# Hello, I need help with this circuit, an RF oscillator

Hello,
Can somebody help, to understand the transistor, what does do 390pF and C(tune), and 4,3µH, they form tank circiuits? and also resistor 1K and capacitor .1, is possible somebody with little words to descirbe how it works please..... Thanks.

#### Attachments

• LOWNOISEOSCILLATOR.png
23.9 KB · Views: 1,587

Baluncore
2021 Award
The 390pF couples energy from the tank to the base and out to the crystal.
The 390pF appears to makes it a common collector Colpitts oscillator.
The base voltage is set to; Vb = 12V * ( 22k / (82k + 22k ) ).
Emitter current is set by the 1k resistor to Ie = ( Vb – Vbe ) / 1k.
The 0.1uF provides very low impedance at RF, much less than the 1k.
The 4.3uH in parallel with 10pF to 265pF is a resonant circuit, like a tank.
The tank circuit will appear as a high impedance at tuned frequency.
The full resonant tank voltage will appear at the emitter.

Merlin3189 and michael1978
The 390pF couples energy from the tank to the base and out to the crystal.
The 390pF appears to makes it a common collector Colpitts oscillator.
The base voltage is set to; Vb = 12V * ( 22k / (82k + 22k ) ).
Emitter current is set by the 1k resistor to Ie = ( Vb – Vbe ) / 1k.
The 0.1uF provides very low impedance at RF, much less than the 1k.
The 4.3uH in parallel with 10pF to 265pF is a resonant circuit, like a tank.
The tank circuit will appear as a high impedance at tuned frequency.
The full resonant tank voltage will appear at the emitter.
Hello Sir, thank you for explained, so .1µf is bypass capacitor, i understand everything, just this 390pf(this is common collector colpitts oscillator) i search on internet colpitts oscillator but they dont look like this one, i dont undertsand where is inductor(sorry maybe i am speaking stupid is difficut to understand),
the 4.3µh in paralell with 10pF to 265pF is a resonant circuit ok is good…
sorry i make you tired with this question, this 390pF(i dont understand)
can you just a litte bit help me, because i understand everything just that no, thanks to you....
of you mean this tank 4.3µf in paralell with 10pF to 265pF resosnace(tank)

Last edited:
Baluncore
2021 Award
For the mode of oscillation of the common emitter Colpitts, see; https://en.wikipedia.org/wiki/Colpitts_oscillator

Your circuit is unusual in that the crystal is being used as a series trap element, driving a low impedance of 12R that is then transformed up to 50 ohm.

Why are you interested in this circuit? Is it homework?

For the mode of oscillation of the common emitter Colpitts, see; https://en.wikipedia.org/wiki/Colpitts_oscillator

Your circuit is unusual in that the crystal is being used as a series trap element, driving a low impedance of 12R that is then transformed up to 50 ohm.

Why are you interested in this circuit? Is it homework?
I am reading one book qrphomebuilder called, is not my homework, but i like to learn about rf design radio tuner,and is my hobby,

Last edited:
I read a little bit before but colppits oscillator is maked through 2 serie capacitor in paralled with inductor, with active device like now with transistor

#### Attachments

• 322px-Cc_colp2.svg.png
5.5 KB · Views: 617
Last edited:
Is like 390pF provides feedback
The 4.3uH in parallel with 10pF to 265pF is a resonant circuit
and formula

Is correct?

#### Attachments

• {displaystyle f_{0}={frac {1}{2pi {sqrt {L{frac {C_{1}C_{2}}{C_{1}+C_{2}}}}}}}_}.png
1.8 KB · Views: 454
Baluncore
2021 Award
I cannot see what makes it oscillate.
Where did you find this circuit diagram ?

I cannot see what makes it oscillate.
Where did you find this circuit diagram ?
GoodMorning,
i wil like to ask you first is correct wat i answer you?
I told you, i am reading this book qrphomebuilder.pdf, (in the book, this circuits they writte is LOWNOISEOSCILLATOR, more not) and i ask you for help, that is all,.

