GM4CXM gets 13cm! (/cont.)
Ray took my advice and invested in one of Pyro Joe’s Spectrian PAs. These come in two flavours . . . Optimised for 26V and optimised for 12V. Ray opted for the high power 26V version. With about 800mW of drive at 2.3GHz, these PAs will deliver up to 60W, but at a price! Being Class-A in operation, they are gloriously inefficient. One estimate is 14%. As a result, a serious heat sink is necessary. Not only that, it is imperative that said heat sink is force-air-cooled. This isn’t a problem except that the heat sink more than doubles the weight of the transverter. That aside, the Spectrian PA is incredibly simple to implement. Two small rows of pins provide input for the enable signal, Drain-current monitoring for the two output devices and a voltage which is proportional to the substrate temperature. This temperature related output can be used to switch on a couple of fans when the amplifier/heat-sink gets hot . . . and believe me it gets hot, and it gets hot fast! The only problem with these boards is that they are bonded on to a solid copper heat spreader. It is easy to solder to the input and output tabs, but soldering to the ground-plane is a near impossibility due to the thermal conduction of the copper (gold-plated) heat-spreader. To get around this, I have devised a screw-down system for attaching the input and output coaxes. Like my own 13cm transverter, this one is built into 2 back-to-back Maplin boxes.
The finished Spectrian PA installed in the upper chassis
The photograph on the left is the fan-control and regulator PEC. Since the total current drawn by the SSB Electronics modules is never more than 800mA, a simple 1A 3-terminal 12V regulator is all that is required to step the 24V down to 12V. The board is mounted onto the rear of the upper chassis in such a way that the multi-turn pot can be accessed through a hole just behind the heat-sink. The 220K resistor in the feedback loop of the LM393 comparator provides about 15 degrees of thermal hysteresis. Thus if the fans are set to come on at 45 degrees Celcius, they will go off at about 30 degrees Celcius.
A DB6NT sequencer is used to control the SMA antenna relay and apply the PA-enable signal. The board in question is the SEQ-3 which is a bit of an over-kill, but the tabs on the MOSFETs make it easy to mount.