Time is no respecter of persons and towards the end of March I reached the big ‘five-oh’, celebrating the event with a holiday. On return there were several non-radio matters requiring my attention, so work on the Lucent PA was put on hold. When the time came to continue with the PA, I verified that the bias was stable at 200mA and connected it up to my 23cm transverter, having first reduced the output to 3W. With 24V on the PA, I applied RF drive, expecting to see something like 30W. But instead I was lucky if I was getting more than 10W out. Slowly, I increased the drive, carefully watching the Collector current for signs of instability and/or saturation. I increased the drive until it reached 10W (maximum permissible drive), at which point the PA output had reached a mere 45W for a Collector current of 5.5A. I had been told that the Collector current should not exceed 6A and that for such, the output should be at least 60W for a maximum drive level of 10W. Yet I was lucky if I was seeing 45W. Still, it was amplifying . . that was something . . . and it was achieving 33% efficiency. I held carrier for about 30 seconds and was pleased to see that the device was stable . . . no tell-tale creeping up of Collector current. I dropped carrier and the collector current dropped back to 200mA. Despite the low(ish) power, I was more or less happy. I re-asserted carrier and was conscious of a little flash of light and the fan on the heat-sink stopped turning . . . The Collector current now read 0mA. The MRF20060R had popped! But why? Back to the data sheet . . .
. . . It was then that I saw it. I knew that the MRF20060R was intended for operation at 1.9GHz, but it was clear that at lower frequencies, the projected gain was going to be considerably lower, the Collector efficiency somewhat lower and internal VSWR higher. Everything pointed to this NOT being a suitable device for use at 1.3GHz. In fact nowhere in Motorola’s data sheet is there any hint that the device can be operated below 1.8GHz. In hind sight, I can only surmise that the internal construction of the MRF20060R renders it entirely unsuitable for use at 1.3GHz . . . Which is probably why there are NO articles on the Internet detailing it’s use on that band.
One final thought . . .
If it wasn’t so easy to obtain Spectrian boards using the Motorola XRF286S, it might just be viable to modify this PA for use at 2.3GHz.