Cheap stable VFO design

[MrSlack]

So I started with grand plans, to build a nice stable but cheap varactor controlled VFO about a month ago that tuned ~7.000-7.040 MHz. Cheap instantly excludes large air variable capacitors, rare FETs, 10 turn pots and other such things.  This went through several iterations and periods of instability as well as about 4 different varactor types. Turns out varactors are expensive as well.

Basic idea is a simple discrete regulator providing an 8.5v reference voltage for the tuning voltage. This is then used to generate a tuning voltage with coarse/fine controls. The voltage is summed and then there is a simple emitter follower. This provides a voltage to the front end of the oscillator. This is a simple colpitts oscillator which uses a 1n4002's reverse biased capacitance vs voltage curve to generate a voltage controlled capacitance. This is used to trim the oscillator.

Drift is about 20Hz/min and is designed to be relatively stable based on battery voltage. Not wonderful but for the general cheapness, adequate.



Course should read coarse above. It was about 2AM when I scribbled that out...

[nugglix]

Hi!

That really looks simple and straightforward.

Will steal a lot ... 

I just discovered that (LTspice) simulation and reality are not necessarily share the same traits.
To my surprise the used varactor (1SV149) was much better in reality than in the simulation.

My current problem is that the transistor distorts the waveform, so I need a way to limit the gain
when the oscillation has started.
I tried 2 anti-par BAT-85 diodes but that introduced a lot of jitter.
I'll do some more experiments tonight - aka build a new oscillator. 

How does the waveform of your circuit look like?
Iow, do you care about that at all?

Cheers
  Guido

[MrSlack]

Yeah I started this design in LTspice. Reality wasn't anywhere near the simulation output.

1n4002 is surprisingly linear for a 0.8-8.5v tuning range. I tried a few BB series varactors and discarded them in the end.

The output waveform is not very clean as you'd expect.. Peaks look fine but troughs are flat, therefore harmonics. I'm adding a buffer amp to it and a low pass with a 3dB point of about 7.2MHz matched to a 50 ohm output to start with. The biasing of the oscillator transistor could be improved as well to keep it in the linear region. In fact version two of this may have a JFET as they are somewhat less tetchy about temperature as well.

Not a bad idea with the diodes. Might have a play with that.

I'll post a picture of the breadboard when I get a few mins. The very definition of fugly.

[nugglix]

My goal is a LO which allows me to cover AM to 160m ham band.
My first IF is 10.7MHz, so I need an oscillator from 11.2MHz to 14.5MHz.
This turns out to be quite a thing -- at least for me.

I went the JFET route today.
See attached files.

The waveform (fft_osc) is nice, I think.
But the bastard quits oscillation on varactor voltages over 3.6V. 
Something the simulation didn't show.
Will add the buffer tomorrow and see if it helps -- but I doubt it.

The clean spectrum amazed me. I'll try this diode limiter again on my
BJT (BF 959) oscillator. Which has a wide tuning range, but also shows
distortion and amplitude issues.

The 2 blue parts at the lower left are 10µH inductors in parallel.
Ahhh... might be that the Q of the inductors is to low.   
Thanks for helping me already
Will try a T68-2 with 30 windings tomorrow.

[T3sl4co1l]

The diode is shorted by the inductor, and the diode is shorting the inductor during half the cycle...

Tim

[MrSlack]

T3sl4co1l: Not 100% sure about that. The thing is throwing 0.36v p-p which isn't enough to make the diode conduct and perform rectification here. Plus it's reverse biased by 0.8v+. If it was a germanium diode at the end of the tuning range perhaps. I'm sticking a couple back to back to see if that does indeed make a difference however.

nugglix: I can't get something with that wide a tuning to actually stable enough not to screw up the audio after the final IF stage hence why I'm trying for 40m CW only. I did an initial cut that tried terribly to span 20-160m but when I opened the window it blew off enough heat to drift by about 10khz. Perhaps AD9850 can save me in the future there! Yours looks good though - pretty clean.

Just blew up my last JFET so will have to wait until Wednesday for more fun

Fugly (tuning was done on a separate board to start with as it had half a varactor experiment stuck to it):

[nugglix]

OH... 20m to 160m is quite a range.
I wouldn't try that, given that I have problems with a tuning range of 3.3MHz.

Ok, last thing for today: try limiting diodes on BJT oscillator.

Back in a minute...     

[nugglix]

Ha! Success!

