requirements should be considered when purchasing a spectrum analyzer: c Stability – Instrument stability is important for maintaining a steady signal display on the screen over time. Instability will manifest itself as a constant drift of the display, especially when lower scan-width settings are used. The scan width refers to the frequency span per division, or the sweep width setting of the instrument. For example, if the scan width is set to 1MHz/div, and 10 horizontal divisions are on the graticule, then the total scan width is 10MHz.
c Frequency range – The instrument must be able to cover the desired frequency range. Typically, the spectrum analyzer used in land mobile radio work should, at a minimum, cover the frequency range of 100kHz to 1,000MHz. More is better-but more expensive, too.
c Scan width – The scan width determines how much (or how little) of the spectrum is displayed on the analyzer screen. At 100MHz/div, the total sweep width is 1,000MHz (for 10 horizontal divisions). Expect to see lots of clutter at that scan width from an off-the-air display-especially with the instrument set for medium to high sensitivity. Such a setting might be used to search for an interference signal or to look at the harmonic of a transmitter signal while simultaneously trying to view the fundamental. The lowest scan-width setting is important, too. For example, to view the individual sidebands of a FM transmitter, modulated by a 1kHz tone, the scan width should be set to 1kHz/div to separate the individual sidebands by one division. Some spectrum analyzers do not provide such a low scan-width setting because they don’t have sufficient resolution required for viewing this type of display.
c Resolution bandwidth – This is an important specification for a spectrum analyzer. The resolution bandwidth determines how far apart (in frequency) two (or more) signals must be to be resolved into separate and distinct displays on the analyzer. For example, if two signals are 1kHz apart, a spectrum analyzer with a resolution bandwidth of 10kHz could not resolve the signals into separate displays. Generally speaking, the resolution bandwidth should be about 10% of the signal separation for good resolution on the analyzer display. For example, to display two or more signals that are 1kHz apart, the resolution bandwidth should be set to 100Hz, and the scan width should be set to 1kHz/div.
c Sensitivity – Spectrum analyzer sensitivity will determine the minimum level of a signal that can produce a usable display. This will, in turn, depend on the noise floor or noise figure of the spectrum analyzer. The minimum detectable signal will not be less than the noise floor of the analyzer. The noise floor will depend on the resolution bandwidth and the video filter used (the greater the resolution bandwidth, the higher the noise floor). All other things remaining the same, decreasing the resolution bandwidth by a factor of 10 will drop the noise floor by 10dB. For example, if the noise floor is 2110dB at a resolution bandwidth of 10kHz, then the noise floor will drop to 2120dBm at a resolution bandwidth of 1kHz. A point is reached where further dropping the resolution bandwidth by a factor of 10 does not result in a 10dB improvement in the noise floor. If a signal input is equal to the noise floor of the analyzer (at a particular resolution bandwidth setting), then a 3dB “bump” will appear on the analyzer display. Because the input signal is equal in level to the noise floor, the two factors combine to be twice as much, or 3dB greater, than the noise floor.
c Input level – It is important that the input level to the spectrum analyzer be kept within the maximum rating of the instrument. The maximum rating is usually noted at the input connector. Observe this precaution at all times. High input levels can also cause overloading, which results in compression of the signal level and production of intermodulation components. According to the specifications for the Tektronix 495P spectrum analyzer, the third-order intermodulation product of two on-screen signals, within any frequency span, will be at least 70dB down.
Typical displays Photo 1 on page 18 shows a typical signal display. At this resolution bandwidth, not much can be learned about the modulation or sideband components of the signal. The scan width is 1MHz/div and the resolution bandwidth is 100kHz. This 10:1 ratio of scan width to resolution bandwidth produces a good display. However, for any modulation sideband components to be viewable, the scan width and the resolution bandwidth must be reduced.
Photo 2 on page 20 shows a frequency-modulated signal (1kHz modulating tone). At this resolution the individual sideband components cannot be seen or studied. Reducing the resolution bandwidth and the scan width allows the individual sideband components to be viewed and studied in detail, as shown in Photo 3 on page 20.
Photo 4 above shows an off-the-air signal centered around 162.5MHz. At the center of the screen, there is one signal (or, possibly, two signals) in close frequency spacing. Photo 5 above shows the same signals with improved resolution. We can now see that there are three different signals here that were obscured by the poor resolution in the display in Photo 4. You might recognize these as weather channels at 162.400MHz, 162.475MHz and 162.550MHz.
