RF vector signal analyzer hardware design - HP 89440A radio frequency baseband analyzer - Technical

Hewlett-Packard Journal, Dec, 1993 by Robert T. Cutler, William J. Ginder, Timothy J. Hillstrom, Kevin L. Johnson, Roy L Mason, James Pietsch

Second Conversion and IF

Following the first IF filter is the second converter assembly which down-converts the 2.446-GHz IF to the second IF centered at 46 MHz. The second LO is a 2.4-GHz signal supplied by the reference. It is amplified to 10 dBm, filtered by a twosection combine filter to eliminate any sidebands at 50 MHz, and then amplified to 13 dBm before application to the second mixer LO port through a 3-dB attenuator. The 46-MHz IF output of the second mixer is amplified by a low-noise amplifier, resulting hi a signal level equal to that of the input signal (-30 dBm full-scale). The noise figure at this point is nominally 23 dB. Following the second converter are three identical cascaded second-IF filter assemblies implemented with capacitively coupled resonators. The filter design is based on a Tchebychev filter with 0.1 dB of ripple. The filter is predistorted and has an insertion loss of 4 dB, The filter has a minimum rejection of 27 dB at 38 MHz (edge of the third-conversion image band), so the three cascaded stages have a minimum of 80 dB of rejection. So that each filter board will have a nominal gain of 0 dB a low-noise amplifier precedes each filter. Three identical filters were chosen over a design in which the entire filter resided on a single board. A single-board design would require shielding between sections of the filter and alignment of the filter would require a complex adjustment procedure. Using smaller identical boards, the alignment procedure is identical for each board and simplified by the fact that there are only four sections to adjust. Shielding is provided by the walls of the card nest in which the boards reside. The three cascaded filters have a 3-dB bandwidth of 8.5 MHz and a combined peak-to-peak ripple less than 1.2 dB.

Third Conversion

The final stage of the RF section receiver is the thirdconverter assembly which translates the 46-MHz second IF to the final IF centered at 6 MHz. A 40-MHz LO is provided by the reference at 3 dBm. Following the conversion are two wideband operational amplifiers, which provide the final gain and buffering before the signal is sent to the IF section. There is a 1-dB gain step that can be switched by the IF section and a manual gain adjustment to compensate for normal gain variance in the manufacturing process.

Local Oscillator

Since the IF section has the ability to tone with millihertz resolution to any frequency within its dc-to-10-MHz input bandwidth, the LO for the RF section does not have to replicate this function. The RF section down-converts the 1800-MHz input span to within the frequency range of the IF section and the digital LO tunes to the desired center frequency. In traditional spectrum analyzers, multiloop LOs are designed to realize millihertz resolution. Multiloop designs mean at least three phase-locked loops in the main LO. The step loop provides coarse frequency resolution over a wide range of frequencies. The interpolation loop tunes across a narrow span but with high frequency resolution. The sum loop combines the outputs of the two loops. In contrast, the HP 89440A RF section has a single-loop LO that tunes in 1-MHz steps. The elimination of the sum and interpolation loops means significant savings in complexity and reduced development risk within the LO section of the HP 89440A. This trade-off is not without consequence because a coarse LO resolution reduces the analysis bandwidth of the instrument. The IF bandwidth of the receiver chain supports an 8-MHz analysis bandwidth but an arbitrary center frequency at the input can only be placed within [ or-] 0.5 MHz of the IF center frequency because of the step size of the LO. Hence, for an arbitrary center frequency the 8-MHz IF bandwidth is reduced to a 7-MHz maximum analysis bandwidth.