Just added the limiting diodes (D3, D4) to the BJT oscillator.
Looks much better (see pics).
I think I'll take this one.

This was a good day at the bench.
Now time to get some sleep.

Cheers!

PS: The amplitude is still enough for a SA612 mixer.

[uncle_bob]

Hi

.... as mentioned above, you have a DC short to ground across your tuning diode. ...

Bob

[edavid]

Haven't you heard about huff & puff stabilizers?

http://www.hanssummers.com/huffpuff.html

[MrSlack]

Ha! Success!

Just added the limiting diodes (D3, D4) to the BJT oscillator.
Looks much better (see pics).
I think I'll take this one.

This was a good day at the bench.
Now time to get some sleep.

Cheers!

PS: The amplitude is still enough for a SA612 mixer.

Looks good. I'm still waiting on my SA612's to arrive. Ordered them a week ago. Considering, as I have a surplus of T37-6's just building a diode mixer and being done with the IC side of things.

Haven't you heard about huff & puff stabilizers?

http://www.hanssummers.com/huffpuff.html

I looked at them myself but have written this off as "too complex". This is designed to be cheap, simple etc. We're getting into DDS/PLL territory there which is a whole different kettle of fish.

Some further thoughts on this:

First idea was to pull the RF energy off the emitter. Unfortunately, not exactly ideal waveform as discussed above (all 100mV/div):



30 mins in a copy of EMRFD and in their progressive "first transmitter" section, they decide to pull it off the base. Quick poke with the scope reveals a cleaner waveform:



So next step is to buffer this, then shove it through a simple 3 pole LC with 3dB at 7.2MHz matched to 50R out. When this is working I'll rebuild it so it's not quite as fugly.

Also borrowing a Tek 494 from a friend on Monday so will have a look at this in the frequency domain too.

[nugglix]

Hi all!

Just a quick follow-up.
Replaced the inductors in the JFET oscillator with a hastily made T68-2 core w/ 25 turns
(~3.5µH). My meter shows a Q of just 30.

Seems to do the job.
Can now tune from ~11.3Mhz to over 16MHz. Which is more than I need.

Will now attach the buffer and do some more measurements.

Link JFET ciruit:
https://www.eevblog.com/forum/rf-microwave/cheap-stable-vfo-design/msg950836/#msg950836

[Radio Tech]

Very nice to see folks building stuff like this. Gives me hope and future plans for myself.

[MrSlack]

Indeed. It's fun doing it this way and that's all that matters

Hi all!

Just a quick follow-up.
Replaced the inductors in the JFET oscillator with a hastily made T68-2 core w/ 25 turns
(~3.5µH). My meter shows a Q of just 30.

Seems to do the job.
Can now tune from ~11.3Mhz to over 16MHz. Which is more than I need.

Will now attach the buffer and do some more measurements.

Link JFET ciruit:
https://www.eevblog.com/forum/rf-microwave/cheap-stable-vfo-design/msg950836/#msg950836

Good stuff. Not used material 2 yet. Got material 6 only.

I just ordered 10 each of T37-6 (final VFO) and FT37-43 (for mixer - need some reactance there!) as well as some more 10k pots and some Altoids tins (this needs to be a pocket rig).

 I increased the tuning range of my VFO by using two 1n4002's. Can now tune the entire 40m band in theory. Buffer circuit time was replaced by garden chores (boo hiss)

[AF6LJ]

Just found this thread this morning a couple of observations.

I would ditch the emitter follower, that is only going to add drift to the circuit.

Your varicap diode shouldn't be a major part of your total C in your tank circuit.
Make it a smaller part of the total C and keep the RF voltage across it to a minimum, if the diode conducts it will dirty up your waveform.

When you have it built and working like you want it to...
Pot it that will make it more stable over temp and less sensitive to mechanical shock.

[MrSlack]

Thanks for the tips - much appreciated.

Definitely going to pot/epoxy the inductor at least on mine - tapping on the table causes drift!

[PA0PBZ]

Definitely going to pot/epoxy the inductor at least on mine - tapping on the table causes drift!

Shout at it and you will have simple FM modulation 

[MrSlack]

That did cross my mind

I tried using candle wax like the old 1970s IF transformer gunk on another experiment a couple of weeks back. Worked as well but buggers up your soldering iron tip if you poke it by accident.

[nugglix]

Hi!

Now everything is in place and the oscillator does it's job nicely.
Wound a new inductor on a T68-2 using 27 windings of #22 (0.63mm) wire.
Meter says Q is 46.

