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Author Topic: Colpitts oscillator.  (Read 17314 times)
DXTUNER
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« Reply #15 on: June 02, 2013, 09:36:24 AM »

Ok, its amended, but I have a concern. What prevents the new 47 pF capactitor from detuning the tank circuit? As in: 47 pF series + 9.56 pF tank capacitance = 7.94 pF new resonance capacitance, for 126.3 MHz?

https://www.circuitlab.com/circuit/xtp8m5/new-circuit-to-try/

Now, as I'm typing this, I see WD4HXG's reply. There's something in his post which answers my original question that began the topic.

For a LC tank I usually set the tap at 50%. For crystals I start out with a 1:10 ratio. The objective is to use as small amount of feedback as possible to get the circuit started and sustain operation over the temperature and operating voltage range.

Thanks a million for that. It's exactly what hadn't clicked in my brain: smaller feedback = better start up. Also, thanks for drawing a Circuit Lab schematic. But I think you need to label it "public" in order for me to see it.

As for getting better than the 1 MHz you are currently experiencing over 15 minutes you are doing pretty darn well already. If you use a crystal you really need an overtone rock.

I kind of though that also, the drift not being that bad for an LC circuit. I was also surprised that it started up immediately. Maybe it's a keeper after all.

https://www.circuitlab.com/circuit/q566f9/works-good/

As far as crystals are concerned, here's what I have available:

  • 115 MHz, 5th overtone. Well used, I pulled it from a 1980's-era VHF converter. Seems to be right on the money still, though.
60 MHz, 3rd overtone. Brand new. Gives me an awesome 7th overtone signal at 140 MHz.
25 MHz, fundamental. Brand new. Gives me a strong signal at 125 MHz, but yes, I realize it's a fundamental crystal.
« Last Edit: June 02, 2013, 09:48:10 AM by DXTUNER » Logged
WB6BYU
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Posts: 17790




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« Reply #16 on: June 02, 2013, 10:32:23 AM »

Quote from: DXTUNER

Ok, its amended, but I have a concern. What prevents the new 47 pF capactitor from detuning the tank circuit? As in: 47 pF series + 9.56 pF tank capacitance = 7.94 pF new resonance capacitance, for 126.3 MHz?



Yes, the other capacitances will affect the resonant frequency - that is one reason
for drift.  Generally you would make the series capacitors (both to the tank circuit and
the coupling to the buffer) as small as possible consistent with good operation.  One of
the main shifts will be the base-to-emitter capacitance of the transistor, which changes
with voltage and temperature and is effectively across the tank circuit (in series with the
47pf isolation capacitor).  All these things do interact, and the stray capacitances become
a larger contribution to the overall frequency of the circuit at VHF where the tuning caps
are smaller.  That's one reason why VHF LC oscillators are rarely used in receivers without
some sort of stabilization because the drift is proportionally worse than at HF.


Meanwhile, it may be enlightening to measure the DC voltages on the transistor pins in
each of your circuits.  That would help to catch problems with base bias, etc.
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DXTUNER
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« Reply #17 on: June 02, 2013, 12:34:19 PM »

Hi Dale, forgive me for being philosophical, but wouldn't the details of your reply be reasons FOR keeping the circuit as simple as possible? It goes right along with what I've found, empirically: circuits with more parts I can't get to 'go', but these Simple Simon things I can at least get a strong oscillation. To me its like having a simply balanced Seesaw. You add something to the Seesaw then you need to add something else, to counteract. And it keeps going on like that, until the Seesaw is no longer really a Seesaw, nor is it balanced. Don't get me going either on how unneccessarily complex our tax code is either. Anyhow, forgive me for that. We all have different approaches to things.

By the way, the Hamtronics 144-146 VHF converter is off the charts. On my little Tecsun PL-600 I get 2-meter transmissions so well that I need to kick in the radio's attenuator. The builder of that deserves an A+, for sure.
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WD4HXG
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Posts: 337




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« Reply #18 on: June 02, 2013, 06:46:18 PM »

Sorry about failing to make the file available. It is now accessible by
the link. I am a newby with the Circuit Labs so thanks for the heads
up on the subtle features.

Also reducing the feedback does not necessarily improve the startup.
Indeed it can make it even harder to start. In general you want to
use as little feedback as possible to prevent IR losses in the components.
It is a balancing act and can be quite tedious.

