J105

[Magnus Danielson]

>>>>> "IF" == Ian Fritz <ijfritz at earthlink.net> writes:

 IF> Magnus --
 IF> An interesting looking device, indeed. I haven't been able to find these
 IF> in small quantities, though. Do you have a source?

I currently doesn't have one, but I think that I actually bougth some
many years ago.

I just checked out the Sales contacts for Sweden, and there are
several of them here, so I would just spend peek at the visay pages to
see if any of them fits your need.

Farnell for instance carries the 2N4391 and J108 (similar to the J105,
another possible choice) among several others.

My main point was that there are a number of devices out there that
migth be considered, each having their merits and it could be a
interesting thing to discuss, one might learn something on the way.

 IF> It would be very
 IF> interesting to see what kind of performance you could get. One other
 IF> point: the input capacitance is about 10x that of the 2N4391.  As Terry
 IF> pointed out, this capacitance causes some spiking. I don't think
 IF> switching speed is much of an issue as both devices are fast compared to
 IF> the time required for discharge.

I know for a fact that the LM311 is a real noise-makes when I turn the
FET of in the ASM-1 VCO. I get all kinds of frequencies. I haven't
cured it but it is time to do so, I am considering a 100 nF or so
sitting across the LM311 body. The LM311 is sitting tigth at -15 V
during the capacitor charging, then as the capacitor is near the
comparator voltages of +5V the LM311 output starts to slowly rise up
to 0V. As it comes near 0V the FET starts discharging the capacitor
while the LM311 still strives towards 0V (but now much more slowly),
then when the cap hits the floor the LM311 falls very quickly to -15V,
and by quick I am talking of some 25-50 ns (BTW, I have this on a
oscilloscope picture that I appended, but I didn't zoom in on the
falltime for more accurate reading). At this time the negative power
line becomes very noisy and you see several strong frequencies pop out
of the box, giving a peak-to-peak distrurbance of 2.4V on raw
sawsignal. A 2.9 MHz signal is to be found as well as a strong 5.3 MHz
signal.

So, in true EMC spirit there is several ways to attack the problem,
one is to ensure that it doesn't pull that much current (keep the
capacitance of the JFET down) or we could slow down the transient.
Actually, if you consider the time margins we have we could reduce the
slope of the LM311 quite a lot before it becomes a real burden on us,
a Miller capacitor (a capacitor in the negative feedback over the
LM311) is one possible way to acheive this. The benefit of reducing
the slopespeed is that the current peak will be less and thus the
source of the spike becomes less.

Just look at the slow slope (the red signal in the picture) as we
start turning the JFET on, why not just let the turn off slope become
something similar to cure away many problems?

Also note that I made this measurement on both extermes of my ASM-1 VCO
both at 88 mHz operation (the picture you got) and on 106 kHz
operation and the LM311 waveform and the noise it induces is the same.

So, to end this little adventure, I think there are several ways to
cure the problem and changing the slope speed rather than keeping to a
low capacitance is one things one can do. The falltime of the raw
sawtooth is 737 ns so a slower turnoff time is reasonble.

As this impulse moves along the -15V line it sneaks in to place we
would not like it and there it can cause a form of syncing of the
other oscillator.

Cheers,
Magnus

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