Oscillator goes from 11.2MHz at ~1.5V tuning voltage to 14.5MHz at ~4.5V.
Of course the linearity isn't the best.
So I better add a super-fine-control for sub-millivolt tuning.   

At 11.2MHz the 1st harmonic is at -32dBc, which should be okay.

In my first BJT oscillator I glued the toroid onto the board horizontally.
Not a good idea as Sue said in another thread.
This time I'll try to glue it vertical on some insulating material.
No clue what I can use...

Nice thread, hope it goes on.
Now for something to eat and then some 

Cheers
  Guido

[AF6LJ]

There are things you can do to improve linearity.
Using a small portion of the tuning range is one way, paralleling diodes is another.
When you are below 30mHZ you have a lot working against you, not the least of which is trying to get a few MHZ of tuning range.

Remember also if these are home brew projects you are not constrained to having to work with voltages less than 12V I am more comfortable with supply voltages in the 18-22V range. This was due to my work at Loral corp, I had other constraints current being one of them.

As for potting; everything plays hell with a hot soldering iron when it comes to potting compounds. We always knew how high to set our VCOs before they got potted and it always worked out just fine, but that was high UHF and we used the high priced potting compound. At least the stuff came in a nice shade of Pink.

[nugglix]

Hi!

There are things you can do to improve linearity.
Using a small portion of the tuning range is one way, paralleling diodes is another.
When you are below 30mHZ you have a lot working against you, not the least of which is trying to get a few MHZ of tuning range.

First, let me correct my statement about the tuning range.
The 3.3MHz range is of course from 500kHz to 3.8MHz, which is AM to the end of 80 meters.
For the end of 160m (2.0MHz) I'd need only 1.5MHz tuning range.
So things are getting better already.   

Remember also if these are home brew projects you are not constrained to having to work with voltages less than 12V I am more comfortable with supply voltages in the 18-22V range. This was due to my work at Loral corp, I had other constraints current being one of them.

At the moment I've set the max. voltage for the supply to 12V.
What would a higher supply voltage give me?

The control voltage for the varicaps is between 1V and 5V.
According to the datasheet for my 1SV149 varicaps this is the most linear region.

Looking at the diagrams I have to admit I missed the f/f_max to V_R diagram.
Discovering new things every day...
Will see if I can make use of it. But this is for the next weekend.

... At least the stuff came in a nice shade of Pink.

Manual drift compensation is completely fine...     


Just attached this
   http://www.aliexpress.com/item/0-1MHz-1000MHz-digital-Frequency-counter-meter-tester-Cymometer-8-digits-0-56-LED-display/32410222299.html
to the oscillator. Looks good.
You can set the IF in the counter and then display the RF you're targeting. Nice thing.

Now preparing for a trip to Ikea, need more shelves... 

Thanks for the hints and that you made me look at the datasheet again.



Edit: spelling

[AF6LJ]

Hi!

There are things you can do to improve linearity.
Using a small portion of the tuning range is one way, paralleling diodes is another.
When you are below 30mHZ you have a lot working against you, not the least of which is trying to get a few MHZ of tuning range.

First, let me correct my statement about the tuning range.
The 3.3MHz range is of course from 500kHz to 3.8MHz, which is AM to the end of 80 meters.
For the end of 160m (2.0MHz) I'd need only 1.5MHz tuning range.
So things are getting better already.   

Remember also if these are home brew projects you are not constrained to having to work with voltages less than 12V I am more comfortable with supply voltages in the 18-22V range. This was due to my work at Loral corp, I had other constraints current being one of them.

At the moment I've set the max. voltage for the supply to 12V.
What would a higher supply voltage give me?

The control voltage for the varicaps is between 1V and 5V.
According to the datasheet for my 1SV149 varicaps this is the most linear region.

Looking at the diagrams I have to admit I missed the f/f_max to V_R diagram.
Discovering new things every day...
Will see if I can make use of it. But this is for the next weekend.

... At least the stuff came in a nice shade of Pink.

Manual drift compensation is completely fine...     


Just attached this
   http://www.aliexpress.com/item/0-1MHz-1000MHz-digital-Frequency-counter-meter-tester-Cymometer-8-digits-0-56-LED-display/32410222299.html
to the oscillator. Looks good.
You can set the IF in the counter and then display the RF you're targeting. Nice thing.

Now preparing for a trip to Ikea, need more shelves... 

Thanks for the hints and that you made me look at the datasheet again.



