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Date: Sun, 29 Dec 2013 22:37:43 +0100
From: =?ISO-8859-1?Q?Stefan_Sch=E4fer?= <Stefan.Schaefer@iup.uni-heidelberg.de>
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Subject: Re: LF: MF PA and VLF...
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Status: O

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 .