On a sunny day (Sat, 27 Jul 2013 13:36:50 -0700 (PDT)) it happened "M. Hamed"
<
mhdp...@gmail.com> wrote in
<
32a280bd-642b-4d82...@googlegroups.com>:
>#4
>---
>
>I realized how much fun oscillators are and how Sine waves are so beautiful=
> so I decided I'm going to up it a notch and take a shot at a 100 MHz oscil=
>lator.
>
>I tried a Colpitts variation of the Hartley oscillator I posted earlier wit=
>h the same MPF102 JFET, a diode, 1 MOhm Res, the tank, and its surrounding =
>capacitors. Inductors and Caps had to get much smaller. So L is in the nH r=
>ange and C is in the low pF.
>
>Couldn't get the thing to oscillate at all no matter how hard I tried. I tr=
>ied too many things to keep track of. I was suspecting the inductance is so=
> low and the inductor Q is inadequate. I tried different types of air-core =
>wire turns. Eventually I moved to the Toroid version but no luck.
1 MOHm ?????? WHERE?
+ 12
|-------
|--- d |
----------------------->| BF245 ===
| | |--- | 100n
5 turns === 47p | ///
8mm diam. |------------|
1 cm length === [ ] 470 Ohm to about 1k
| | 100p |
/// /// ///
You need the right kind of JFET, BF254 are around for a few cent on ebay and
go up to 700 MHz.
>Next, I decided a JFET isn't the thing for this and decided to move to BJT.=
> I used a 2N3904. I played a lot with the number of turns and capacitor val=
Would it not be simpler to DC bias from the cold side of the coil?
+
|
|--------
|/ |
--------------------| ===
| | |>\ | 100n
( === 47p | ///
( L |------------|
+ ( === [ ] 1k
| | | 100p |
33k [ ] | /// ///
|------|
| |
15k [ ] === 10n
| |
/// ///
Now L is no longer damped by 33k paralel with 15 k, only by the transistor Zi, and that is about beta x 1k (Re).
>My L is 3-5 turns on a 68-6 toroid. The value on the circuit is just a gues=
>s based on the frequency I'm getting. I had the following observations:
At many MHz it is unusual to use toroids, those have a high Al, you want low.
Toroids can have huge losses too at higher frequencies, depends on the material,
>- I had a big 300p cap for C2 this killed the ability to oscillate (later I=
> realized it had to do with the amount of feedback. If you reverse C1 and C=
>2 however, simulation shows the voltage swing is low. I am guessing it has =
>to do with transistor gain. Increasing C1 should probably also mean decreas=
>ing R3. I am taking shots in the dark here, I will try to do some analysis =
>later).
>- Even with C2=300p, increasing C3 to 66p restored oscillation.
>- After reducing C3 back to 33p, I was able to reduce C3 back to 33p and ge=
>t oscillation.
Confusing circuit, C3, IF you must have it, should be on the other side, connected to R1, R2,
and a much higher value (10nF), as it purpose is ONLY decoupling,
It should not be part of the tuning parameters.
C2 C3 should then be connected to the coil.
>- I kept on trying to reduce inductance to increase frequency but that came=
> with a price of reducing the voltage swing at the tank.
>- I still do not have a unified theory of how the values of L and Cs affect=
> frequency and voltage swing but it seems I should strive for having higher=
> L for better stability and voltage swing. This means I need to reduce the =
>C's to as low as I can get.
>- In the end I was able to reach 67 MHz as the max I could squeeze out of m=
>y circuit but it's not very stable.
Use an air coil, as drawn in the above diagram, for 100MHz and up, like in this picture:
http://tvforfree.files.wordpress.com/2012/09/georgesmartfcd.jpg
>- Even with everything laid out on a perf-board and with no power supply by=
>pass you still can see 67 MHz of oscillation that is clean looking on a sco=
>pe. Stability is another matter.
>- Soldering and de-soldering is a pain when you have try different things.
>- a plain vanilla 2N3904 can go long ways!
>- I don't like JFETs. I love BJTs.
JFETS are cool, they make good low noise amps too.
I once did a cascode JFET input amp that beat the better BJTs at that time.
It also depends on what make MPF120 you have,
one datasheet here shows a graph that its gain is flat to 700 MHz.
Remember that when using a source follower as gain element the VOLTAGE gain of the JFET stage
itself is always a bit less than 1.
So to get > 1 feedback you need to step up with either a transformer (tap on coil or extra turn),
or as in this case with the right ratio of capacitors, AT LEAST 2 to 1, or even higher.
try again.
>-------------------------------------------------
>Next steps:
>
>- Add more turns to my toroid
No toroids!!!!
>- Try a better a toroid core, may be a 2 or 6 Mix
Air core!!!
Use a trimmer capacitor, or a small coil former with a core you can screw in and out for adjustment.,
>- Reduce caps to the minimum possible.
No, no, that will only create instability,
Around 100 pF should be fine.
>- Hope for 100 MHz.
100 MHz is nothing,
how do you measure frequency? Not loading that LC with a counter I hope?
Use an FM radio tuned to 100 MHz, and a 50 to 250 pF (or there about) antique
air spaced variable capacitor, of if not in possesion of such a wonderful piece on engineering,
bend the air coil, and turn or bend until you hear 'flop flop' sounds from that radio.
And make sure that is not just an harmonic...