Baluncore
2021 Award
GoodMorning,
i wil like to ask you first is correct wat i answer you?
My post #4 was at 6:44AM. It was dark, sub-zero, with frost on everything. I was tired after a busy 22 hours, so I finally got some sleep after sunrise. That explains why I wrongly referred to the “common emitter Colpitts oscillator” in post #4.

The phone started ringing at 9:45AM, with questions from interstate, so I had less than 3 hours sleep. I needed more information to answer your question when, after two power cuts, I got back to it at at 11AM.

The equation you gave is from wikipedia. If the circuit you have works like the wikipedia Colpitts circuit, then the equation is probably correct. But your circuit is different. When I simulated it with LTspice it rings on startup, but then dies. It does NOT continue to oscillate. If your circuit from QRP homebuilder is not a common collector Colpitts, then that equation cannot be applied. A Colpitts oscillator needs two series capacitors as a divider across an inductor. That is not present, so it does not oscillate. So what is it? and why the weird series crystal trap?
I am sorry about the delay, but I need more sleep before I untangle what is going on in that circuit.

Maybe some other PF member out there will recognise the circuit while I snatch another couple of hours sleep.

berkeman and Tom.G
Tom.G
Gold Member
i am reading this book qrphomebuilder.pdf
Hello Michael,
Can you tell us where we can find the book you are reading, qrphomebuilder.pdf? It is an unusual circuit and we hope if we can read the same book we can see how it works.

My research so far found a vacuum tube circuit called an "electron coupled oscillator" that is almost the same. But I can not find good details on how it works for your circuit.

The tank circuit is the 4.3uH coil and the 10-265pF variable capacitor. The 390pF capacitor will change the resonant frequency by some small amount. The main reason for the 390pF capacitor is to feed some of the energy from the tank back to the transistor base so the transistor can oscillate.

I think the complete circuit uses the tank, the 390pF, and the crystal to set the phase shift for the base feedback to allow oscillation.

Cheers,
Tom

My post #4 was at 6:44AM. It was dark, sub-zero, with frost on everything. I was tired after a busy 22 hours, so I finally got some sleep after sunrise. That explains why I wrongly referred to the “common emitter Colpitts oscillator” in post #4.

The phone started ringing at 9:45AM, with questions from interstate, so I had less than 3 hours sleep. I needed more information to answer your question when, after two power cuts, I got back to it at at 11AM.

The equation you gave is from wikipedia. If the circuit you have works like the wikipedia Colpitts circuit, then the equation is probably correct. But your circuit is different. When I simulated it with LTspice it rings on startup, but then dies. It does NOT continue to oscillate. If your circuit from QRP homebuilder is not a common collector Colpitts, then that equation cannot be applied. A Colpitts oscillator needs two series capacitors as a divider across an inductor. That is not present, so it does not oscillate. So what is it? and why the weird series crystal trap?
I am sorry about the delay, but I need more sleep before I untangle what is going on in that circuit.

Maybe some other PF member out there will recognise the circuit while I snatch another couple of hours sleep.
Hello, are you tired? and how it works….do you see dificult circuit?

Hello Michael,
Can you tell us where we can find the book you are reading, qrphomebuilder.pdf? It is an unusual circuit and we hope if we can read the same book we can see how it works.

My research so far found a vacuum tube circuit called an "electron coupled oscillator" that is almost the same. But I can not find good details on how it works for your circuit.

The tank circuit is the 4.3uH coil and the 10-265pF variable capacitor. The 390pF capacitor will change the resonant frequency by some small amount. The main reason for the 390pF capacitor is to feed some of the energy from the tank back to the transistor base so the transistor can oscillate.

I think the complete circuit uses the tank, the 390pF, and the crystal to set the phase shift for the base feedback to allow oscillation.

Cheers,
Tom
Hello Tom, here is the link https://nt7s.com/files/QRPHomebuilder.pdf page 268

Baluncore
2021 Award
I changed the way the inductor and resistor are connected to the minimum to make it a Colpitts oscillator.
It now self oscillates at about 7.2MHz. I have not yet implemented the crystal trap or output stage.

