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eHam Forums => HomeBrew => Topic started by: DXTUNER on May 31, 2013, 09:31:21 AM



Title: Colpitts oscillator.
Post by: DXTUNER on May 31, 2013, 09:31:21 AM
I've come across a few templates for a Colpitts oscillator that I think a simpleton such as myself can make. But I need some advice please.

1. What (roughly) should the C1:C2, i.e. top:bottom, ratio be, for VHF? Can they be equivalent?
2. For the whole tank circuit, should the L:C ratio be rather small, since its a parallel circuit?

Thank you for answering.


Title: RE: Colpitts oscillator.
Post by: KB1GMX on May 31, 2013, 09:52:20 AM
1. What (roughly) should the C1:C2, i.e. top:bottom, ratio be, for VHF? Can they be equivalent?

Varies with active device used,  the tuned circuit, and layout (important at VHF).

2. For the whole tank circuit, should the L:C ratio be rather small, since its a parallel circuit?

Varies with overall scheme of the above.  For VHF the stability of the osc is likely not going to
be enough for most uses other than a raw signal source.  There are enough design variables
that not knowing a lot of exacts (transistor, frequency and all) that any answer is a guess.

Allison



Title: RE: Colpitts oscillator.
Post by: DXTUNER on May 31, 2013, 11:42:32 AM
Ok, I've drawn it - in case you want to look at it & then answer. Its just a real basic Colpitts oscillator meant to be an l.o. in a VHF converter I'm building. Please note: I just want to make a simple thing, for fun. I'm not trying to compete with Palomar or Palstar here.

Please note 2: I'm not asking for biasing advice or anything like that. I can already bias it. I'm just wondering generally what the C1:C2 ratio should be, at VHF. This is my first Colpitts circuit attempt.

Thank you much.

https://www.circuitlab.com/circuit/9jd4zd/proposed-colpitts-oscillator/


Title: RE: Colpitts oscillator.
Post by: WB6BYU on May 31, 2013, 12:46:10 PM
When in doubt, make both capacitors equal for a starting point.  Then vary
the ratio once it is working to see if you need less feedback.

Values?  I'd probably start with reactances in the 100 - 500 ohm range,
depending what I have on hand.


Comments on the initial circuit:

The tank coil will conduct DC from the collector to base, throwing off your bias.

The center tap from the capacitors should go directly to the emitter rather than
to ground (though that will work if the emitter resistor is bypassed.)

A popular way to reconfigure the oscillator is to:
1) remove the RF choke in the collector lead.  Connect the collector directly to
the +12V supply with a bypass capacitor to ground

2) disconnect the tank coil from the collector and tie it to ground instead.  This
is still effectively connected to the collector because both are at RF ground.

3) use a larger emitter resistor (1K or so).  Tie the capacitor center tap directly to
the emitter instead of ground.  Take the output from the emitter through a small
coupling capacitor.

4) put a series capacitor between the base bias resistors and the tuned circuit
so the coil doesn't provide a DC path that messes up the bias.


Also note that, while the MPS-H10 is rated for 25V, some of the other VHF/UHF
transistors have lower voltage ratings, and may be best run at 6V or 9V rather
than 12V.


Title: RE: Colpitts oscillator.
Post by: G3RZP on May 31, 2013, 02:50:05 PM
I usually look at the emitter to ground capacitor being twice the value of the base to emitter cap. But a lot depends on the transistor gain. If you are worried at all about phase noise, an un-bypassed 27 ohm emitter resistor helps.

The old W3JHR 'synthetic rock' is a useful variation, too.


Title: RE: Colpitts oscillator.
Post by: KB1GMX on May 31, 2013, 08:08:53 PM
Interesting..

two errors.  One its a Pierce variant of colpitts.  The other is the coil
shorts the bias from collector to base.

The coil should have a cap in series to block DC.  You can redraw the circuit
as a Pi network connecting the collector to the base (through a DC blocking cap)
and the transistor acting like a amplifier.

Now the collector side cap is likely to be the smaller value and the base side
the larger.  There is a lot of latitude in values and what hits frequency is
more important, the right values will tend to be more stable in frequency as well.

W6byu gives some good starting values.

Note when you say VHF that is any frequency between 30 and 300mhz, so a
more specific range is nice to know.

Allison


Title: RE: Colpitts oscillator.
Post by: DXTUNER on June 01, 2013, 09:57:51 AM
Thank you all very much. Well, maybe it was dumb luck, but I put together a simpler Colpitts-like circuit which, to my surprise, works rather good. It isn't perfect but the output signal is strong, from start up. There are a couple of drawbacks that I'd like fix, if possible.

1. There's a drift of about 1 MHz on start up, i.e., the signal will travel back and forth between 121.500 MHz and 122.500 MHz. It settles down a bit after 15 minute or so.
2. Connecting a simple J310 buffer circuit decreases the output signal, to a level that I'm not comfortable with.

