Technical Insights > RF + Microwave

Three Tricks for Measuring Low-Level Signals with Signal Analyzers

2019-02-05  |  5 min read 

Finding and measuring low-level signals can be an elusive and frustrating task. Low-level signals love to hang out where we can’t see them, like in the noise or the skirts of higher power signals. These signals could be interfering with your transmitter’s performance, but they can be very tricky to pinpoint.  Here are three easy tips to become a guru at finding those pesky little spurs:

  • Minimize your input attenuation
  • Narrow your resolution bandwidth
  • Add a pre-amp to your test setup

Let me walk you through how these three simple test configurations enable you to find and measure low-level signals.

Minimize your input attenuation

A signal analyzer generates its own noise after the attenuator, which will mix with your device’s signal.  Attenuation lowers your signal’s power, which is useful if you have a large signal – it gives you the ability to put in signals larger than what the internal components of the analyzer can accept. But with small signals, this becomes a problem. When you attenuate your signal, your signal gets smaller, but the analyzer’s noise stays constant. This reduces your signal-to-noise ratio, making it harder for you to see the difference between your signal and the noise of the analyzer.

If you are measuring a low-level signal, you probably don’t need to worry too much about blowing the front end of the signal analyzer, so drop down that attenuation and let your low-level signal be as strong as possible, allowing it to show itself over the noise of the analyzer.

Narrow your resolution bandwidth

A narrow resolution bandwidth (RBW) lowers the displayed average noise level (DANL), improving signal-to-noise ratio, getting you another step closer to finding your elusive low-level events. When you have a wide band, you have more frequencies in which to catch noise.  A narrow RBW covers a smaller frequency range, and therefore is going to limit the amount of noise at every instance of the measurement sweep, allowing those low-level signals to shine. 

RBW is also an important setting if you are trying to find a low-level signal that might be hidden by a high-level signal’s skirt. A narrow RBW will, as its name suggests, increase resolution, narrowing the skirts of signals and allowing nearby signals to be resolved separately on the analyzer’s screen.

 

Figure 1 - The yellow trace is a measurement result with a wide RBW.  The blue  trace is the same measurement with a narrow RBW, which has reduced the noise and resolved the signal of interest clearly

 

Add a pre-amp to your test setup

If these first two tips don’t do the trick, you can add an external component to boost your device’s signal.  Adding a pre-amplifier to your set-up can increase your signal-to-noise ratio. It’s the same concept as reducing attenuation. You want to increase your signal’s strength before it gets mixed with the noise of the signal analyzer. Just make sure to choose an amplifier with low noise and high gain. If the noise of the amplifier is too high, it will dominate the measurement.

In summary, find low-level signals by reducing input attenuation, narrowing resolution bandwidth, and connecting a pre-amp. By following these three simple tricks, you can transform your elusive signal from a snake in the grass to a giraffe in the grass. I hope these three tricks help you find the signals that once eluded you.