#### Attachments

• Modified_to_Colpitts.png
48.1 KB · Views: 963
• Now_Oscillates.png
38.2 KB · Views: 974
Klystron and berkeman
I changed the way the inductor and resistor are connected to the minimum to make it a Colpitts oscillator.
It now self oscillates at about 7.2MHz. I have not yet implemented the crystal trap or output stage.
View attachment 227567
View attachment 227568
Thank you sir, for time, thank you for explained…… i understand.....so that circuit in the book was wrong?so for the rest everything stay the same, like you explaind #2, 0.1uf stay the same?

Baluncore
2021 Award
so that circuit in the book was wrong?
The circuit is not yet proven “wrong”.

When I have time I will implement the series crystal and output load, to see what effect it has, and if it makes the original circuit oscillate.

Tom.G
Gold Member
@michael1978 , @Baluncore
http://faculty.frostburg.edu/phys/latta/ee/ranger/schematic/xtaloscillator/rangerxtaloscillator.html
(you can click on the circuit to get a bigger image, and then click on a component (part) for a detailed description of what they do.)

The above is a detailed explanation of an Electron Coupled Oscillator implemented with a vacuum tube. It is the equivalent to the circuit in https://nt7s.com/files/QRPHomebuilder.pdf page 280. The circuit @michael1978 is asking about in post #1 is merely an extension of the above circuits with a tank in the Emitter.

Cheers,
Tom

Michael, that QRP Homebuilder is an interesting book, I am saving it for future reference.

michael1978
Baluncore
2021 Award
A more complete diagram is on page 270 of 945. The diagram @michael1978 posted is only a partial.
I have the book and circuit. The relevant part of the circuit still has the same tank.

The question comes with how the crystal effects the unusual oscillator. The low impedance output suggests the crystal is being 'held down' at one end, so as to have a greater effect on the base. Also, there is a question, how far does the crystal 'pull' the tank.

@michael1978 , @Baluncore
http://faculty.frostburg.edu/phys/latta/ee/ranger/schematic/xtaloscillator/rangerxtaloscillator.html
(you can click on the circuit to get a bigger image, and then click on a component (part) for a detailed description of what they do.)

The above is a detailed explanation of an Electron Coupled Oscillator implemented with a vacuum tube. It is the equivalent to the circuit in https://nt7s.com/files/QRPHomebuilder.pdf page 280. The circuit @michael1978 is asking about in post #1 is merely an extension of the above circuits with a tank in the Emitter.

Cheers,
Tom

Michael, that QRP Homebuilder is an interesting book, I am saving it for future reference.
Hi Tom Thnx Man, i dont understand the electron tube, can you please tell me about circuit posted in #1
did you explained in post #13

Last edited:
The circuit is not yet proven “wrong”.

When I have time I will implement the series crystal and output load, to see what effect it has, and if it makes the original circuit oscillate.
Ok thnx

Merlin3189
Homework Helper
Gold Member
I can't say I understand this cct and, as others, would be interested in the EMRFD source, if it gives some explanation. But my 2 cents, FWIW
View attachment 227564
I'm struggling to see this as a Colpitts. It has two capacitors: but they're not in the right place. It's all very well to say that if we move things around a bit we could make it into a Colpitts, but ...? And are the values ok? At different positions of the tuning capacitor?

The whole of the left hand side is a two terminal device as far as the crystal is concerned, so it seems to me it must be a circuit which presents a negative resistance at the operating frequency. (That doesn't actually help me a lot, as I don't know anything about the design of negative resistances around a transistor. The only place I've seen it is with Gunn diodes and op-amp circuits.)

But, I do see references around to instability in emitter followers with a capacitive load. I can't find a detailed treatment of this (and I'm really not up to doing one myself !) but a TI application note for a wideband buffer amp IC makes a fairly unambiguous comment re.emitter follower cct,
Compensation: The three buffer amplifiers are inherently stable in applications with resistive loads and adequate supply bypassing. However, oscillation may occur in cases where a capacitive load of 100 pF or more is present. A series input resistance of 50Ω–300Ω will prevent this oscillation by compensating the negative input-resistance seen as a result of the reflected capacitive load. All source, cathode, or emitter-followers are subject to this phenomenon which is a result of transit time through the active region of the devices.

Whether that helps us here or not, is another question.
We have a parallel LC emitter load. This will become capacitive above its resonant frequency. If that creates a negative resistance input impedance, then the crystal could oscillate at its serial resonant frequency.