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





Title: RE: Colpitts oscillator.
Post by: G3RZP on June 01, 2013, 10:04:54 AM
What circuit for the buffer? At that frequency, you probably need a source follower - anything with a grounded source and drain load will have problems caused by Miller effect, which will decrease input impedance.


Title: RE: Colpitts oscillator.
Post by: KB1GMX on June 01, 2013, 10:28:40 AM
Again you have the coil connecting the collector to the base The only thing
limiting the current is that 1K resistor.  The fact that it oscillates is not that surprizing
but that it has no stability is not.

You need a cap between the coil and the base of the transistor.  Any value like .001uf
will help.

Use 9V and regulate it.  Any voltage change will result in frequency change.

The more output level you try for the more heating of the transistor and the more drift.

If you can get the drift down to around 50khz your doing ok.  10Khz is exceptional.
It will never be as stable as a crystal at 121mhz.

FYI: that is the aircraft band (108-135mhz) and radiating a signal there can be serious. 
An oscillator there should be well shielded.

Using a fet as a buffer is problematic due to gain and power limiting.
 

Allison


Title: RE: Colpitts oscillator.
Post by: DXTUNER on June 01, 2013, 01:12:32 PM
Again you have the coil connecting the collector to the base The only thing limiting the current is that 1K resistor.  The fact that it oscillates is not that surprizing but that it has no stability is not.
Allison

I want to be sure I follow you. Are you saying the L and C should be flipped? So that the 2 capacitors are closer to the B-C bridge? Or are you suggesting that the tank circuit should be put somewhere else entirely? Yes, sorry I wasn't clear before, but I want it to oscillate in the 115-125 MHz range. Its the only way I can see how to make a VHF converter for the air band (to SW)

My designs look weird, I'm sure, but I'll explain the method. I want to get an oscillation first, then build off of that. Initially I was constructing published circuits having 12-20 components. Such as, but not limited to:
http://www.qrp.pops.net/butler.asp
And this one, ("Starts first time, every time." See figure captioned "local oscillator".):
http://homepage.eircom.net/~ei9gq/vhf_conv.html

Well, neither of them worked for me at all. I spent considerable time tweaking the values afterward. Still, nothing. And I'm looking at those circuits, seeing parts that don't make any sense *to me*. So, I take small sections of certain circuits which make sense to me, leaving behind remainders I don't understand, and blend them into a simple hybrid that will at least oscillate. I mean, what good is an oscillator that doesn't oscillate? My goal is to use a simple circuit as a starting point. But I want to least get an oscillation, first. Which I'm getting. I hope somebody understands my reasoning, if only one person.

What circuit for the buffer? At that frequency, you probably need a source follower - anything with a grounded source and drain load will have problems caused by Miller effect, which will decrease input impedance.

It was a source follower, using a J310. From my results I'd have to agree with KB1GMX that JFET's are gain killers. I want my l.o. to have as strong a signal as possible, because I don't want any HF bleed through. On my earliest attempts making a VHF converter I got too much from the RF input, and not enough from the local oscillator.

Thank you for continuing to throw me advice and tips. Try to ignore that I appear stubborn.








Title: RE: Colpitts oscillator.
Post by: DXTUNER on June 01, 2013, 02:47:08 PM
I said too much in reply before fully absorbing all of your posts. Ok, I'm going to try this:

https://www.circuitlab.com/circuit/j4r25a/try-this/




Title: RE: Colpitts oscillator.
Post by: WB6BYU on June 01, 2013, 07:04:23 PM
Making progress, but there are still some critical components missing.

1) The coil shunts the base bias to DC ground.  You can run an FET with zero volts
on the gate, but a bipolar isn't happy in that condition.

Fix:  add two resistors:  one from +6V to base, one from base to ground.  If you
use 33K from 6V to base and 10K from base to ground you should get about
1mA collector current in the transistor.  But you also need a SERIES capacitor
between the coil and the junction of the two resistors with the base to prevent
the coil from shorting out the bias voltage.  (There are other configurations you
can try, but that is the simplest.)

2) The emitter needs a DC connection to ground.  A 1K resistor will suffice.
In fact, take the one out of the collector circuit and use it.  Connect the
collector directly to +6V AND MAKE SURE YOU HAVE AN RF BYPASS CAPACITOR
CONNECTED DIRECTLY FROM COLLECTOR TO GROUND.  Something like 470pf or
1000pf.

3)  You still need a series output capacitor between the emitter and the
buffer stage, otherwise the DC levels will be thrown off.

4)  I'd probably choose something smaller than 22pf for the tuned circuit,
perhaps 10 to 15pf, and then roughly 1x to 3x that value in place of the 82pf.
You'd have to use a larger coil, of course, and if that is all you have then
you can live with the existing values.


For each circuit you have to make sure that both the DC and RF circuits are
set up properly, and that they don't interact.  Neither will work properly without
the other.  The DC circuit requires appropriate voltages on the collector (6v),
base (roughly 1.5V) and emitter (about 1V based on the recommended values.)
Note that a coil looks like a zero ohm resistor at DC, while a capacitor is an open
circuit.  If connecting the next stage messes up the bias then that needs to be fixed.