In LC circuits the LC losses will have little effect generally with
oscillators in the 10 mW range. With crystals it can be a real
problem child creating thermal drift issues. If to high then the
quartz can be damaged to the extent that the quartz will crack.
Keep in mind that overtone rocks are usually very thin and much
more susceptible to thermal and mechanical stresses due to their
thinner plate. If you dissect an overtone crystal you will find the
center of the plate is typically thin enough to see through.

73

Chuck WD4HXG
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DXTUNER
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« Reply #19 on: June 02, 2013, 10:09:17 PM »

Ok, it didn't work.

https://www.circuitlab.com/circuit/xtp8m5/bah-humbug/

But, when I worked my way backwards, piece by piece, arriving back at my 'dum-dum topology' (resonator at collector & base, self biasing scheme, and 1 K resistor at the collector, not the source), then I had something which worked.

https://www.circuitlab.com/circuit/98b5q9/this-also-works/
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DXTUNER
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« Reply #20 on: June 03, 2013, 04:58:57 AM »

I just found this published Colpitts circuit, the resonant inductor place in the collector. I'm going to see if I can use this simple scheme at 115-125 MHz, using an MPSH10 or an MPS901 instead.

http://commons.wikimedia.org/wiki/File:NPN_Colpitts_oscillator_collector_coil.png

Not that I've had any luck at all with a C-tap at emitter/ground level, but I'm willing to keep trying.
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WD4HXG
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Posts: 337




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« Reply #21 on: June 03, 2013, 05:15:18 AM »

Check this link again. It is marked public. Just rechecked.
It may be that when I changed the status from private
to public the URL changed.

https://www.circuitlab.com/circuit/5rtwm4/charlie-colpitts/


Do not give up on the configuration with the resonator in the
base-emitter circuit. It is a proven design. It works.

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WD4HXG
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« Reply #22 on: June 03, 2013, 06:30:04 AM »

Are you using + 6 volt, -6 volt and Ground?

If so you need an AC ground path (capacitor)
from the -6 volt to the circuit ground. The
reason I ask in the schematic you show the
collector of the MPSH transistor with a bypass
cap connected to ground. If you are indeed
running a differential supply then you will need
a bypass from each both supply rails to the
circuit Ground reference.

Also in your schematic:

https://www.circuitlab.com/editor/#?id=xtp8m5

if you draw the line directly from the resonator
cap tap and drag the line to the emitter it will
connect to the emitter plus place a jumper over
the line for the bias resistor line. It will look similar
to 1/2 of a sine wave if it works correctly.

I think the following link reflects what you are
trying to do:

https://www.circuitlab.com/circuit/2m44sw/your-circuit-revised/

The following is the circuit using a differential supply.

https://www.circuitlab.com/circuit/a9ahdg/your-circuit-revised-differential/

73

Chuck WD4HXG
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DXTUNER
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« Reply #23 on: June 03, 2013, 10:22:00 AM »

Are you using + 6 volt, -6 volt and Ground?

Perhaps I was wrong with my convention. The 6 V - is from the ground terminal of the 6 volt lantern battery.

Honestly I'm not in favor of DC-blocking or DC-bypass capacitors at all. Not when using a battery, anyway. I could see it if I were using an AC/DC transformer, but not with a battery. I've been adding them recently only because you good folks on here tell me that I should. But I can't see where they improve anything. In a few cases my circuits worked better without any DC-blocking capacitors.

I appreciate everything. I'm learning a lot. Don't lose your patience with me. I do get aggravated when these published designs don't work, though. If its published it should be easily reproducable.
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WD4HXG
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Posts: 337




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« Reply #24 on: June 03, 2013, 05:55:14 PM »

The reason for using the caps is so as to not defeat
the biasing. To get the transistor to work you need
current to obtain conduction from the collector to
the emitter. No base current, no collector current.
It is possible to get a transistor to oscillate with the
bias off from optimal but you usually wind up with
other unwanted effects, such as higher than expected
or wanted harmonic levels.

The resistors in the emitter, collector, and base circuit
set the bias for optimal amplification. When you add an
inductor which shorts the base to ground the class of
operation of the stage is changed, usually for the worse.