If Michael and [Edit: change Anorlunda to Baluncore } are building or simulating this cct, maybe they can see from the data it generates, whether this is happening. It would seem to me that if it were the case, the LC circuit would need to have its nominal resonant frequency below that of the crystal.

If the cct is actually a VFO with a crystal filter on the output, as the QRP Home Builder article seems to suggest, presumably the VFO will continue to oscillate as it is tuned away from the crystal frequency, but the output will drop due to the crystal filter? There may be some "grid dip" effect in the VFO that would show up in the simulation?

#### Attachments

• 322px-Cc_colp3.png
16.8 KB · Views: 504
Last edited:
Baluncore
2021 Award
I am satisfied that I have correctly modelled the circuit given. I have found that it does not oscillate, even with a crystal on the output, with higher, lower or matched frequencies between the tank and crystal.

I now question how or why it worked well enough to be written up. Why have a crystal in series with the output? This is an amateur design, so why are there no other similar circuits out there in the literature? Why the weird low-Z output network, followed by the impedance transformer to 50 ohms?

Without other information, I can only assume that the output transformer was coupling back into the tank circuit. That would make it a feedback oscillator.

Tom.G
Gold Member
Check out the post by @Merlin3189 above. The TI link he gave, http://www.ti.com/lit/an/snoa725a/snoa725a.pdf, notes that a capacitive load on an Emitter follower is reflected as a negative input impedance due to the Electron transport time in the Base-Emitter junction.
Without other information, I can only assume that the output transformer was coupling back into the tank circuit.
The frostburg.edu article article explains the circuit as used in the Johnson Radio Viking Ranger Ham radio set.

As the frostburg.edu shows, the B-E capacitor and the E-Gnd capacitor form a voltage divider for feedback. In the OPs circuit the E-Gnd cap is replaced with a tank. In that case, the tank circuit tuning could be rather critical.

Other possibilities; the simulation may use a simplified transistor model without that delay, or the transistor gain could be too low.

Cheers,
Tom

Hello, sorry but i realy cant follow u, can somebody more clear to explain me,
But Baluncore explain in begin very easy to understand, after i said the is colppits oscillator i change the position of components, it was easy to understand
Now i dont understand nothing

Baluncore
2021 Award
Now i dont understand nothing
You should not study this oscillator because it has no description or analysis and it is not used.
You should study the Colpitts oscillator because that is well understood and established.

A vacuum tube works like a FET. Specifically, like an N-channel, depletion mode, FET.

Circuits can be designed that raise the output voltage as more current is drawn. That is called negative resistance.
Negative resistance provides gain, which therefore makes oscillation possible.
If the BJT in the unusual oscillator circuit had negative resistance, then oscillation would be possible.

michael1978
You should not study this oscillator because it has no description or analysis and it is not used.
You should study the Colpitts oscillator because that is well understood and established.

A vacuum tube works like a FET. Specifically, like an N-channel, depletion mode, FET.

Circuits can be designed that raise the output voltage as more current is drawn. That is called negative resistance.
Negative resistance provides gain, which therefore makes oscillation possible.
If the BJT in the unusual oscillator circuit had negative resistance, then oscillation would be possible.
Like you said in post #post 2? is that correct?

Last edited:
You should not study this oscillator because it has no description or analysis and it is not used.
You should study the Colpitts oscillator because that is well understood and established.

A vacuum tube works like a FET. Specifically, like an N-channel, depletion mode, FET.

Circuits can be designed that raise the output voltage as more current is drawn. That is called negative resistance.
Negative resistance provides gain, which therefore makes oscillation possible.
If the BJT in the unusual oscillator circuit had negative resistance, then oscillation would be possible.
Like you said in post #post 2? is that correct? can you please answer me because i am happy i learn new thing complicated for me i cant whait

Last edited:
Hello,
Can somebody help, to understand the transistor, what does do 390pF and C(tune), and 4,3µH, they form tank circiuits? and also resistor 1K and capacitor .1, is possible somebody with little words to descirbe how it works please..... Thanks.View attachment 227534
When converting a fundamental oscillator type (like Colpitts) into one mediated by a crystal, they sometimes get developed to exploit the crystal as a narrow filter to deliver an output with very low phase noise, which is a critical feature when the output is to be used in receiver mixer stages.

