RadioBanter - Bad Colpitts osc Waveform

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Bad Colpitts osc Waveform

Hi All,
Please see pictures and diagram at ABSE.

I duplicated a circuit for someone because they got a bad
output waveform.
Well I did too.
Capacitor = air variable set at 365pf
inductor = 41uh aircore
With freq set at 1.3Mhz I have the voltages as follows;
Collector - 11.8vdc
Base - 3.1vac riding on top of 3.2vdc
Emitter - 2.5vac riding on top of 3.9vdc (Don't be critical, it's not ac
if it sets on dc)

As I shift up in freq at about 2.7Mhz the voltages jump.

Capacitor = air variable set at 40pf
inductor = 41uh aircore
Freq = 3.7Mhz
Collector - 11.8vdc
Base - 10vac riding on top of 1.8vdc
emitter - 1.1vac riding on top of 9vdc

The base waveform in both senerios is much more sinewave (ish) than the
output.

Any ideas how to get a decent waveform? The next step will be a buffer,
but...

Note; original schematic used a crystal, I substituted the LC. Could this
be the
cause of the distortion?

Thanks, Mike

Bad Colpitts osc Waveform

wrote in message
ups.com...

amdx wrote:
Hi All,
Please see pictures and diagram at ABSE.

I duplicated a circuit for someone because they got a bad
output waveform.
Well I did too.
Capacitor = air variable set at 365pf
inductor = 41uh aircore
With freq set at 1.3Mhz I have the voltages as follows;
Collector - 11.8vdc
Base - 3.1vac riding on top of 3.2vdc
Emitter - 2.5vac riding on top of 3.9vdc (Don't be critical, it's not
ac
if it sets on dc)

As I shift up in freq at about 2.7Mhz the voltages jump.

Capacitor = air variable set at 40pf
inductor = 41uh aircore
Freq = 3.7Mhz
Collector - 11.8vdc
Base - 10vac riding on top of 1.8vdc
emitter - 1.1vac riding on top of 9vdc

The base waveform in both senerios is much more sinewave (ish) than the
output.

Any ideas how to get a decent waveform? The next step will be a buffer,
but...

Note; original schematic used a crystal, I substituted the LC. Could
this
be the
cause of the distortion?

Thanks, Mike

Mike ,

What is ABSE? Need a circuit before comments can be made.

alt.binaries.schematics.electronic

Bad Colpitts osc Waveform

Most of the current newsgroup servers have blocked the binaries groups. A
lot of us can't get them.

Jim

"amdx" wrote in message
...

Mike ,

What is ABSE? Need a circuit before comments can be made.

alt.binaries.schematics.electronic

Bad Colpitts osc Waveform

Well, offhand, I don't like the way the base is biased... That
transistor will be clamping on the negative voltage swing which probably
answers why the waveform is so poor.

If you make a voltage divider for the base voltage, the transistor
oscillato ought to give a better waveform.

Perhaps something like 27K from +12 VDC to the basen, and 9.1 K from base
to ground.

This should improve the waveform shape....

Jim Pennell
N6BIU

--

11:40 Pacific Time Zone
Jan 21 2007

International Time
19:40 UTC
21.01.2007

Bad Colpitts osc Waveform

amdx wrote:

Hi All,
Please see pictures and diagram at ABSE.

I duplicated a circuit for someone because they got a bad
output waveform.
Well I did too.
Capacitor = air variable set at 365pf
inductor = 41uh aircore
With freq set at 1.3Mhz I have the voltages as follows;
Collector - 11.8vdc
Base - 3.1vac riding on top of 3.2vdc
Emitter - 2.5vac riding on top of 3.9vdc (Don't be critical, it's not ac
if it sets on dc)

As I shift up in freq at about 2.7Mhz the voltages jump.

Capacitor = air variable set at 40pf
inductor = 41uh aircore
Freq = 3.7Mhz
Collector - 11.8vdc
Base - 10vac riding on top of 1.8vdc
emitter - 1.1vac riding on top of 9vdc

The base waveform in both senerios is much more sinewave (ish) than the
output.

Any ideas how to get a decent waveform? The next step will be a buffer,
but...

Note; original schematic used a crystal, I substituted the LC. Could this
be the
cause of the distortion?