Edit: spelling

Always glad to help out when I can.
I've seen those counter boards around, it is hard to believe they can produce them for that cost.

[MrSlack]

I think that's literally the cost of those things. We have a very abstracted cost over the top of everything whereas in China they have just the material cost. Profit per item isn't as important as profit on a volume of stuff.

My frequency counter, which I will describe in another thread, didn't cost more than that either. Pretty much stolen from EMRFD with some resolution modifications (more digits).

Awaiting postman at the moment who will bring me gifts of toroids ... I hope ... but probably bills.

Just a quick note: with respect to tuning, when my VFO eventually turns into a transceiver, the idea is to skip back to tuning capacitors as I have just obtained a large box of very nice ones cheap from ebay with reduction drives. The transmit and receive frequencies are offset in the mode I'm using so the plan is to have a separate receive and transmit capacitor and switch them in and out on TX/RX. Using some tips on a video from VK3YE, you can zero beat the TX capacitor easily enough with a switch. So tune until you receive a CW signal with the RX capacitor, switch the TX capacitor in, zero beat it and then switch it back. Using a solid state auto TR switch and semi-break in should make it usable after it is set up if it doesn't drift too much.

I'm going very low tech here for ref

[AF6LJ]

Sounds like this is going to work out well.
Keep us posted on your progress.

[MrSlack]

No further progress. T37-6 toroids I ordered on the 23rd of May haven't turned up yet. Some FT37-43's turned up for the mixer so will have a bash at that and recycle the T50-2's for now. FFFfffff ebay and Royal Mail do my head in.

Small diversion as a pile of SMD stuff turned up from CPC. Designed a simple push pull audio amplifier for the receiver. Built it using dead bug through hole. Worked fine first time. Simple gain stage, rubber diode and push pull with some negative feedback. Decided to try and build an SMD version - spent the best part of two hours manually cutting traces on the board with an X-acto and soldering it up. Powered it up with current limiting and nothing happened. So an hour of debugging turned out that some of the MMBT3906's I ordered were actually MMBT3904's. Managed to eventually sort them all out and solder a replacement on.

Small amplifier - after replacement transistor:



Not RF or VFO related but it will be connected after the mixer and AF filter

Edit: just noticed I missed a transistor base solder joint and ALL the tants are soldered on the wrong way round (doh!). Still works though and didn't blow up in my face

[StillTrying]

The diode is shorted by the inductor, and the diode is shorting the inductor during half the cycle...

Hi
.... as mentioned above, you have a DC short to ground across your tuning diode. ...

FWIW, I agree, is there a mistake in the oscillator part of the diagram?

[MrSlack]

It's not shorted. The diode doesn't conduct until Vforward is above ~1.1v on a 1n4002 and it is seeing about 600mV p-p. Also the diode doesn't get turned on because it can't switch at 7MHz because it's a basic rectifier. The only effect we see here is capacitance from the size of the depletion region. Also the voltage source for the diode is reverse biasing it so it doesn't conduct.

So no the thing is right.

[uncle_bob]

It's not shorted. The diode doesn't conduct until Vforward is above ~1.1v on a 1n4002 and it is seeing about 600mV p-p. Also the diode doesn't get turned on because it can't switch at 7MHz because it's a basic rectifier. The only effect we see here is capacitance from the size of the depletion region. Also the voltage source for the diode is reverse biasing it so it doesn't conduct.

So no the thing is right.

Hi

Pardon me, but as drawn, the coil clamps the diode at a DC voltage of zero. You can not bias the diode in that configuration. You can not tune the VCO with the tuning diode shorted at DC. It can have interesting AC voltage on it, but it's DC must be zero.

Bob

[StillTrying]

Pssst. If there's no one watching I'm still with you uncle_bob.

At first I thought it was tuning by stretching the negative peaks a few mV into the diode's conduction, - in sensitive parts silicon diodes start conducting at about 330mV for me. Then I saw that these diode's revV/cap graph is quite steep near 0V. But now I think most of the tuning is done by the changing characteristics of the tuning voltage's emitter follower. But I'm not at all good with analogue RF stuff!**

But it works somehow, - LT version oscillates at least.

**I came across this thread while searching to see if anyone had used a diode to edge a cheap 100ppm or 50ppm xtal a little bit closer.

[MrSlack]

I'll pop a scope on this later and show you what it's doing. The diode doesn't behave like a diode here at all.

You couldn't put a signal diode in here - it would behave differently.

