Fortunately, we can make things better—for your signal analyzer
If you use even a gently aged spectrum analyzer, you may sometimes wonder why its amplitude accuracy above about 4 GHz is slightly worse than when it was new or after it has been freshly calibrated. Personally, I sometimes wonder why I cannot do the things I did when I was 20 years old.
In both cases aging is occurring. Although nothing is technically broken, we can make things better without magically locating a certain DeLorean car equipped with a flux capacitor and then driving 88 mph, hoping for a lightning strike, and traveling back in time.
What is a preselector, and why would it drift?
If we assume the instrument is a Keysight X-Series signal analyzer, what has probably happened is the preselector, sometimes called a YIG-tuned filter or tracking filter, has drifted a bit—but not enough to cause an out-of-specification situation. In an X-Series analyzer, the preselector is located in the signal path between the input attenuator and the first mixer, and it is used only at tuned frequencies of 3.6 GHz and higher.
The filter bandwidth should be wide enough to measure the desired signal, yet narrow enough to reject image frequencies and undesired signals (which may overload the first mixer). Depending on the tuned frequency, the bandwidth of the filter ranges from about 40 MHz to 75 MHz. Filter shape and ripple across the passband also vary with tuned frequency. As the analyzer tunes, the preselector filter tracks the change and provides a “centered” passband at the current frequency, as shown below.
Typical passband response of a YIG-tuned preselector
Instrument software automatically handles most of this preselector tuning; however, careful adjustment of the instrument will help deal with the rest.
Ensuring better performance
As the instrument ages, especially its preselector assembly, the filter bandpass will drift. As a result, the signal being measured might fall in an area of passband ripple or on a steeper portion of the filter response. Here are three tips to help ensure the best performance:
- For the absolute best amplitude accuracy, the Preselector Center function (accessible via the front panel or SCPI) uses internal calibration signals to vary the preselector filter tuning in real time and obtains the best possible tuning. Be forewarned that this routine is time-consuming. If you need the very best amplitude accuracy using the preselector, then re-center the preselector at each measurement frequency.
- Every three to six months, apply the Characterize Preselector routine. This performs “preselector centering” at various pre-determined frequencies up to the maximum frequency range of your analyzer. The analyzer stores the tuning values and automatically uses them the next time the analyzer is tuned to those frequencies. One advantage: after this routine runs, you may not need to rely on the slower Preselector Center routine (above). No external equipment is required: simply press System, Alignments and Advanced then select Characterize Preselector.
- Bypass the preselector filter. If your instrument contains option MPB, microwave preselector bypass, you can select the bypass path and remove the preselector from the signal path. The downside: the instrument is no longer filtering the input signals (i.e., it isn’t “preselected”). Depending on the span setting, you may see image frequencies that are not being rejected by the preselector and so appear at the first mixer. The advantage: the bandwidth is about 800 MHz at the first mixer, preselector drift is no longer an issue, and measurement speed may increase because the instrument is no longer trying to avoid oversweeping the preselector filter.
More detail is available in our preselector tuning application note.
Three closing comments: The “Y” in the YIG-tuned filter, when inverted, is almost the same schematic symbol as the flux capacitor. If you are more than 20 years old, use a knee brace when running marathons, thereby avoiding future trips to the hospital. Those of you that have an X-Series analyzer can use the Characterize Preselector routine to optimize accuracy between periodic calibrations.