If you have noticed inconsistent rejection data when testing filters with a USB signal generator, unfiltered harmonics might be the culprit. In this video, Robert demonstrates how RF harmonic filtering can correct skewed measurements and explains why using a Low Pass Filter (LPF) is essential for achieving lab-grade accuracy. This guide is perfect for anyone using the analyzer portion of their TPI units for insertion loss measurements.

Key Takeaways:

• The Harmonic Challenge: Learn why versatile USB generators often output harmonics along with your selected frequency, which can interfere with your detector readings.
• Rejection Floor Issues: Understand how these harmonics can affect your readings, making a filter’s rejection look significantly worse than it actually is.
• The LPF Solution: See a live demonstration of how adding a 500 MHz Low Pass Filter helps you achieve a more correct response by suppressing unwanted harmonics.
• New Product Announcement: Learn about the upcoming set of low pass filters (500 MHz and 1.2 GHz) currently in development to support future measurements.

Step-by-Step Walkthrough:

When performing absolute forward response measurements, precision is key. In this demonstration, Robert utilizes the TPI 10006A to walk through a common pitfall in RF testing: harmonic interference.

1. Setting Up the Baseline The process begins by connecting the RF output to the RF detector input. The first critical step is “zeroing out” the cable so it is not part of the measurement. By storing the cable’s baseline response, you can switch to insertion loss mode to ensure the cable is excluded from the actual filter measurement. Robert sets his markers and grid for a 433 MHz SAW filter with a 200 MHz span.
2. Identifying the Harmonic Issue USB signal generators are incredibly versatile, but they do not filter harmonics. If you select 100 MHz, you are also getting 200 MHz, 300 MHz, and all subsequent harmonics. When testing a 433 MHz SAW filter, these harmonics can cause the detector to show a rejection of only about -29 or -30 dB—a result Robert knew was incorrect for this specific filter.
3. Applying a Low Pass Filter To fix this, Robert inserts a 500 MHz Low Pass Filter into the signal path. This ensures that while the 433 MHz fundamental frequency passes through, the second harmonic and others are suppressed before they can affect the detector.
4. Analyzing the Results With the LPF in place, the results are immediate. The rejection improves to the -49 to -50 dB range, which is the limit of the current y-axis setting. While the LPF adds a small amount of extra insertion loss (moving the peak from -3.5 dB to roughly -4 dB), the resulting data is far more accurate because the harmonics are no longer causing interference.
5. The Future of TPI Generators To assist users in the future, Robert announced that he has developed a set of low pass filters that are currently in process. These filters—one at 500 MHz and one at 1.2 GHz—will be added to future sales to help with 315, 400, 800, and 900 MHz measurements, ensuring users get the most accurate results right out of the box.