[GK]

The inductance value of that torodial inductor would be sensitive to the amount of DC current flowing through it. I have an old spectrum analyser that frequency sweeps the 2nd (LC) LO this way.

[G0HZU]

The inductance value of that torodial inductor would be sensitive to the amount of DC current flowing through it. I have an old spectrum analyser that frequency sweeps the 2nd (LC) LO this way.

In this case I don't think a few mA of current will affect the inductance of that T50-6 inductor significantly (as in enough to tune the VFO across a band) because it has a powdered iron core but it's not really a wise thing to do in a circuit like this. So the DC current should be avoided.

The overall circuit is poor.  There's a 1k resistor damping the Q of the inductor for a start. It should be possible to make a basic narrowband VFO here with phase noise in the ballpark of -155dBc/Hz at 10kHz offset.

I'd expect that design to be maybe 20dB worse than this and I agree that the varactor diode is shorted at DC by the inductor. Maybe it does still tune over a short range but this isn't a good VFO circuit IMO.

[GK]

The inductance value of that torodial inductor would be sensitive to the amount of DC current flowing through it. I have an old spectrum analyser that frequency sweeps the 2nd (LC) LO this way.

In this case I don't think a few mA of current will affect the inductance of that T50-6 inductor significantly (as in enough to tune the VFO across a band) because it has a powdered iron core

Well the "varactor" diode has no significant DC voltage impressed across it, so what do you propose? The OP should remove the diode altogether and report on the results.
 
 

[G0HZU]

The inductance value of that torodial inductor would be sensitive to the amount of DC current flowing through it. I have an old spectrum analyser that frequency sweeps the 2nd (LC) LO this way.

In this case I don't think a few mA of current will affect the inductance of that T50-6 inductor significantly (as in enough to tune the VFO across a band) because it has a powdered iron core

Well the "varactor" diode has no significant DC voltage impressed across it, so what do you propose? The OP should remove the diode altogether and report on the results.

TBH I'm not sure how/if the circuit will tune in its current state. It just looks bad from a design point of view and it kind of hurts just to look at the circuit. I wouldn't expect to see the inductance change (at 7MHz) with a few mA passing through the coil.

If it still tunes without the varactor then there are other possibilities that cause the 'tuning'. eg the loading changes caused by the emitter follower? I'd be very surprised if the inductance of that coil changes (enough to cause band tuning) with a few mA DC. If it does then maybe it isn't a genuine T50-6 core? I'd expect to see so little change in inductance it would be hard to measure.

[MrSlack]

Some notes:

1. It doesn't tune without the diode. That was the intial state of the oscillator. It sits around 7.15MHz.
2. The 1k resistor was a 22k trimmer to start with. This was reduced until the circuit oscillated. Initially it didn't, I suspect because the reverse bias wasn't low enough impedance to overcome the diode's action and allow the oscillator to start.
3. It is a genuine T50-6 core.
4. Tuning isn't very linear. At the bottom end of the tuning range ~6.95MHz it compresses. This is the price of using a 1n4002 instead of a proper varactor diode. If I could be bothered to plot capacitance vs voltage I could pick a more linear region to operate in but this will not result in the tuning range reqiured.
5. The emitter current on the transistor is stable at all frequencies so this isn't loading. This is deduced from reading the voltage across the resistor with a scope.

For ref, this is purely an experiment to ask the following questions:

1. Can you build a VFO with ridiculously cheap parts that everyone has (the T50-6 will be eliminated as well)? Apparently yes.
2. Is it stable enough to be able to tune CW? Apparently yes.
3. Would I be happy putting this near a transmitter? Hell no.

I can only ask people to review the outcome not the design. The design is horrid and makes me cringe. Purely an exercise in minimalism. It started with op amps, Infineon varactor diodes, FETs, buffers, filters, power supply regulators and evolved towards zero.

[GK]

Some notes:

1. It doesn't tune without the diode. That was the intial state of the oscillator. It sits around 7.15MHz.

If it doesn't tune without the diode then that also rules out the suggestion that "loading changes caused by the emitter follower" are responsible.

With zero DC bias the 1n4002 probably has a lot of junction capacitance; whether that changes enough to tune the oscillator over the observed range with a variation or maybe a mV or a fraction of a mV effective variable DC bias is another question.

[MrSlack]

I measured the 1n4002 with an LCR meter standalone outside the oscillator and the numbers add up. I don't have my notes handy to check this at the moment as they are at home.

