[sdiy] More VCO fun

[Ian Fritz]

At 02:45 PM 12/3/2009, David G. Dixon wrote:
>Only 1nF, with a ramp height of 5V.  It should drain out in a jiffy!

Yes it should.  Is something else limiting your measurement?  You are using 
a high enough frequency scope and good probes?


>By the way, subsequent simulations suggest something new (or at least not
>shown in your schematic): Putting a small cap (say, 47pF) between the
>comparator inputs helps dramatically to bring the reset right down to ground
>at significantly lower RC constants, and just generally cleans up the saw
>reset response.  I'm definitely going to add said cap to my circuit as soon
>as I get home tonight to confirm.

I can't remember seeing an improvement with that cap.  Nowadays I usually 
put diodes across the 311 inputs.


>I found some handy-dandy little test circuits in
>an application note on JFETs (Motorola AN211A) for measuring the forward
>transadmittance y_fs (essentially the same as the transconductance g_m) and
>some other things, which I might throw together on breadboard for selecting
>the best JFETs.  Incidentally, this value (which the application note
>suggests is THE figure of merit for JFETs) is essentially the same for
>2N5485 (3.5mS (min) to 7mS (max) @ V_DS = 15V, I_S = 1mA) and 2N4391 (6 mS
>(typ) @ V_DS = 20V, I_S = 1mA).

It's worth having a test circuit for pinchoff voltage also.


>Yes, JFETs are not the most consistent little bastards, are they?  I must've
>stuck a relatively crummy one in my circuit lastnight.

It's difficult to control the pinchoff voltage in manufacturing.  MOSFETS 
seem fairly consistent from what I've seen.

   Ian