Things to Improve

The following are possible improvements to consider if ever the amplifier is redesigned.


Housing and DC Bias Circuit Board

The overall dimensions of the amplifier were determined by the DC bias circuit board in the back cavity. Because the amplifier is not too much thicker than the diameter of the Hirose connector, it is unlikely that one would want to make it thinner. However, the DC bias circuit board is probably larger than necessary. If this board were made smaller, then the amplifier housing itself could be both narrower and shorter.


Microstrip to Coplanar-Waveguide Transition Circuits

The center conductor on the CPW end of the transition should probably not go all the way to the edge of the board, to prevent the possibility of short-circuiting. Rather, it might be pulled back from the edge by perhaps 50 microns. I would leave the ground planes going all the way up to the edge of the board.

Also, it would have been easier to have had the transitions already diced to the proper lengths. If this had been done, then it would have been a good idea to also have the microstrip end of the board pulled back from the edge, perhaps 100 microns.


The QMMIC Chip

The QMMIC could be redesigned to consume less power. The primary means to accomplish this end would be to replace the resistors on the drain bias circuits with inductors, or possibly leave the resistors for tuning purposes but not run any current through them, adding inductors for the biasing.

I would also make the chip smaller. I would try to get the chips diced with less border around the edges, especially at the input and output. The goal would be to keep the chip as small as possible so that substrate modes cut off at higher frequencies. As an added benefit, the yield from a wafer could be larger.

I would also think more about how much gain is needed. Having more gain would reduce the noise contribution from room temperature IF systems. The ideal gain would be about 35 dB. More gain would have negligible effect on the system noise temperature, but might have the undesirable effect of dissipating more heat at the cold reservoir. On the other hand, already with 30 dB of gain, difficulties are encountered from reflections, substrate modes, etc. A possible solution might be to use two chips with about 20 dB of gain each. The first would be optimized for low noise and good output match. The second would probably need a very good input match to avoid standing waves rather than tuning for minimum noise. With this dual-chip system, one chip could ultimately be integrated with the mixer, and the second put in a separate housing outside of the mixer block. Such a system would be good from several viewpoints. First off, integrating the amplifier would eliminate noise from cabling and isolator loss, and eliminate cable ripples. Having less gain on the integrated amplifier chip would reduce problems with cavity modes, etc. in the mixer block. It would also reduce the amount of heat being dumped into the mixer block in close proximity to the mixer chip. The isolator would no longer be necessary, and the second amplifier could be located far away from the mixer block if necessary, possibly at a higher temperature.


Integration With Isolator

This amplifier has been used with an isolator at the input. In this configuration, there are several centimeters of connectors with about 5 interfaces between the isolator and the amplifier. The interfaces probably introduce mismatches and reflections, while the long distance introduces loss, noise, and passband ripple. A potential improvement would be to integrate the isolator and amplifier into a single package. This would have the additional advantages of reducing overall size and failure modes.