I'm going to stick a decoupling capacitor from the cathode of the diode to the rest of the oscillator in line with other designs. That will eliminate the DC bias current from the picture and implicate only the diode in the tuning - how does that sound?

[G0HZU]

As a side note, you could probably tune the T50-6 core 'thermally' if you passed a lot of DC current through the winding. This core material is fairly stable over temperature but you probably only need to pass 10mA through it to heat it slightly. That would cause some frequency drift in a VFO but probably much less than 1kHz (at 7MHz) and the response time would be very slow

[uncle_bob]

I measured the 1n4002 with an LCR meter standalone outside the oscillator and the numbers add up. I don't have my notes handy to check this at the moment as they are at home.

I'm going to stick a decoupling capacitor from the cathode of the diode to the rest of the oscillator in line with other designs. That will eliminate the DC bias current from the picture and implicate only the diode in the tuning - how does that sound?

Hi

A blocking cap sounds like a real good idea.

To get reasonable Q out of the diode, you want it reverse biassed.

Bob

[MrSlack]

Will sort out this evening. Approximately 8 hours away for me

[GK]

I measured the 1n4002 with an LCR meter standalone outside the oscillator and the numbers add up. I don't have my notes handy to check this at the moment as they are at home.

I'm not sure that the numbers do stack up. Here is the capacitance versus reverse-bias plot for the 1N4002:



It's a pity the graph doesn't go all the way down to zero volts, but unless something seriously funky is going on <0.1V I'm at a loss to explain how a <=mV or so of variation could cause enough of a capacitance change to result in the frequency tuning range that you are reporting.

[uncle_bob]

I measured the 1n4002 with an LCR meter standalone outside the oscillator and the numbers add up. I don't have my notes handy to check this at the moment as they are at home.

I'm not sure that the number do stack up. Here is the capacitance versus reverse-bias plot for the 1N4002:



It's a pity the graph doesn't go all the way down to zero volts, but unless something seriously funky is going on <0.1V I'm at a loss to explain how a <=mV or so of variation could cause enough of a capacitance change to result in the frequency tuning range that you are reporting.


In any case the 1N4001 looks like a poor varactor diode for a conventional 40m oscillator. A reverse bias span of 2V to 20V only yields a 9pF variation in capacitance.

Hi

As soon as you forward bias the diode, that curve changes a lot. Since an LCR meter (or an oscillator) could be putting several volts p-p on the diode, you may start to "see" forward bias at the low reverse bias levels.

One way to look at it:

By the time the diode gets to 0.7V forward bias, it's basically a short RF wise. As it approaches the "zero ohm" point, it's capacitance goes up quite a bit.

Bob

[GK]

Hi

As soon as you forward bias the diode, that curve changes a lot. Since an LCR meter (or an oscillator) could be putting several volts p-p on the diode, you may start to "see" forward bias at the low reverse bias levels.

One way to look at it:

By the time the diode gets to 0.7V forward bias, it's basically a short RF wise. As it approaches the "zero ohm" point, it's capacitance goes up quite a bit.

Bob

The junction capacitance does typically rise at an accelerated rate as 0V DC bias is approached, but the slope in this case would have to be ridiculously steep to effect the capacitance variation required to tune this oscillator with the limited range of variable DC bias that the OP must be getting with the diode shorted by the DCR of the inductor.

[uncle_bob]

Hi

As soon as you forward bias the diode, that curve changes a lot. Since an LCR meter (or an oscillator) could be putting several volts p-p on the diode, you may start to "see" forward bias at the low reverse bias levels.

One way to look at it:

By the time the diode gets to 0.7V forward bias, it's basically a short RF wise. As it approaches the "zero ohm" point, it's capacitance goes up quite a bit.

Bob

The junction capacitance does typically rise at an accelerated rate as 0V DC bias is approached, but the slope in this case would have to be ridiculously steep to effect the capacitance variation required to tune this oscillator with the limited range of variable DC bias that the OP must be getting with the diode shorted by the DCR of the inductor.

Hi

.... which is why several of us have been pointing out the short circuit issue. Of course, there are many ways to get a circuit to change frequency.

Bob

[GK]

Hi

As soon as you forward bias the diode, that curve changes a lot. Since an LCR meter (or an oscillator) could be putting several volts p-p on the diode, you may start to "see" forward bias at the low reverse bias levels.

One way to look at it:

By the time the diode gets to 0.7V forward bias, it's basically a short RF wise. As it approaches the "zero ohm" point, it's capacitance goes up quite a bit.

