Hello Wolf, VLF
That sounds good! Let us see some images then, and the schematic.
Meanwhile my VLF PA is ready to go,coherent keying should be possible now.
https://dl.dropboxusercontent.com/u/19882028/VLF/20131229_222857.jpg
https://dl.dropboxusercontent.com/u/19882028/VLF/20131229_222959.jpg
I saw the IR2110 has a schmitt trigger input for the shutdown pin, so i
can key this input with an analog circuit, this one:
https://dl.dropboxusercontent.com/u/19882028/VLF/PTTnew.jpg
And i want to use this for the VOX:
https://dl.dropboxusercontent.com/u/19882028/VLF/VOXvlf.jpg
Seems we're all soldering the whole day, Clemens too, hopefully (10 days
left!).
73, Stefan/DK7FC
Am 29.12.2013 19:39, schrieb wolf_dl4yhf:
Hi Stefan,
Hi Wolf,
What type of FETs are you using? The standard IRFPs? Many of them
have internal Z diodes, so no addional RC network or Z idode is
needed. The losses are there anyway, of course.
I'm using HUF 75343 P3 : 75 A, 55 V, 9 mOhm RDs on, and a hefty gate
input capacitance.
Just for historic reasons (on LF), and because I still have a tube
with over a dozen of them.
But the TC4452 has sufficient driving power to switch them rapidly
enough, even at 470 kHz (and presumably more).
For the DC modulator (with 0.1 V ... 13 V output, depending on the PWM
duty cycle), there is another HUF 75343, and one IRF 4905, working
together as a complementary push-pull stage at 100 kHz with a PWM DC
of "almost" zero (due to the limited speed of the comparator + driver)
to 100 percent. After a simple choke, and a few microfarads (ceramic),
this provides the 0... 13 Volts for the MF final stage.
In case of mismatch or excessive drain current (actually measured as
source current in the PA), the PWM DC is gradually reduced in a
control loop... and the control loop design turned out quite tricky,
because the loop gain depends on the load, and the loop has a dead
time (due to the 100 kHz switching-mode modulator) which can cause the
loop to oscillate under certain conditions. This is actually the
reason why the PA is not "on air" yet...
Cheers,
Wolf .
|
