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X-Series Signal Analyzers Preselector Tuning for Amplitude Accuracy in Microwave Spectrum Analysis Application Note 1586 Introduction While modern spectrum analyzers have been evolving to provide ever- better accuracy at RF frequencies, their accuracy at microwave frequen- cies has not generally been improving. Indeed the requirements placed on the analyzer user to achieve the best accuracy have become harder to real- ize with modern signals such as wide- band digital modulation. Hardware and algorithms in the X-Series signal analyzers help you maintain the best accuracy without requiring a specific kind of input signal or external test signal. Microwave spectrum analyzers use preselectors to improve performance by removing unwanted mixer images Table of Contents and responses to local oscillator (LO) harmonics. Unfortunately these prese- Introduction ...1 lectors have instabilities and must be Low band, high band, and retuned frequently, and the preselector preselection ...2 tuning previously required a signal of Problems with YIG Preselectors ...3 near-CW statistics at the frequency of interest. In the X-Series signal analyz- Operation of and requirements ers, noise-source tuning removes this for preselector centering...5 difficult-to-satisfy requirement. A new centering capability ...6 Using the X-Series preselector centering capability ...7 Summary ...7 Low Band, High Band, and Preselection Modern spectrum analyzers operating 1st IF 2nd LO up to 26.5 GHz have a "low band" and LPF amp a "high band" signal path, as shown in the simplified block diagram, Figure 1. LO LO IF HI BPF HI out The "low band" in the X-Series signal analyzers covers frequencies up to 3.6 GHz. In low band, the signal YIG-tuned LO is upconverted to a high IF (5.1225 preselector HI GHz), then downconverted to an IF of 1st LO 322.5 MHz. Thi