Microcircuits for the HP 83750 series sweepers - microcircuit design in HP's 83750 microwave sweepers

Hewlett-Packard Journal, April, 1993 by Eric V.V. Heyman, Rick R. James, Roger R. Graeber

Four custom microcircuits provide the basic output signal, the RF band, signal switching and distribution, amplification, ALC and pulse modulation, power amplification, and two stages of YIG filtering.

This article discusses the design of the four custom micro-circuits designed for the HP 83750 Series sweep oscillators. The microcircuits are:

* The dual YIG oscillator (DYO)

* The switched amplifier filter detector (SAFD)

* The 0.01-to-2-GHz heterodyne band microcircuit (HetBand)

* The combiner modulator amplifier (ModAmp).

Dual YIG Oscillator

The signal for the HP 83750 Series sweepers is generated in the dual YIG oscillator microcircuit. The DYO is actually two YIG oscillators in one magnetic housing. One oscillator covers the span from 2 to 11 GHz and the other covers from 11 to 20 GHz. The output power exceeds 20 mW from separate outputs for each band.

The high-band 11-to-20-GHz oscillator consists of a YIG resonator and a single GaAs IC chip that contains both the oscillator and buffer stages. Fig. 1 is the schematic diagram. The IC is fabricated using an HEMT GaAs IC process with an [f.sub.T] of 50 GHz and an f[.sub.max] of 100 GHz. The chip (Fig. 2) measures only 960 by 960 [micro]m.

The oscillator stage consists of a 200-[micro]m FET in a source follower configuration. The feedback is generated by a 0.2-pF thin-film capacitor connected between the source and ground. This feedback generates an impedance looking into the gate of the device that has a negative real part and thus has a reflection coefficient greater than 1, which is a necessary condition for oscillation to begin.(1) The condition for oscillation to begin is:

[gamma.sub.device] [gamma.sub.resonator] > 1, were [gamma.sub.device] and [gamma.sub.resonator] are the reflection coefficients of the device and resonator, respectively. The condition at oscillation is:

[gamma.sub.device][gamma.sub.resonator] = 1.

This relationship is achieved as [gamma] "device is reduced by limiting during the buildup of oscillation. The phase condition is satisfied by a shift along the resonator curve, possibly allowing the oscillation to occur somewhat off resonance.

The source follower configuration has the potential to oscillate at undesired frequencies above or below the desired band. These undesired oscillation conditions, called lockup modes, are a result of the interaction of the YIG coupling loop parasitics and the active device. The oscillator circuit must be designed so that there is insufficient reflection gain to support the lockup mode. At the low end of the band this is accomplished by placing an inductor in parallel with the source feedback capacitor. This has the effect of reducing the capacitance on the source and thus the reflection gain at lower frequencies. In addition, a high-impedance coupling loop is used that does not provide the proper phase relationship for the lockup mode. To avoid lockup at the high end of the band the transmission line between the device and the resonator is kept short.

The buffer amplifier stages consist of a 300-pro FET followed by a 400-[micro]m FET, both in the common source configuration. The oscillator stage is matched to the buffer amplifier using a short length of transmission line. The primary purpose of the buffer stages is to provide isolation and therefore a stable match to the oscillator stage, making the oscillator frequency independent of the load.

The YIG (yttrium iron garnet) resonator provides the high-Q tuning circuit for the oscillator. The high-band resonator is constructed of a 300-[micro]tm-diameter, undoped YIG sphere centered in a multiturn coupling wire. The ratio of sphere to loop diameters is a trade-off between suppression of spurious resonances and oscillation strength. The YIG resonator provides a resonance that tunes linearly with an applied magnetic field.(2)

The 11-to-20-GHz high-band oscillator is built on a 0.010-inch molybdenum carrier (Fig. 3). This carrier is held to the lid by studs inserted in the lid. The carrier's function is to provide a continuous ground plane. The YIG GaAs IC is soldered to a small heat spreader and then epoxy-attached to the carrier. A 0.010-inch fused silica microstrip circuit is epoxy-attached between the YIG resonator and the IC to provide the proper transmission line length. The output circuit is a 0.010-inch sapphire microstrip circuit which provides output matching and transition to a right-angle SMA connector.

The low-band 2-to-11-GHz oscillator consists of a YIG resonator, a bipolar transistor oscillator stage, a matching network, and a broadband buffer amplifier (Fig. 4). The oscillator stage uses a silicon bipolar transistor with an [f.sub.max] of 22 GHz.

The transistor is configured as a common base circuit with an inductor in series with the base terminal. The inductor is realized as a length of transmission line. This inductor transforms to a negative real impedance at the emitter port and thus meets the above criteria for oscillation. The collector port is terminated in a matching network that provides load conditions to optimize oscillation strength, harmonics, and linearity. The buffer stage consists of a 2-to-20-GHz traveling-wave GaAs IC amplifier. The YIG resonator consists of a 600-[micro]m-diameter 550-gauss Y/G sphere in a half loop of 950-[micro]m-diameter wire.