For curiousity sake.... how much do these commercial units go for? Why?
--snip--
I've read good reports on the web, but have absolutely no personal
Since no one else has chosen to answer your first question,
The scheme you got suckered in is VERY old........ At least you had the
I don't think you understand what's happening. The JFET's gate has zero
That's why I said "slight".
In another posting John pointed out that the reactive and resistive components combine
You've really piqued my interest now. I can't imagine what would aspect of this
Well, I guess that eliminates that issue. My capacitors came out of the parts drawers
Yes. That's what comes of writing this at 3:00 AM.
But as I pointed out in another post, this also means that you have lots of feedback at
Here's a suggestion. Get rid of the components you have in the source lead now. Make
I may put a couple 1 uF mylars in parallel to see if there's any diff,On Fri, 7 Jan 2005 10:18:28 -0800, "Watson A.Name - \"Watt Sun, the
Dark Remover\""
NOSPAM@dslextreme.com> wrote:
"The Phantom" <phantom@aol.com> wrote in message
Just to make sure we're talking about the same thing, is the
circuit
under discussion here?
+9V
|
.-.
| |
33k| |
'-'
|
.----------------------------------------------o----o Vout
| |
| o-------o |
.-. | | |
| | | 4.3M .-. |
| | 330k === | | +------o V
out
'-' GND | | |
| '-' |
| +|| | |-
o---/\/\---o---/\/\--o--/\/\----o---||--o--->| JFET MPF102
| | | | || |-
| 330k | 330k | 330k | 10u |
| | | | o----.
| | | | | |
| | | | .-. |
--- 1.0u --- 1.0u --- 1.0u --- 1.0u | | | +
--- --- --- --- 1.0k | | --- 2
caps
| | | | See '-' ---
3200uF
| | | | note | | total
=== === === === below === ===
GND GND GND GNF GND GND
The circuit is essentially the same, except that I've changed the
source
bias to three Si diodes in series, and a 1k resistor. I think I'll
lower the 1k even lower, to get the bias point to where the sine wave
swings are more symmetrical.
I suggested using a cap because I think you do in fact have some
DC
bias at the gate. It is possible that you are getting some slight
rectification of the AC on the gate (since it's a junction FET),
and
With the attenuation of the RC netwrok being 18, I don't see how the
signal swing could be more than 9V / 18 or 1/2V peak-to-peak.
That's why I said "slight".
the 10 uF capacitor looks to be an electrolytic (I infer this from
the
+ sign on the left plate), and probably has enough leakage to give
some gate bias.
I used a larger cap before, and it was leaky, which upset the bias.
I
changed it to 10uF, and the leakage problem went away. I'm not
saying
that the 10 uF has none, just that it seems that it's much, much less
than the larger cap. As a precaution, I could change it to a
different
cap. However I don't have a 10 uF in a non-'lytic that's anything
reasonable in size. I can parallel a few 1 uFs instead. The
reactance
of the 10 uF is about 24k at .67Hz. If I go to a 1 uF the reactance
will be 240k, which is more than 5 percent of the 4.3M gate resistor,
and adds appreciably to the attenuation. In this circuit, with its
damped osc problem, that can make the difference between sustained
oscillation and damped.
In another posting John pointed out that the reactive and resistive
components combine
in quadrature, so that the ratio of 240K to 4.3 Meg doesn't by itself
indicate what the
change in gain requirement will be for a change to 1 uF. As you
pointed out to him in
response, there is the changed phase shift to consider. One really
has to analyze the
full phase shift network. The 10 uF and 4.3 Meg you have there now
changes the required
gain also, and I thought I would calculate the numbers for the cases
we're talking about.
For a network of 4 equal 330K/1uF sections, the theoritical
required gain is -18.3878
For the four sections plus the 10uF/4.3Meg the required gain
is -20.0796
For the four sections plus the 10uF/4.3Meg and with 11 Megs
additional to ground at the
gate, the required gain is -20.7562
For the four sections plus 1uF/4.3Meg the required gain
is -21.7218, so changing the 10
uF to 1 uF raises the required gain by 21.7218/20.0796, or 8.2%.
And, of course, the frequency of oscillation changes, about +7.7%
from the first to
last case described above.
Well, there's 3200 uF across the source resistor, and the currenmt thru
I made up a jig with a socket for the FETs. I used two 9V batts, one
Well, with impedances of megohms and a load resistor of 33k, the
Or pronounce "Worcester" ;-)
480)460-2350 | |480)460-2142 | Brass Rat |It's hard to believe this is caused by fields from the microwavesnip
I tried this Pete and it seems to be the problem. I moved it towards
the screen and it got worse and visa versa. Thanks alot for the help.
j b
It may be hard to believe, but as I noted in my other response
Unfortunately, magnetic fields are not that easily contained.
It's certainly A likely source, but probably not the only
Yes I know,
But this is supposedly a very small normal leakage from a standard
Were those all meant to be links?
They haven't been the other 200 times he's posted it: