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

The HP 89440A has one additional feature not found in traditional swept analyzers. A feed-forward LO feedthrough nulling circuit has been added to reduce the level of LO feedthrough in the first IF. Beyond the second converter, the second-IF fiters remove the feedthrough term. Without this LO feedthrough nulling, LO feedthrough referred to the input could be 20 dB higher than a full-scale input signal. This could result in residual responses and increased secondharmonic distortion at input frequencies below 15 MHz.

Cyanate Ester Printed Circuit Boards

The input attenuator and several other RF receiver, LO, and RF source boards that operate at frequencies beyond 1 GHz use cyanate ester printed circuit board material. Cyanate ester was chosen in place of the standard glass epoxy printed circuit board (HP FR4) because of its lower loss tangent, which results in lower losses in the board. The dielectric constant of cyanate ester is 4.0 at 2 GHz while glass epoxy is typically specified at 4.5. All cyanate ester boards in the RF section are 0.030 inch thick rather than the standard 0.060 inch. The thinner printed circuit board material has two advantages. Many receiver, LO, and source boards in the RF section use surface mount parts with microstrip construction. With the thinner printed circuit board material, ground vias are shortened by 0.030 inch, reducing parasitic inductance in surface mount components needing a return to ground. In addition, microstrip transmission lines are narrower. A 50-ohm microstrip transmission line is nominally 0.060 inch wide on the 0.030-inch printed circuit board material while the same line on 0.060-inch printed circuit board is 0.110 inch wide. The disadvantage of 0.030-inch printed circuit board is less rigidity. However, all of the cyanate ester boards are mounted on "plates" and are well supported (see "Microwave Plate Assembly" on page 50).

Attenuator

The main signal path of the HP 89440A RF section starts with a step attenuator assembly that provides 0 to 55 dB of attenuation in 5-dB steps and is followed by the first converter. The step attenuator has an input for a calibration signal from the HP 89410A, a mode to terminate the user input during calibration, and a bypass mode to bypass the RF section. The bypass path connects the RF section receiver input connector directly to the IF section receiver input for frequencies below 2 MHz.

First Conversion

Following the input attenuator is the first converter. Input signals are converted to an IF centered at 2.446 GHz. The first mixer is a variant of the single-balanced design used in the HP 8590B spectrum analyzer. It is preceded by a 15section low-pass filter with a cutoff frequency of 1.8 GHz. The input low-pass filter eliminates input image frequencies (4.89 GHz to 6.69 GHz) as well as spurious components (spurs) resulting from out-of-band inputs. The first-converter LO supplies a 20-dBm signal between 2.452 GHz and 4.242 GHz, which is attenuated by 3 dB at the LO port of the mixer to improve match. Following the mixer is a microstrip directional coupler, where the LO feedthrough cancellation signal is introduced. This is followed by a diplexer and a 4.5-GHz low-pass filter (not shown in Fig. 1) to eliminate mixer products and LO harmonics which can produce residual responses when mixed with the LO of the second converter. The entire mixer is built on Duroid board (Rogers Corporation) which has a dielectric constant of 2.33 0.05 and a loss tangent of 0.001 at 1 GHz. The board thickness and dielectric constant are tightly specified so that printed circuit board microwave filters, couplers, and transmission lines with repeatable performance can be produced. Early in the design it was recognized that skin effect losses in the input attenuator and the input low-pass filter preceding the first mixer would result in frequency dependent loss that is about 4 dB at 1.8 GHz. This unflatness can be calibrated and removed, but it results in a displayed noise floor that is unflat, and it reduces the effective dynamic range of the ADC by 4 dB. An amplitude equalizer (not shown in Fig. 1) was added between the attenuator and the input low-pass filter to eliminate this effect. This has the added benefit of reducing the level of multiple tones in the first IF at low input frequencies. Multiple tones are present because the sum and difference products and the LO feedthrough are only separated by the input frequency and are not eliminated by the first IF filter if the input frequency is low.