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DXTUNER
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« Reply #25 on: June 03, 2013, 06:23:59 PM »

It is possible to get a transistor to oscillate with the bias off from optimal but you usually wind up with other unwanted effects, such as higher than expected or wanted harmonic levels.

Thanks. Well that explains the numerous harmonics I usually get, then. I'm about to try this one that I posted about above. My frequency adjusted variation:

https://www.circuitlab.com/circuit/7ykx8p/definitely-worth-trying/

Unfortunately the 'master file' that I'm emulating doesn't have an explanation. So I'm wondering if the collector inductor (please see) is actually resonating with the capacitance of the 2N2222 transistor shown. I'm going to use an MPS901, but that's getting off point. I believe I'd seen somewhere an oscillator circuit where the transistor's capacitance is used to resonate with a coil. I'm not sure if that's what going on the scheme I'm copying. Nah, I really don't think the 2N2222 has a capacitance of 50 pF.

http://commons.wikimedia.org/wiki/File:NPN_Colpitts_oscillator_collector_coil.png

By the way Charlie, I see you're up in Sterling. I'm down in Richmond.
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DXTUNER
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« Reply #26 on: June 03, 2013, 08:20:17 PM »

Ok, for the first time I got something using an emitter-oriented Colpitts configuration. So there might be hope. However, it was on my second attempt. My first try didn't work at all. That was with the 33:33 and 0.1 uH set up I posted above. But when I changed it 18:15 and 0.22 uH I got something. Probably the worst sounding signal I ever heard, it sloshed in & out at will, but it was something.

I think I have bias adjustments to do on this. Then maybe I'll get the signal good. I'm open for biasing critique.

https://www.circuitlab.com/circuit/kh5a2x/second-version/




« Last Edit: June 03, 2013, 08:23:35 PM by DXTUNER » Logged
WD4HXG
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Posts: 337




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« Reply #27 on: June 04, 2013, 01:48:20 PM »



Thanks. Well that explains the numerous harmonics I usually get, then. I'm about to try this one that I posted about above. My frequency adjusted variation:

https://www.circuitlab.com/circuit/7ykx8p/definitely-worth-trying/

Unfortunately the 'master file' that I'm emulating doesn't have an explanation. So I'm wondering if the collector inductor (please see) is actually resonating with the capacitance of the 2N2222 transistor shown. I'm going to use an MPS901, but that's getting off point. I believe I'd seen somewhere an oscillator circuit where the transistor's capacitance is used to resonate with a coil. I'm not sure if that's what going on the scheme I'm copying. Nah, I really don't think the 2N2222 has a capacitance of 50 pF.

http://commons.wikimedia.org/wiki/File:NPN_Colpitts_oscillator_collector_coil.png

By the way Charlie, I see you're up in Sterling. I'm down in Richmond.

If you built the circuit above (definitely worth trying) per the schematic then you
may want to use a cap from the Vcc end of the inductor to circuit ground. About
470 to 1000 pf is good. While batteries are suppose to in theory look like a short
to a voltage at RF they can be quite an impedance.

One thing to start thinking about is the concept of the circuit DC ground and
an AC ground. They are usually the same point but spots like the Vcc need
essentially a large capacitor that looks like a short to the signal (120 MHz).
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DXTUNER
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« Reply #28 on: June 04, 2013, 04:07:30 PM »

While batteries are suppose to in theory look like a short to a voltage at RF they can be quite an impedance.One thing to start thinking about is the concept of the circuit DC ground and
an AC ground. They are usually the same point but spots like the Vcc need essentially a large capacitor that looks like a short to the signal (120 MHz).

Thanks Charlie. I need to step back and absorb a few things that I don't get. Including -- and I know this makes me look bad, I don't care -- transistor biasing. I don't fully understand it. Normally I have to use online "biasing calculators" to make a circuit.
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WD4HXG
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Posts: 337




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« Reply #29 on: June 05, 2013, 10:52:24 AM »

There are two worlds in play within radio circuits. One is
DC, the other is AC. Transistors are biased within the
DC realm. In oscillators the general setup is Class A
biasing.  You are setting the stage for the typical Class
A amplifier configuration. That establishes the playing
field for the addition of components that are evaluated
with the AC view. There are interactions between the
two but in general are fairly easily managed.

All of us started out without knowledge of the circuits
but learned over time. You are following the same
learning curve as predecessors. Don't let it put you
off.
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