I have tried to reduce the number of parameters acting on the functioning and came to a schematic without transformer, and even without ferrite:
At the begining, L1 and L2 can be wound with spaced turns and adjusted with a ferrite core. Then their inductance can be increased by compressing the turns, and the ferrites can be removed. It doesn't a matter, there is no difference, so we can conclude that the ferrites were not saturated and play no role here except increasing the inductance.
Here are the results.
The pulse width is around 6.5ns and the measurement is made at a repetition rate of 20 Khz.
Under 20v, 61mA, i.e. 1.22W only for the MOSFET power consumption, the pulse peak voltage is around 600v under 50Ω. This mean an instantaneous peak power of 600
2/50=7.2KW.
Now with my radio receiver I observed that the harmonics are at least 20 dB down the fundamental near 77 Mhz. They are negligible. So we must not consider that the pulses are square, we must rather approximate them with half-sine. Consequently their rms value is 600/√2 and the rms power in the 6.5ns period is: 600
2/4*50=
1.8KW.
The instantaneous peak power is not instructive. Only the rms power allows us to know how much energy is carried by the pulses: E = rms Power*6.5ns.
It is also interesting to evaluate the mean rms power. The duty cycle is 6.5ns/(1/20Khz) = 0.13*10
-3. The mean rms power is therefore 0.13*10
-3*1.8KW=0.23W. The input power being 1.22W, no overunity
. In fact the efficiency is higher due to energy in the ripples after the pulse. The load heats significantly although it is made of two resistances in series of 23.5Ω, 1/2W, followed by the third of 4.7Ω for the output divider.
Above 20v, the consumption increases but not the pulses.
With 2 diodes in parallel or with 3 diodes in series, I didn't notice any improvement. With two diodes in series, under 28v, 94mA, i.e. 2.63W for the MOSFET power consumption, the peak pulse voltage is around 800v under 50Ω, meaning a rms power in the 6.5ns period of: 800
2/4*50=
3.2KW, and a mean rms power of 0.4W. Nevertheless the load resistances become hot, so we can consider that my power calculus is underestimated (probably because of insufficient pass-band of the scope) and the mentioned powers should be considered as minimum.
What I may conclude:
- 600v/50Ω pulses are easily reached with this setup, provided that the diode is as good as the FUF5404.
- L1, C2 and the pulse width are the critical parameters, and they depend on each other.
- L2 can have to be also adjusted but is of less importance
- the MOSFET must be fast and able to handle high voltages (500-900v) rather than high currents
- the better DSR pulses are obtained when they are synchronized with the falling edge of the back emf
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Topic: DSRD pulse generator (Read 200808 times)