Bob

The junction capacitance does typically rise at an accelerated rate as 0V DC bias is approached, but the slope in this case would have to be ridiculously steep to effect the capacitance variation required to tune this oscillator with the limited range of variable DC bias that the OP must be getting with the diode shorted by the DCR of the inductor.

Hi

.... which is why several of us have been pointing out the short circuit issue. Of course, there are many ways to get a circuit to change frequency.

Bob

You are talking about something completely different now. That there are numerous things wrong and sub-optimal in this design is beyond question. Precisely how the described circuit can function exactly as claimed is what I am trying to figure out.
     

[uncle_bob]

Hi

As soon as you forward bias the diode, that curve changes a lot. Since an LCR meter (or an oscillator) could be putting several volts p-p on the diode, you may start to "see" forward bias at the low reverse bias levels.

One way to look at it:

By the time the diode gets to 0.7V forward bias, it's basically a short RF wise. As it approaches the "zero ohm" point, it's capacitance goes up quite a bit.

Bob

The junction capacitance does typically rise at an accelerated rate as 0V DC bias is approached, but the slope in this case would have to be ridiculously steep to effect the capacitance variation required to tune this oscillator with the limited range of variable DC bias that the OP must be getting with the diode shorted by the DCR of the inductor.

Hi

.... which is why several of us have been pointing out the short circuit issue. Of course, there are many ways to get a circuit to change frequency.

Bob

You are talking about something completely different now. That there are numerous things wrong and sub-optimal in this design is beyond question. Precisely how the described circuit can function exactly as claimed is what I am trying to figure out.
   

Hi

Well, that's not exactly the point of the DC short either.

How can it work:

1) The schematic and the circuit are not the same

2) The parts on the breadboard are not what they are "supposed to be".

3) The measurements are in error.

That's a short list.

Bob

[GK]

Hi

Well, that's not exactly the point of the DC short either.

How can it work:

1) The schematic and the circuit are not the same

2) The parts on the breadboard are not what they are "supposed to be".

3) The measurements are in error.

That's a short list.

Bob

Sherlock Holmes I presume?

[StillTrying]

A few random notes from last nights simulations on the off-chance someone finds them interesting, or even useful. - I simulated(on and off) for over 2 hours so you don't have to. LOL

Despite it only going down to 0.1V, GK's 1N4002 junction capacitance/reverse voltage graph above is very close to what I 'measured' in LT. A 1Amp diode starts off at 35pf at 0V, and applying a real 1V to 8.5V tuning voltage gives only a 2:1 change in capacitance.
LT's variactor starts at ~52pF, - still with a 2:1 range.

I 'measured' the tuning diodes capacitances by probing the 5.4Mhz current through it and it's parallel 30pF, and compared the 2 pk-pk currents ratio.

In the simulation if I fix 'the short' so that the tuning voltage of 1V to 8.5V gets across the tuning diode it makes very little difference to the frequency or its full range - it's still about 5.4Mhz with about a 250Khz change.

You'll like this next one.

Now knowing the tuning diodes capacitances, if I swap out LT's variactor diode for a fixed 50pF with ESR 10R does the frequency still change with the 1 to 8.5V? - Yes! But in the opposite direction - at 8.5V it drops about 1Mhz.

Just for some colour, using LT's variactor, no short, green =output, red =tuning voltage, blue = emitter-collector.

[G0HZU]

Obviously the impedance at the emitter of the 'tuning' transistor/buffer will change with changes in bias. At very low bias voltages the impedance at the emitter will be much higher and the (potentially severe) damping impact of the 1K resistor will be less because there is a high impedance at its cold (emitter) end. So there should be more RF voltage developed across the T50-6 inductor at low tuning voltages because of the reduced damping. But I think the diode/transistor may interact as a form of crude RF clamp here with the side effect of changing the RF frequency downwards as this ALC/clamping happens. So maybe you don't 'see' this increased RF voltage because the diode is doing something in the form of ALC/limiting. Maybe the side effect of this ALC process is how it appears to tune with changes in tuning voltage (at the emitter) despite the diode being shorted at DC by the inductor? The diode will be a different animal if this clamping/limiting is happening and the limiting probably only happens strongly/significantly at low tuning voltages?

Ideally, the emitter node should be bypassed at RF so maybe some of the tuning strangeness will go away when there is a decent RF decoupling cap across the 10k resistor in the emitter? This would clamp this node at RF and this would define the damping effect of the 1k resistor across all tuning voltages and frequencies. A 1k resistor across the inductor is going to damp the loaded Q of the system quite a bit and this will degrade the phase noise and drift/stability.

