Using a tracking generator

-September 17, 2012

Many spectrum analyzers include tracking generators, but some new engineers my not completely understand what the purpose is and how they can help with certain measurements. In a nutshell, tracking generators are variable, or swept, RF generators that "track" with the spectrum analyzer sweep frequency. Some tracking generators have a limited frequency sweep as compared with their mating spectrum analyzer, so you need to know any frequency limitations in advance. For this example, I'll use the new Rigol DSA815TG and the built-in tracking generator indeed sweeps the same frequency range (9 kHz to 1.5 GHz) as the spectrum analyzer.

When activated, the tracking generator and spectrum analyzer function as a scalar network analyzer - that is, you may display |s21|, or amplitude, but not phase. While not quite as versatile as a true network analyzer, a scalar analyzer is useful for measuring filters, small-signal amplifiers, adapters, coax loss and attenuators. Let's test a simple 150 MHz low pass filter and a 10 dB attenuator.

The tracking generator in the Rigol DSA815TG is a bit limited in that it may be adjusted between only 0 and -30 dBm, however, as you'll see, this is still useful for the above measurements. First, let's take a look at the low pass filter.

150 MHz Low Pass Filter (Mini-Circuits BLP-150)

First, set the desired frequency range on the analyzer in the normal way. I used 1 to 400 MHz. Next, connect a short coax cable from the tracking generator output to the analyzer input. Press the TG button and turn on the tracking generator. Set the output amplitude to 0 (zero) dBm. The Rigol has a "normalize" feature, which will remove the effects of any connecting cable(s) and adapters. So to normalize the sweep, press TG > Normalize > Stor Ref. Now enable normalization by pressing TG > Normalize > Normalize On. You should now have a straight line sweep at the top of the display.

Figure 1 - The first step should always be to normalize the system.

Finally, disconnect one end of the cable and insert the filter. You should see the resulting response with a cutoff frequency near 150 MHz. Press the Marker button and using the dial, set the marker for -3 dB and read off the "3 dB bandwidth" frequency of 153.95, just within the advertised tolerance of 155 MHz.

Figure 2 - The Mini-Circuits 150 MHz filter is inserted between the tracking generator output and analyzer input.

Figure 3 - Filter response of the Mini-Circuits 150 MHz filter.

Rigol - North America has several application notes on how to use the DSA815TG tracking generator for filters and preamplifier measurements, as well as many other useful helps.

10 dB Attenuator (Mini-Circuits 15542-HAT-10+, rated to 2 GHz)

In this case, I'd like to see the frequency response over the entire range of the analyzer (9 kHz to 1.5 GHz), so, set the upper and lower frequencies. Because I'd like to see a high resolution display of the attenuation factor, I chose a scale per division of 2 dB (AMPT > Scale/Div). Then normalize as above using a short coax cable between the tracking generator output and analyzer input. Inserting the attenuator should ideally display as straight line at -10 dB.

In this case, the attenuator has some ripple (about +/- 0.4 dB) - still within tolerance and probably to be expected for a $20 device.

Figure 4 - Measurement of the Mini-Circuits 10 dB attenuator.

Figure 5 - 10 dB attenuator measurement.

For those who are managing semi-anechoic chambers, one measurement I highly recommend performing at regular intervals is a "system check" of the coax and all connectors between the measurement antenna and spectrum analyzer. By disconnecting the antenna and connecting the antenna coax cable to the tracking generator output (after normalizing with a short cable), you'll read the actual cable and connector losses between the antenna and analyzer.

Of course, this data could be imported into the analyzer as a correction factor, so that any cable losses are factored out. But perhaps the more important reason is to check the coax and connectors for frequency "suck-outs", which could indicate a loose connector, broken connector or damaged coax. We used to find issues regularly enough that it really was worth the time and trouble.

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