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 frequency range of the LO in the RF section basically starts at the first IF frequency and tunes to a frequency 1.8 GHz above that. A commercial YIG (yttrium iron garnet) oscillator was selected to cover 2.4 to 4.3 GHz. The output of the YIG oscillator goes to the LO distribution amplifier and is also fed back to the synthesizer phase-locked loop (see Fig. 2).

Since the output frequencies involved are beyond the reach of programmable counters, the feedback path includes a down-conversion stage. The LO frequency range of 2.452 to 4.242 GHz is down-converted with one of three offsets (2.4, 3.0, or 3.6 GHz). The offset frequency is chosen to produce a down-converted signal between 42 and 642 MHz, which is within the range of the RF section programmable counter.

The value of the programmable counter (N) is chosen to divide the counter input frequency down to 1 MHz (N = 42 to 642). The output of the divider is phase-detected against a 1-MHz signal derived from the 40-MHz reference. This establishes the 1-MHz step size for the LO. The sign of the phaselocked loop must be switchable because the YIG oscillator tunes above and below the offset frequencies. The sign is switched by swapping the reference and feedback signals at the phase detector.

To compensate for the wide range of the loop gain (because N ranges from 42 to 642), a programmable gain block with 25-dB gain variation is added to the loop. Finally, a DACdriven coarse tuning signal is used to steer the YIG oscillator into the lock range.

The LO is distributed across four printed circuit boards and one microwave plate assembly. The circuit boards contain the 600-MHz reference, the 40-MHz reference, the frequency counter, the phase detector, and the YIG driver. The circuits on the microwave plate are fabricated with cyanate ester printed circuit boards and include the frequency multipliers and the YIG down-conversion.

LO Distribution

The LO distribution amplifier is built around a packaged GaAs MMIC amplifier designed by HP's Microwave Technology Division. This amplifier has dual outputs which are used to supply the LO signal to both the RF receiver and the RF source sections.

LO Offsets

The three offset frequencies (2.4, 3.0, and 3.6 GHz) are generated by multiplying 600 MHz by integer values. The 600 MHz comes onto the LO microwave plate assembly and is split to provide signals for both the offset multipliers and the second-LO multipliers. P-i-n diode switches select the path to the activated offset multiplier and each path has its own final stage of amplification before the multiplier. Schottky diodes are used as the hamionic generating devices in each of the multipliers. At the output of each multiplier is a twosection coaxial filter to suppress the adjacent 60 0 -MHz harmonic. The second LO (2.4 GHz) is generated in a similar manner.

YIG Down-Conversion

Down-conversion is implemented with a 7-dBm microwave mixer. Because of the low-level signals involved and the wide frequency span of the output IF (10 MHz to 700 MHz), the noise figure of the postconversion amplifier is important. The broadband noise within the 700-MHz span is sampled down by the digital dividers (10log[700 MHz/1 MHz] = 28.5 dB noise gain) and can contribute to the phase noise pedestal.