You are right in that the defining feature of a Colpitts sort is the tuning tank of 2 capacitors in series and an inductor across them.
The variant you have is not quite like that, but that it also has the equivalent circuit of a crystal in series with 220pf and 12 Ohms, all across the resonant part, as in parallel as well.

I can recognize that you are also using LTSpice simulation tool from Linear Technology. I have used this tool before to simulate crystal oscillators. It does require the crystal be substituted by the crystal equivalent circuit.

My first impression of how the circuit operates is that the shunt impedance of the crystal with the 220pf and 12 Ohms, put on the transistor base, will allow the feedback to be effective for oscillation only at the crystal frequency. The very small sample in series with the crystal, taken off the 12Ohms/220pf combination, driving the matching transformer amounts to minimal loading. The signal at this point will be from the resonant ringing in the crystal quartz, in effect a very narrow filter, and the waveform should have very low phase noise.

I am not completely sure of the inductor connection on the variable and 390pF capacitors. Not quite what one would call a "Colpitts" sort.

I have information (PDFs) on most aspects of crystal oscillator design and simulation, and when I can find it, I may come back to this thread and post again.

Baluncore
2021 Award
It does require the crystal be substituted by the crystal equivalent circuit.
I designed the crystal using spice capacitor parameters. This models the fundamental, but does not model overtones. The code needed to compute crystal equivalent spice capacitor parameters is as follows;

' Crystal specifications wanted.
Fp = 7.040e6 ' crystal parallel resonant frequency, Hz
Q = 20e3 ' crystal Q factor
R1 = 60 ' crystal series resistance, ohms
C0 = 5e-12 ' crystal shunt capacitance, farads

' variables needed for initial temp approximation and final value.
Fs_tmp, Fs ' series resonant frequency, Hz
Wo_tmp, Wo ' angular frequency, rad/sec
C1_tmp, C1 ' crystal motional capacitance
L1 ' crystal motional inductance

' initial computation.
Fs_tmp = Fp
Wo_tmp = Fp * TwoPi
C1_tmp = 1 / ( Q * Wo_tmp * R1 )
Fs = Fp / ( 1 + C1_tmp / ( 2 * ( C0 + CL ) ) )
Wo = TwoPi * Fs

' Refine to final values.
C1 = 1 / ( Q * Wo_tmp * R1 )
L1 = 1 / ( Wo^2 * C1 )

' It generated these capacitor parameters;
Basic capacitance = 18.8393 f F = 0.0188393 pF.
Series inductance = 27.149 mH.
Series resistance = 60.0 ohm.
Parallel capacitance = 5.0 pF.

For a parallel frequency of 7.040 MHz @ Load capacitance = 20 pF
The series frequency is 7.037348 MHz
Here is the crystal model and simple output response.

Once simulating the circuit, change the tuning capacitor of the tank, to sweep across the synthetic crystal. Watch the initial transient ringing to see it decay, which implies it has loss not gain.

I have information (PDFs) on most aspects of crystal oscillator design and simulation, and when I can find it, I may come back to this thread and post again.
I have searched many of the texts on oscillator design and have not found any similar design, ECO with a tank, or other similar topology.

I find it hard to believe this is an electron coupled oscillator with 390pF between the base emitter terminals.

There is a problem with crystal series traps in that, even when not resonant at the oscillator frequency, it will pass all the distortion and noise to the output.

#### Attachments

• xtal_model_1.png
25.1 KB · Views: 844
• Xtal_resp_1.png
76.1 KB · Views: 860
Merlin3189 and dlgoff
Tom.G
Gold Member
@Baluncore
Can you get your transistor simulation to show the negative input impedance as mentioned by:
But, I do see references around to instability in emitter followers with a capacitive load. I can't find a detailed treatment of this (and I'm really not up to doing one myself !) but a TI application note for a wideband buffer amp IC makes a fairly unambiguous comment re.emitter follower cct,
I suspect that is really needed for the circuit to function.

(Looks like you got the crystal simulatoin nailed.)

