How do I use the MDO3000 to generate a frequency response curve?

The MDO3000 boasts a 50 MHz noise generator as a feature in the function generator, which can be used in conjunction with the wide-capture-bandwidth spectrum analyzer to produce a frequency response curve for an electronic system or device. The noise generator produces a signal at all frequencies up to 50 MHz that provides us with a baseline against which we can compare the frequency response of the device.

For this example, I have a Transciever Preselector, which can attenuate or amplify signals in different bands in the low MHz range. I’m generating a 50 MHz noise signal on the MDO3000 and am inputting that signal into the preselector. I am then connecting the output of the preselector to the spectrum analyzer input of the MDO3000. The preselector is currently turned off, so we see the baseline produced by the noise generator. To get a cleaner signal, I’m going to turn on the average trace on the spectrum analyzer by going to the RF menu, selecting detection method, selecting Manual, and changing the Average Trace mode to Average using the multipurpose knob A. Select the Spectrum Traces softkey, toggle the Normal trace Off and toggle the Average trace On. To get a good balance between update rate and displayed signal integrity, set the number of averages to 32. Then go into the frequency/span menu to adjust the span to the area of interest. When I turn on the preselector, you can see its frequency response. Any signal above the baseline is amplification and any signal below the baseline is attenuation. For example I see a -14 dBm peak at 2.6 MHz. Since the baseline signal was about -50dBm, I can see that I’m getting 36 dB of gain at the previous setting. As I adjust the gain and tuning on the preselector, I can see the instrument’s frequency response change accordingly.