colin wrote:
"Anthony Fremont" wrote in message
...
colin wrote:
"Anthony Fremont" wrote in message

The drive from the amplifier will inevitably be heavily clipped,
this is cuased by the need to have sufficient gain to start
oscillation, the clipping is the efffect of getting rid of the
excess gain.

Ok, I guess that makes sense. You want plenty of gain, and with
plenty of gain comes clipping.

theres two types of clipping as youve probably figured out, current
clipping where the transistor turns off completly, and voltage
clipping where the transistor saturates,
with a bjt the later cuases problems with storage time, wich is
variable with temprature, hence makes for unstable HF oscillators.
the later is better but is difficult to ensure this happens before
the former with out some form of level feedback.

with jfet however either form of clipping isnt so bad, if you notice
the circuit I put here there is a smal amount of positive gate
current at clipping. but I just slapped those values together.

Ok, I think I'll stick with JFET based oscillators from now on. I like the
waveform, much better anyway. ;-)

The current waveform is usually quite distorted,
usualy this is perfect to generate frequency multiples with a
suitable tank.

This would be for overtone oscillators?

well not crystal overtones exactly, but frequency multipliyng
oscilators, say to generate over 100Mhz -
crystals tend to be difficult to get much higher than this even with
5th overtone.

I have some crystals for 2M HAM stuff in an old Kenwood TS-700A. I assume
these really don't oscillate at ~150MHz, but are in the 30MHz range but cut
to produce lots of harmonics.

There was a low distortion oscillator discussed in here not so long
ago. I think you do better with the inductor in the collector,
at least you need to take the output from the tuned circuit with a
capacitive divider,
but not via the transistor,
wich is easier with the inductor in the collector.

Also a jfet is ideal as it has a much softer clipping.

I found an MPF102 here and doctored up the circuit for it, much nicer
waveform. Much less voltage in the tank though, but it starts fine.
This is fun. ;-) I'm going to wind myself a lower inductance coil
and see if I can get it to run at 10MHz.

you can still easily get nearly 2x full supplly swing accros the tank
with a jfet.
the resistor can also be replaced by an inductor getting you to
exactly 2x supply.

Ok, I've been doing allot of tinkering yesterday and have some neat stuff
working here. :-) I picked up some 1mH inductors to use in the oscillator
and buffer. Beautiful results, until the kickback from the 74HC14, but I
can live with it.


have you given up with the ne602 and the crystal ?

Just side tracked for now. ;-) The crystal is kinda restrictive,
but I'm going back to the 602 soon. I'm going to try to make a
single conversion superhet for WWV reception at 10MHz. I have some
IF cans of various colors, so I'm sure I have some for 455kHz. If I
can make an LO that will stay within 1kc of 9.545 at room
temperature, I'm going to go with it to beat against WWV.

For interest try putting the crystal across the inductor when it is
tuned at the right frequency and see what happens, then see what
difference it makes at different frequencies. Ive played about with a
bag of10mhz crystals to find 2 that were precisly a some khz apart,
eventually I made one lower by degrading by heating it with a
soldering for some time, its to easy to seriously degrade it though.

unfortunatly most crystals are at a peak of frequency at room
temperature, above and below this temp they start to fall in freq,
untill you reach the higher flex in the tempcooef curve where the freq
starts to rise again.
this can be quite hot and the tempcoeef is then rather steep.

another trick is to look for crystals that will give you a correct
frequency at overtone, 3,5,7th
or use a frequency multiplier x2,x4 etc I spent ages doing this once.
you can even go up in freq then divide by 1-256 with ttl etc.
ofc a pll is the the other option.

Thanks for the info, I'll have to get back to the crystals, but I'm having
to much fun with coils now. ;-)

Right now my prototype circuit is drifting back and forth over a
1.5kc range while running at 4MHz. I'm using ordinary ceramic caps
to set the frequency, and everything is on a breadboard. The 2kc
drift seems to coincide with the air conditioning induced
temperature changes in the room. Plenty of room for improvement on
that. ;-)

The transistor capacitance changes with temperature, not to mention
cheap ceramic caps too,
god knows what the temp coef of breadboard cap is lol.
not to mention anything other than air cored inductors.
with a low capacitance transistor you would get better stability,
with a lower inductance the transistor cap has less effect too.
you can do a whole lot better than that with just the right parts.

I have a nice temp compensated oscillator wich I got with some other
stuff but not a usefull frequency.
unfortunatly a different crystal didnt match the temp comensation,
but you can use capacitors wich have the opposite temp cooef.
they also make trimmers like that too, although you can just put an
ordinary trimmer in series with it.

I added a PIC chip for stability. ;-)

I just wound another coil for 5uH. Things get more picky as the
frequency goes up, but I got it running at 9.5Mhz and it's staying
within a few kcs. Interesting stuff. :-)

last colpits I built was a wide range 2:1 1-2ghz VCO with a PHEMT
(fet) this had an extremely low capacitance, but is probably a bit
over the top for 10mhz.
I had 2 wich I tried to keep 455khz apart, very difficult, they liked
to sync up if they got closer than 1Mhz.
(were a few threads about it.)

Cool, but way beyond my reach. Yeah mechanical systems are like that. I
collect wind-up watches and many beat at the same rate (18,000 BPH). You
can put a bunch in say a sock drawer (box with hard, flexible bottom to
carry vibrations) and they'll all sync up after a while.