Hello Luis,
I am at work but will draft a more detailed post later. The new
CD74HCT74E flip flop board shows similar horrible traces, so I think
you may well be on to something here! Thank you. My doubt though is
using an HP signal generator to inject a X2 frequency also shows the
same dire waveforms, I would have *THOUGHT* its signal would be pretty
clean? My USB scope has an FFT function and the gates and drains don't
look good on it, using the U3S (no output stage sig straight from
CLK0 pin of the Si5351A) but it's a bit of a toy in that mode. I could use an
SA to look at the input and fiddle with a LFP on it I suppose. Thanks
again.
Sunday, June 2, 2019, 9:38:07 PM, you wrote:
> Hi Chris
> One of the modifications I have done to the LF PA is related to the input at
> HEF4013
> I was using the typical 220nF in series and a 12K and 10K resistors to +12V
> and GND
> The exciter is also an U3, but the old one with DDS. Anyway, the
> problem was that the input was too
> sensible (too high impedance) and any small signal drived the HEF4013. The
> drain waveforms were
> also not nice at all, and not squared. I even found that just
> touching the input of the PA with my
> finger drived it out at make the mosfets to work in a very unstable and ugly
> way
> So modified the input components to work as low pass filter and provide lower
> impedance
> The 220nF was changed to 47pF and the 10K to GND replaced with a 5K
> adjustable trimmer
> The 12K to 12V is still in use. You will have to use different values for MF
> to get it working
> a 950KHz
> I adjusted the trimmer to work with a 270KHz input signal at 8V5
> p-p. Higer frequency signals do not drive
> the HEF4013 at all. Now this is a completely different PA. Squared
> drain waveforms and absolutely stable
> Seems that garbage in = garbage out rule do also apply here ;-)
> Have a try and let us know. It is an easy test
> 73 de Luis
> EA5DOM
> ________________________________________
> De: [email protected]
> [[email protected]] en nombre de Chris Wilson
> [[email protected]]
> Enviado: domingo, 2 de junio de 2019 16:11
> Para: [email protected]
> Asunto: LF: HEF 4013 flip flop anomaly
> Hi, I thought a separate thread best for this question
> U3S exciter for MF, taking the X2 frequency output @ circa 950 kHz
> from the Si5351A CLK0 output pin to a level converter using a
> SN74AHCT125N buffer between the si5351a and the HEF4013 flip flop,and
> we are seeing something we do not understand, maybe someone can
> explain?
> We have an adjustable voltage regulator off the main 12V supply for the
> HEF4013, next to it on my little driver board, and by chance, we found
> that as we lower the voltage to the HEF4013 the drain waveforms clean
> up, and just before the 4013 shuts down they are relatively good. I'll
> link to scope shots.
> We have the driver board grounded right by the driver IC ground points
> to as near the FET source pins as the layout will allow (10 mm or so,
> copper sheet strap) Before we go further we would like to know why
> reducing the voltage to the 4013 improves things! We also see the gate
> waveforms go negative when the FET's have power to them, without
> voltage applied they are perfectly square. You can see this effect in
> the screenshots. Gates are lower traces, drains upper. Thanks.
> We are building a board using a CD74HCT74E flip flop in the meantime,
> we suspect a 4000 series is not the best choice...
> http://www.chriswilson.tv/4-point-8v-on-4013-25v-on-drains.png
> http://www.chriswilson.tv/6v-on-4013-25v-on-drains.png
> --
> Best regards,
> Chris mailto:[email protected]
--
Best regards,
Chris mailto:[email protected]
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