If your spectrum analyzer lacks a tracking generator, here's a low-cost method for plotting the frequency response of filters, coax cable loss, and other 2-port RF components.
One of the advantages in being active on LinkedIn is the opportunity to meet many new friends and colleagues, not to mention learning about new and interesting products. In this case, one of my connections, Michael Schutten (GE Global Research), posted a low-cost noise source in the EMC Troubleshooters group .
A noise source is simply a circuit that produces a very broad band of “shot noise” energy over a range of frequencies. This one is advertised to cover 200 kHz to 2 GHz and is available from eBay for $29 . The circuitry is mounted in a sturdy aluminum box with 12-V power, an LED power indicator, and SMA RF output. The universal power plug supplied includes screw clamps for attaching wires, a nice touch. The circuitry is merely a Schottky diode biased to produce shot noise, then amplified by three stages of broadband amplifiers.
Figure 1 Here’s the noise source with its universal power plug.
Figure 2 With the noise source cover removed, you can see the major circuit functions.
Now, what would one use a noise source for? Well, as an inexpensive replacement for a “tracking generator.” A tracking generator is simply an RF signal generator that produces a CW signal and tracks with the sweep of a spectrum analyzer. Many spectrum analyzers include an optional tracking generator and I always recommend clients adding this on to their purchase. I wrote an article on how to use tracking generators a few years ago . They can help plot out the frequency response of an RF filter, for example.
Figure 3 The frequency response of the noise source shows the system noise floor (aqua trace) and the output of the noise source in dBuV (yellow trace). We’re looking from 10 MHz to 6.2 GHz.
Figure 3 shows the RF output of the noise source and you can see it extends well beyond the advertised 2 GHz. In this case, we’re looking from 10 MHz to 6.2 GHz. Markers are placed near the knee of the plot (1.23 GHz) and at 2, 3, and 6 GHz. The noise spectrum is only down 12.7 dB from the reference marker (MR).
So, let’s measure a sample filter. I happened to have a bandstop filter designed for the AM broadcast band (540 to 1710 kHz). Placing the filter in series with the output should show us the bandstop frequencies and can be used to characterize the filter.
Figure 4 The test setup shows the bandstop AM broadcast filter in series with the noise source RF output.
I’m using the Tektronix RSA306B USB-powered spectrum analyzer , but any spectrum analyzer with the appropriate frequency range may be used.
Figure 5 The two outer markers are positioned at the limits of the AM broadcast band. The others mark the dips in the response. The aqua trace is the system noise floor, the yellow trace is the response without the filter, and the green trace is the filter response.
We can easily inspect the frequency characteristics of this filter and see that the response is at least 20 dB down, or more.
So, here’s a low-cost method for plotting the frequency response of filters, coax cable loss, and other 2-port RF components. The noise source should also be useful for measuring the gain characteristics of low-level preamplifiers, but you’ll likely need to reduce the RF output by at least 40 dB with an attenuator, so the preamp is not overdriven. One other use would be a measurement of shielding effectiveness . If your spectrum analyzer lacks a tracking generator, this would be an quick and inexpensive solution!
—Kenneth Wyatt is president and principal consultant of Wyatt Technical Services.