Photo 6 on page 22 shows a display resulting from the equipment setup shown in Figure 2 above. Here, a broadband noise generator, followed by a broadband RF amplifier, is connected to the input ofthe spectrum analyzer. With the scan width set to 10MHz/div and the center frequency set to 100MHz, the spectrum scan is from 50MHz to 150MHz. The display shows the response of the broadband noise generator and amplifier combination. The display in Photo 6 is quite noisy because no video filter is inserted. Photo 7 top, left, shows the same setup with the insertion of a 3kHz video filter as indicated at the bottom of the CRT screen. The filter provides a much cleaner and thinner trace for observing the frequency vs. amplitude response of frequency-sensitive devices, such as filters. The amplifier that is used in Figure 2 has a built-in FM-band notch filter that can be switched “in” or “out” as desired. In Photo 8 at the left, the FM broadcast band filter has been switched “in,” which is evident from the response curve shown on the CRT.
Photo 9 on page 26 shows a display resulting from the equipment setup shown in Figure 3 on page 28. This setup is used to check the bandpass response of a notch filter in the VHF highband. In Photo 9, you will notice that the frequency marker (the bright dot indicated by the red arrow) is placed over the deepest point in the notch response so that the notch frequency can be determined more accurately. As shown on the frequency marker readout, the frequency of the notch is 154.1MHz.
Another way that the filter response could be checked is shown in Figure 4 on page 28. Here, a signal generator (VFO type) is manually tuned across the notch frequency of the filter while the spectrum analyzer is set to maximum hold mode. As the signal generator is tuned across the notch frequency of the filter the filter’s response is “painted” on the screen and “frozen” on the display until the “max hold” is disabled. See Photo 10, top, right.
Photo 11 at the right shows an off-the-air display. Here, a taxicab company operating at 152.450MHz competes with a digital paging signal at 152.480MHz. (Looks like a little conflict here.) Imagine a taxicab operating near the paging transmitter and trying to hear the base station. The taxicab signal can also interfere with a pager that may be near the cab company’s transmitter location.
Tips and techniques Some precautions should be observed, and special techniques can be used, to get the most benefit from the spectrum analyzer and to avoid some common pitfalls:
c Scan loss – Using a sweep rate that is too high can cause serious amplitude distortion of the CRT display. When this happens, the amplitude of the display is less than it would be for a proper sweep rate. This is called scan loss. To check for a scan loss condition, reduce the sweep rate and watch for an increase in the amplitude on the display. If the amplitude of the display increases when the sweep rate is reduced, then a scan loss condition was occurring, and the sweep rate must be lowered for an accurate amplitude display.
For large scan widths, the sweep must be slow or the resolution bandwidth must be wide. Some spectrum analyzers have interlocking controls so that the proper sweep rate is selected for the scan width and for the resolution bandwidth. Other analyzers have auto resolution controls that automatically select the proper resolution bandwidth for selected sweep rate, scan width, video filter, etc. Some have warning indicators that warn the operator when the display is not calibrated because the sweep rate, scan width, resolution bandwidth or video filters are not properly set.
c Overload – If the spectrum analyzer becomes overloaded, intermodulation distortion products can be generated. These might be mistaken for real signals on the display. To determine whether a signal is real or false, simply increase the attenuator setting. A real signal will show a decibel-for-decibel drop. A second-order intermod product will show a 2:1 drop. A third-order intermod signal will show a 3:1 drop. So, if the signal display drops more than the attenuator change, it is a false signal, generated internally. Some spectrum analyzers have special provisions for identifying false signals.
c Dynamic range – When you are trying to view a weak signal in the presence of a strong signal, the limited dynamic range of the analyzer may cause the weak signal to be reduced in amplitude. A high-pass filter can be used to reduce the amplitude of the fundamental signal while passing the harmonic(s). The insertion loss of the filter at the fundamental frequency must be factored into the calculations to determine the level of the harmonic, relative to the fundamental frequency.
c Sensitivity – To improve the sensitivity of the spectrum analyzer, a wideband preamplifier can be placed ahead of the spectrum analyzer input. Let’s say that a wideband RF amplifier with a gain of 20dB and a noise figure of 4dB is used ahead of the spectrum analyzer, as shown in Figure 4 above. Suppose that the spectrum analyzer has a noise floor of 2100dBm at a resolution bandwidth setting of 10kHz. This would represent a noise figure of 34dB. Adding the preamplifier ahead of the spectrum analyzer (as previously described) would yield a total system noise figure of 14.4dB. This represents an improvement in the overall noise figure of almost 20dB. Thus, the sensitivity improvement is significant.
The right tool for the right job The requirement for a spectrum analyzer depends, of course, on how the spectrum analyzer is to be used. How precise are the measurement requirements? Is portability important? What about power supply requirements? Must the analyzer run on battery power? If the analyzer is to be used in the field frequently, it must be rugged and lightweight.
With the crowded spectrum in which we work, interference issues are popping up more often. The spectrum analyzer is an excellent tool to search out and identify possible interference signals. The best advice when purchasing a spectrum analyzer is to buy the best instrument that your budget will allow. Then, learn everything you can about the operation of the particular instrument so that the maximum use can be made of the instrument. Read the manual. Observe all precautions, and treat the instrument with care and respect. It can really save the day when properly used.
Until next time . stay tuned!