The circuit just looks dodgy to me... I'd bin it and start again with a JFET and connect the tuning diode(s) differently to prevent clamping and also aim for a higher loaded Q (I.e. less damping of the main resonator!).

[uncle_bob]

A few random notes from last nights simulations on the off-chance someone finds them interesting, or even useful. - I simulated(on and off) for over 2 hours so you don't have to. LOL

Despite it only going down to 0.1V, GK's 1N4002 junction capacitance/reverse voltage graph above is very close to what I 'measured' in LT. A 1Amp diode starts off at 35pf at 0V, and applying a real 1V to 8.5V tuning voltage gives only a 2:1 change in capacitance.
LT's variactor starts at ~52pF, - still with a 2:1 range.

I 'measured' the tuning diodes capacitances by probing the 5.4Mhz current through it and it's parallel 30pF, and compared the 2 pk-pk currents ratio.

In the simulation if I fix 'the short' so that the tuning voltage of 1V to 8.5V gets across the tuning diode it makes very little difference to the frequency or its full range - it's still about 5.4Mhz with about a 250Khz change.

You'll like this next one.

Now knowing the tuning diodes capacitances, if I swap out LT's variactor diode for a fixed 50pF with ESR 10R does the frequency still change with the 1 to 8.5V? - Yes! But in the opposite direction - at 8.5V it drops about 1Mhz.

Just for some colour, using LT's variactor, no short, green =output, red =tuning voltage, blue = emitter-collector.

Hi

So what does the schematic look like now?

Bob

[GK]

A respectable 40m Colpitts oscillator can still be made with a BJT. One benefit a properly biased BJT gives over a self-biasing JFET is predictability in operation particularly with regards to signal amplitude. In this kind of circuit the biasing point of the transistor strongly influences the amplitude of oscillation and with JFETs this can vary quite significantly due to the large parameter spreads of those devises.

I've made a few quick mods to the presented circuit to bring it more or less in line with what it should look like. To keep the tank circuit Q decent with the effective loading you should stick to an inductance <=2uH in value. You can compute the theoretical best case Q simply by dividing the parallel resistance across the tank circuit by the reactance of either the L or the net parallel C (they are both the same) at the resonant frequency. Basically >C with <L = >Q. But with >C you'll need a varactor with >variation in C too, which will be a limitation when using generic diodes like the 1N400x series as the amount of C variation over a practical range of reverse bias voltage is not particularly great.
   
Note this is only a SPICE sim and a real world example might need some empirical tweaking of the transistor DC bias point to get the amplitude of oscillation right - you don't want the varactor diodes to forward bias at the minimum DC tuning voltage (which should limited to perhaps no less than a volt or two).   

[G0HZU]

Yes, a bipolar transistor will be the low risk choice for mass production. I'd still prefer to make a JFET VFO/VCO for a one off homebrew design though...
Some people have claimed great success using a Vackar circuit using a JFET (or a bipolar) device so maybe the OP could google for this?

Because the OP's original circuit locks the varactor bias at 0V I think the RF waveform will explore the region towards forward bias on part of the waveform.

I think the diode will then begin to resemble a very non linear capacitor and the capacitance will increase a lot towards the (negative) waveform peak. So this will be what pushes the frequency down. So I think the tuning mechanism for the original circuit will come from tuning the damping resistance via the emitter follower. The waveform won't have to get fully towards forward bias to give the 'tuning effect' and so it won't be obvious (on a scope) what is actually happening in the circuit.

For this reason I'd expect the OP's circuit to be very sensitive to temperature changes so this is another reason the design is poor. It would probably 'tune' if you forgot about applying a tuning voltage and removed the emitter follower (and the associated bias voltage) and simply replaced the 1k resistor with a passive trimmer resistor to ground. I suspect that you wouldn't have to change the resistance very much to see the frequency change by the required 40kHz. See the image below.

I wouldn't recommend anyone to make/build the circuit below because it will be just as poor as the original circuit. The circuit below is not a good one

[StillTrying]

So what does the schematic look like now?

I dunno, as far as I knew 2 circuits had been built, and there was just me trying the simulation.
I was only interested in the tuning diode operation, even/especially if it was unconventional, as a way to edge a digital circuit's clock xtal in the right direction, so this latest simulation version is almost the same as the one I posted above, but you can certainly have it.