For RF you need to make sure that every terminal is properly and intentionally
connected as needed to make a complete circuit.  If the battery or power supply
is not properly bypassed, the connecting leads may become part of the tuned
circuit, and the frequency will vary when they are flexed.  If bypass capacitors
are too large they will look like inductors (due to the lead length, which must be
kept short).  That's why we're recommending 1000pf rather than 10nf bypass
capacitors at this frequency.


Title: RE: Colpitts oscillator.
Post by: DXTUNER on June 01, 2013, 11:41:10 PM
Like this?

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


Title: RE: Colpitts oscillator.
Post by: WD4HXG on June 02, 2013, 06:59:01 AM
Here is what I typically use. The bias values for the
resistors on Q1 may be off and have to be adjusted.
Working from a 62 year old memory here.

Q2 is a buffer (emitter follower) which minimizes the
connections downstream reflecting their load back
to the oscillator and causing frequency pulling.

I usually do not use a resistor in the collector of
the buffer or oscillator. The Colpitts as shown is
an emitter follower which provides POWER gain.

For a LC tank I usually set the tap at 50%. Thus
if you have 100 pF as the resonating cap then use
200 pF for each. Similarly if your resonating cap
is 50 pF then use 100 pF for each.

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. The less feedback through the
crystal the less heating and less drift. (Yes even
the small microwatt feedback levels on quartz
create enough heating to shift the quartz
frequency of resonance).

As for getting better than the 1 MHz you are
currently experiencing over 15 minutes you are
doing pretty darn well already. LC tanks can be
fairly stable over temp with tolerable drift up to
5 to 10 MHz (my personal experience). At 100
MHz you really need either a quartz resonator
or a PLL system otherwise the drift is likely to
be way more than you want to tolerate. Radios
from the 50's through the 70's used phase locking
techniques to keep the receiver LO locked to the
incoming station the listener tuned to.

If you use a crystal you really need an overtone
rock. 5th or 7th overtone will get you between
the 100 and 200 MHz range. Don't try to use a
fundamental crystal in an overtone application.
The harmonic resonances in fundamental crystals
are much lower amplitude than the fundamental.
You will find yourself chasing your tail trying
to add selectivity in the oscillator to narrow
the amplification bandwidth so the other resonances
do not pull you off frequency. Also the harmonic
resonances are often not exact multiples of the
fundamental. They will be close but off just
enough to be maddening.

Hope this helps.

73

Chuck  WD4HXG

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


Title: RE: Colpitts oscillator.
Post by: WB6BYU on June 02, 2013, 08:14:27 AM
Like this?

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



Not quite:  the base bias resistors can't deliver any DC bias to the base because
the 47nf capacitor blocks it, and the coil still shunts the base to ground for DC.

Connect the bias resistors directly to the base lead, then put the series capacitor
(47nf is way too large:  47pf would be better) between the bias connection
and the tuned circuit to isolate the coil from DC.

Then to make it an overtone crystal oscillator, adjust the LC circuit so it resonates
close to your desired frequency and add the crystal in series in the lead between
the emitter and the two capacitors in the tuned circuit.


Title: RE: Colpitts oscillator.
Post by: DXTUNER 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.


Title: RE: Colpitts oscillator.
Post by: WB6BYU 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.


Title: RE: Colpitts oscillator.
Post by: DXTUNER 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.


Title: RE: Colpitts oscillator.
Post by: WD4HXG 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


Title: RE: Colpitts oscillator.
Post by: DXTUNER 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/


Title: RE: Colpitts oscillator.
Post by: DXTUNER 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.


Title: RE: Colpitts oscillator.
Post by: WD4HXG 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.



Title: RE: Colpitts oscillator.
Post by: WD4HXG 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


Title: RE: Colpitts oscillator.
Post by: DXTUNER 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.


Title: RE: Colpitts oscillator.
Post by: WD4HXG 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.




Title: RE: Colpitts oscillator.
Post by: DXTUNER 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.


Title: RE: Colpitts oscillator.
Post by: DXTUNER 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/






Title: RE: Colpitts oscillator.
Post by: WD4HXG 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).


Title: RE: Colpitts oscillator.
Post by: DXTUNER 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.


Title: RE: Colpitts oscillator.
Post by: WD4HXG 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.


Title: RE: Colpitts oscillator.
Post by: DXTUNER on June 06, 2013, 07:20:51 PM
Thank you, Charlie. I've stepped back from the project for a few days because of (a.) family priorities and (b.) my only HF receiver going kapoot. Or is it '"kaput"? My goal is to make a VHF converter that I can use on an HF radio. I have a Uniden scanner that receives well, but its no fun to me because it doesn't really have a tuning knob. I like to be hands on when dx'ing and monitoring.