Thanks, Mike

This circuit doesn't look very good from any standpoint other than
(possibly) being cheap. It's questionable for a crystal oscillator in
anything but el-cheapo consumer electronics, and just bad for a coil &
cap oscillator. Most of the points have been mentioned already; I'll
just reiterate them with my own spin:

The base biasing is _wrong_. Biasing a bipolar transistor that way will
make it extremely sensitive to device and temperature variations. You
can somewhat get away with it in an oscillator because you're working
with a class C device, so the emitter voltage will rise with rising
oscillation strength, making sort of a poor-man's AGC (with emphasis on
the 'poor').

Bias should be with a resistive divider on the base, adjusted so that
the oscillator starts up nicely yet keeps the emitter voltage from
rising to within 1 volt or so of the collector voltage.

The ratio of C1 and C2 is also extreme. I'm surprised that you get
oscillation at all. The rule of thumb for a Colpitts is to choose
feedback caps with reactances at frequency of around 150 ohms. You can
improve the waveform (and load the circuit less) by retaining the 1nF
"C2", increasing the capacitive reactance to the emitter with a smaller
cap in series to the transistor emitter, and retaining the take-off
point at the hot end of C2.

I wouldn't build an oscillator like this without following it with at
least one buffer stage designed to minimize the effect of following
circuits on the oscillator, and following _that_ with an amplifier
stage. You're not building tube equipment, transistors don't cost that
much -- go ahead and use a few!

If you don't get a book dedicated to RF circuit design (like Hayward's
"Experimental Methods") then at least get a copy of the ARRL handbook
and follow the guidelines there. You can go wrong with them, but not
too much.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

Bad Colpitts osc Waveform

"Tim Wescott" wrote in message
...
amdx wrote:

Hi All,
Please see pictures and diagram at ABSE.

I duplicated a circuit for someone because they got a bad
output waveform.
Well I did too.
Capacitor = air variable set at 365pf
inductor = 41uh aircore
With freq set at 1.3Mhz I have the voltages as follows;
Collector - 11.8vdc
Base - 3.1vac riding on top of 3.2vdc
Emitter - 2.5vac riding on top of 3.9vdc (Don't be critical, it's not
ac
if it sets on dc)

As I shift up in freq at about 2.7Mhz the voltages jump.

Capacitor = air variable set at 40pf
inductor = 41uh aircore
Freq = 3.7Mhz
Collector - 11.8vdc
Base - 10vac riding on top of 1.8vdc
emitter - 1.1vac riding on top of 9vdc

The base waveform in both senerios is much more sinewave (ish) than the
output.

Any ideas how to get a decent waveform? The next step will be a buffer,
but...

Note; original schematic used a crystal, I substituted the LC. Could
this
be the
cause of the distortion?

Thanks, Mike

This circuit doesn't look very good from any standpoint other than
(possibly) being cheap. It's questionable for a crystal oscillator in
anything but el-cheapo consumer electronics, and just bad for a coil & cap
oscillator. Most of the points have been mentioned already; I'll just
reiterate them with my own spin:

The base biasing is _wrong_. Biasing a bipolar transistor that way will
make it extremely sensitive to device and temperature variations. You can
somewhat get away with it in an oscillator because you're working with a
class C device, so the emitter voltage will rise with rising oscillation
strength, making sort of a poor-man's AGC (with emphasis on the 'poor').

Bias should be with a resistive divider on the base, adjusted so that the
oscillator starts up nicely yet keeps the emitter voltage from rising to
within 1 volt or so of the collector voltage.

The ratio of C1 and C2 is also extreme. I'm surprised that you get
oscillation at all. The rule of thumb for a Colpitts is to choose
feedback caps with reactances at frequency of around 150 ohms. You can
improve the waveform (and load the circuit less) by retaining the 1nF
"C2", increasing the capacitive reactance to the emitter with a smaller
cap in series to the transistor emitter, and retaining the take-off point
at the hot end of C2.

I wouldn't build an oscillator like this without following it with at
least one buffer stage designed to minimize the effect of following
circuits on the oscillator, and following _that_ with an amplifier stage.
You're not building tube equipment, transistors don't cost that much -- go
ahead and use a few!

If you don't get a book dedicated to RF circuit design (like Hayward's
"Experimental Methods") then at least get a copy of the ARRL handbook and
follow the guidelines there. You can go wrong with them, but not too
much.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Thanks Tim,

The base biasing is _wrong_.
I tried Jim's voltage divider biasing arrangement, it didn't help the
waveform, but I understand
the temperature sensitivity of the original.

