A high-speed microwave pulse modulator - design of the optional circuit for Hewlett-Packard's HP 8360 family of microwave synthesized sweep oscillators

This optional fast pulse modulator uses an unequally spaced diode topology to achieve a wide bandwidth and a high on-off ratio without resorting to performance-limiting diode saturation.

FAST PULSE MODULATION, Option 006 for the HP 8360 synthesized sweep oscillators, offers improved pulse modulation performance over the standard instrument block diagram. In this configuration, the instrument has an additional pulse modulator at the output of the RF deck. Pulse modulating after the YIG multiplier substantially improves performance. A new wideband modulator is the key to this improved performance.

Standard instrument

In the standard HP 8360, the modsplitter microcircuit performs the pulse modulation (see article, page 36). A linear modulator and a pulse modulator cover the 2-GHz-to-8-GHz frequency range. These are two-octave modulators. They consist of equally spaced diodes shunted across a microstrip transmission line. The diodes are turned on to attenuate the signal and turned off to pass it. Ideally, these circuits have a two-octave attenuation response centered on the frequency where a quarter wavelength equals the spacing between the diodes.

In the standard HP 8360, pulse modulation occurs before the YIG multiplier, and the bandwidth limitations of the YIG multiplier typically result in pulse rise and fall times of about 15 nanoseconds.

Wideband Modulators

Often applications requiring more than a two-octave bandwidth use the equally spaced diode topology. The spacing between the pin diodes is set for a quarter-wavelength maximum-attenuation response at the high-frequency end of the band. To achieve greater attenuation at the low-frequency end of the band, the diodes are driven into saturation and have less shunt resistance. The disadvantage of this scheme is that the rise and fall times of the pulsed waveform depend on the amount of stored charge in the i layer of the pin diodes. As the diodes saturate, the stored charge in the i layer increases and the modulator takes longer to change state.

The new modulator does not depend on diode saturation. Instead, the diodes are spaced unequally so that the minimum attenuation of a given set of diodes is cancelled by other sets.(1)

High-Speed Wideband Modulator

Fig. 1 shows the block diagram of the fast pulse modulation option. At the input and the output of the modulator are high-power amplifiers that provide power, gain, and good reverse isolation characteristics. These amplifiers are monolithic microwave integrated circuits (MMICS) developed by the Hewlett-Packard Microwave Technology Division.

Fig. 2 is a photograph of the fast pulse modulator.

The modulator circuit is constructed on a 0.010-inch-thick alumina substrate with laser-cut holes for precise diode placement. Chip mesa pin diodes are epoxied to the baseplate of the package in these laser-cut holes. Gold mesh is used to connect the diode anode to the circuit pattern. A bias structure and a video feedthrough filter are added to the circuit. The bias circuit extends from the modulator to a O-pF dc feed. The bias circuit consists of a discrete five-pole low-pass filter with a cutoff frequency of 700 MHz. This bias circuit will pass a fast pulse drive waveform and reject RF energy in the operating bandwidth of the assembly. The video filter is a seven-pole discrete high-pass filter. The series capacitors are fabricated with microstrip technology on sapphire substrate. Shunt inductors are aircore coils wound with 0.0015-inch gold wire. A frequency dependent attenuation pad follows the video filter to compensate for the gain slope of the component. Following the output MMIC amplifier, a 22-GHz low-pass filter improves the above-band harmonic response of the component.

An electronic switch at the output of the modulator multiplexes the low-band lo-MHz-to-2.35-GHz frequency range and the high-band 2.35-GHz-to-20-GHz frequency range. Modulator Performance

The fast pulse modulator's performance was characterized at the microcircuit level with the following results. On-Off Ratio. Fig. 3 shows the on-off ratio for the pulse modulator. The on-off ratio is 95 dB from 2.0 GHz to 20 GHz. Output Power. Fig. 4 shows the typical output power of the component.

Gain. Fig. 5 shows the typical gain of the component. Pulse Performance. Fig. 6 shows the pulsed waveform created by the five-diode pulse modulator. The modulator's rise and fall times are typically better than 2 nanoseconds.

Conclusion

The HP 8360 with Option 006 offers pulse modulation with high power, fast rise and fall times, wide dynamic range, and broad bandwidth. The heart of the pulse modulation system is a new modulator design. The power, gain, and reverse isolation are achieved using HP MMIC traveling wave amplifiers. The fast pulse modulator, at the output of the RF chain of microcircuits, improves the maximum power and pulse performance of the standard HP 8360 instrument.

Reference

1. M. K. Koenig, "A Non-Commensurate Line Length Modulator," Microwave journal, March 1991.

COPYRIGHT 1991 Hewlett Packard Company
COPYRIGHT 2004 Gale Group