Total posts : 45366
In my previous post about the use of tubes for Part 15 AM, I stated that improved efficiency can be obtained if the RF drive to the final stage is obtained from a generator of narrow pulses rather than a quasi-sinusoidal waveform. At the time of my previous post, I did not know how to make the grid pulser.
As I stated in another thread (“Great technical thread,” initiated by Neil), I am investigating the loss in efficiency due to a parasitic diode from drain to source of transmitters using power MOSFET output stages. This is called the “Turn-off behavior ” of a power MOSFET. When the MOSFET is on, and is then turned off, ringing is produced. The ringing waveform is a train of positive pulses at the drain with the negative half cycles clipped by the parasitic diode. It occurred to me that I can make a good grid pulser by keeping a MOSFET on for most of a cycle, but turning it off long enough to generate a single positive-going pulse. The turn-off time is established by logic circuits. In my circuit, I had set the turn-off time to be 0.1 the period at 1.6 MHz. The inductance and capacitance at the drain should be set to be so that the turn-off time is pi[SQRT(LC)]. The inductor should be wound on a large core so that the energy storage in the inductor would be large. I used an IRF 510 MOSFET with a .95 uH inductor wound on a 3″ diameter low-permeability ferrite core. The LC product is smaller than would be needed for a half cycle of the ringing to be a tenth cycle at 1.6 MHz, but the pulser works well, nevertheless.
Using the pulser, I was able to get the load impedance at the plate to match the parallel-equivalent resistance of the ground resistance. The smaller the ground resistance is, the larger the parallel-equivalent resistance will be. A larger load resistance requires a greater DC plate voltage and a smaller plate current. The use of high voltage can be a problem. I zapped a couple of 12AX7 tubes when working with more than 500 VDC.
A vacuum tube with a grid pulser is probably the only Part 15 AM transmitter design that can give very high efficiency over a wide range of output impedances.