The ratio of C1 and C2 is also extreme.
This circuit and all values was taken from Joseph Carr's "Secrets of RF
Design"
I did replace the crystal with the LC. He calls this a 1 to 20 Mhz crystal
osc.

You can improve the waveform (and load the circuit less) by retaining the
1nF "C2", increasing the capacitive reactance to the emitter with a smaller
cap in series to the transistor emitter, and retaining the take-off point
at the hot end of C2.
Sorry, I don't know if I understand what you suggest; Do you mean to say
connect
a small cap from the connection point of C1 and C2 and the connection point
of the
emitter and R2?

I have seen several Colpitts osc. that have an inductor in series with the
emitter resistor,
would that be useful? If so what value for 3 to 6 Mhz.

On another note; I noticed the base waveform is pretty good. Has anyone
seen a design that
lightly couples a signal from the base to maybe a FET and buffers it. Just a
thought!

Thanks,
Mike

Bad Colpitts osc Waveform

"RST Engineering" wrote:

Most of the current newsgroup servers have blocked the binaries groups. A
lot of us can't get them.

20 cents a day will get your binaries back...

Bad Colpitts osc Waveform

amdx wrote:
On another note; I noticed the base waveform is pretty good. Has anyone
seen a design that
lightly couples a signal from the base to maybe a FET and buffers it. Just a
thought!

The collector waveform will always be much richer in harmonics than the
base waveform. If you want to multiply up this is Good. If you see some
random circuit in a handbook keep in mind that they may very well have
intended it to be used for multiplying up 2x, 3x, 4x, etc. so the bias
etc. may have been chosen just to make the waveform at the collector
look un-sine-ish!

There are numerous oscillators that take the output signal from a
tapped-down tank if they want a minimum of harmonic content. Tapping
down helps minimize the effect of loading on the tank Q, but taking the
signal from there will have at least some effect on oscillator
stability. Another place that might like look slightly nicer from a
waveform view (and may be lower impedance) is the emitter. And there
exists oscillators that put a smallish resistor in series with the tank
- the resistor value is chosen small to not degrade the tank Q very
much, and the small voltage across the resistor looks very sine-wavish.

Tim.

Bad Colpitts osc Waveform

On Sun, 21 Jan 2007 09:27:36 -0800, "RST Engineering"
wrote:

Most of the current newsgroup servers have blocked the binaries groups. A
lot of us can't get them.

Jim

I have recently subscribed to a service from the Agent Newsreader
folks at http://www.forteinc.com/agent/support.php
For $2.95 per month I get 7G of upload/download. Where your usage
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Usenet. It works for me.

BTW, are you the RST Engineering that used to make the Aviation
Electronics in the 1970's?

John Ferrell W8CCW

Bad Colpitts osc Waveform

"John Ferrell" wrote in message
...

I have recently subscribed to a service from the Agent Newsreader
folks at http://www.forteinc.com/agent/support.php
For $2.95 per month I get 7G of upload/download. Where your usage
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Yeah, and $3 a month here and $5 a month there for stuff that wildblue ought
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BTW, are you the RST Engineering that used to make the Aviation
Electronics in the 1970's?

And still do. www.rstengineering.com

Jim

Bad Colpitts osc Waveform

BTW, are you the RST Engineering that used to make the Aviation
Electronics in the 1970's?

And still do. www.rstengineering.com

Jim

Nice products. I still have a couple of your VOR testers that I mainly
used to verify my radios worked before departing the Bahamas on an
international IFR flight plan. After the plane had set a couple of
weeks it was nice to know the Navs were working before getting
airborne.

I probably ought to EBAY them. I have not used either for over 20
years and will have no use in the future!

John Ferrell W8CCW

Bad Colpitts osc Waveform

I duplicated a circuit for someone because they got a bad
output waveform.
As I shift up in freq at about 2.7Mhz the voltages jump.

If the voltages suddenly jump with frequency change then this is a sign of
instability. Not seen the circus digramus confusicus, but if it is a colpits
then reduce the gain by adjusting the ratio of the two divider capacitors. I
assume, of course, that you have adequately decoupled the supply line, and
that there is no inductive coupling/feedback.

Instability causes eratic frequency and/or severe distortion. In
particularly bad conditions it can cause unwanted visitors (CBers,
neighbours, DTI, etc.).