"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.

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.

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.

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.

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 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.)

Colin =^.^=