shorty wrote:I binned them as I thought they were junk.
The 2SC1947 is a wonderful device. It's got a huge amount of gain compared to the previous generation devices (like the MRF237). The only real issue with them was the need for a big heatsink. The "Stentor" approach, which puts a big piece of earthed metal between the oscillator and the output, is probably the best one.
I looked at the "Stentor" and wondered if that approach could be modernised, getting rid of the trimmers and adding a PLL. I came up with a three transistor exciter, with a dual-gate FET as the oscillator, with its output amplitude adjusted by a diode-pump RF sensing circuit. The buffer stage was a BFR96, giving about 300mW. The final was a 2SC1947A, using the input filter trick that Stephen Moss developed so that the gain was fairly flattened across the band.
I used a Schottky Diode rectifier to derive a voltage proportional to the output power, and used this to control the gain of the dual-gate FET in the oscillator - this guaranteed exactly even power right across the band. It also gave the option of adding an extra transistor to turn off the power to the buffer stage if the SWR was very wrong.
The PLL was based on the "Pira" SAA1057 design, but with the code slightly modified to enable the "locked" output function. The modulator was scaled for 1V5 p-p for 75kHz deviation, and uses the control voltage to adjust the deviation sensitivity - it's not perfect, but the maximum for 75kHz deviation only varies by about 0V05 across the band.
The board develops 4 Watts exactly at 12V65 supply (a 7812 with a silicon diode in the ground lead gives exactly the right supply voltage!)
The whole thing fits on a 2½"×4½" PCB, and has a fairly big metal flange as the heatsink for the output transistor which also provides the screen between the output filter and the rest of the board. The cleanliness of the output is pretty good - no harmonics worse than -50dBc - and the in-band noise is very low. The only flaw is the PLL re-tuning noise. It's very low level, but it's irritating, and I don't really see a way of getting rid of it!