When converting a fundamental oscillator type (like Colpitts) into one mediated by a crystal, they sometimes get developed to exploit the crystal as a narrow filter to deliver an output with very low phase noise, which is a critical feature when the output is to be used in receiver mixer stages.

You are right in that the defining feature of a Colpitts sort is the tuning tank of 2 capacitors in series and an inductor across them.
The variant you have is not quite like that, but that it also has the equivalent circuit of a crystal in series with 220pf and 12 Ohms, all across the resonant part, as in parallel as well.

I can recognize that you are also using LTSpice simulation tool from Linear Technology. I have used this tool before to simulate crystal oscillators. It does require the crystal be substituted by the crystal equivalent circuit.

My first impression of how the circuit operates is that the shunt impedance of the crystal with the 220pf and 12 Ohms, put on the transistor base, will allow the feedback to be effective for oscillation only at the crystal frequency. The very small sample in series with the crystal, taken off the 12Ohms/220pf combination, driving the matching transformer amounts to minimal loading. The signal at this point will be from the resonant ringing in the crystal quartz, in effect a very narrow filter, and the waveform should have very low phase noise.

I am not completely sure of the inductor connection on the variable and 390pF capacitors. Not quite what one would call a "Colpitts" sort.

I have information (PDFs) on most aspects of crystal oscillator design and simulation, and when I can find it, I may come back to this thread and post again.
Thnx MAN

I designed the crystal using spice capacitor parameters. This models the fundamental, but does not model overtones. The code needed to compute crystal equivalent spice capacitor parameters is as follows;

' Crystal specifications wanted.
Fp = 7.040e6 ' crystal parallel resonant frequency, Hz
Q = 20e3 ' crystal Q factor
R1 = 60 ' crystal series resistance, ohms
C0 = 5e-12 ' crystal shunt capacitance, farads

' variables needed for initial temp approximation and final value.
Fs_tmp, Fs ' series resonant frequency, Hz
Wo_tmp, Wo ' angular frequency, rad/sec
C1_tmp, C1 ' crystal motional capacitance
L1 ' crystal motional inductance

' initial computation.
Fs_tmp = Fp
Wo_tmp = Fp * TwoPi
C1_tmp = 1 / ( Q * Wo_tmp * R1 )
Fs = Fp / ( 1 + C1_tmp / ( 2 * ( C0 + CL ) ) )
Wo = TwoPi * Fs

' Refine to final values.
C1 = 1 / ( Q * Wo_tmp * R1 )
L1 = 1 / ( Wo^2 * C1 )

' It generated these capacitor parameters;
Basic capacitance = 18.8393 f F = 0.0188393 pF.
Series inductance = 27.149 mH.
Series resistance = 60.0 ohm.
Parallel capacitance = 5.0 pF.

For a parallel frequency of 7.040 MHz @ Load capacitance = 20 pF
The series frequency is 7.037348 MHz
Here is the crystal model and simple output response.
View attachment 227696
View attachment 227697

Once simulating the circuit, change the tuning capacitor of the tank, to sweep across the synthetic crystal. Watch the initial transient ringing to see it decay, which implies it has loss not gain.

I have searched many of the texts on oscillator design and have not found any similar design, ECO with a tank, or other similar topology.

I find it hard to believe this is an electron coupled oscillator with 390pF between the base emitter terminals.

There is a problem with crystal series traps in that, even when not resonant at the oscillator frequency, it will pass all the distortion and noise to the output.
Thank You Very Much, for helping me, and loosing time for me....

Merlin3189
Homework Helper
Gold Member
First, apologies to Baluncore for confusing him with Anorlunda.
Well done on all the simulation work. I really ought to try to learn to use this Spice stuff myself.

Second, I think I can now see the Colpitts configuration, with the crystal and 390pF & VC across it. If the 4.3uH is bundled in with the VC, there will be a setting where, at the crystal frequency the combination appears as a capacitor of the right value to get the unit+ loop gain. Quite how the inductor helps, I'm not sure yet.

What would be very helpful, would be someone with a copy of the ARRL "Experimental Methods in RF Design" which seems to be the source of this cct.

And since my interest has been piqued in the idea of emitter follower negative resistance, I'd be very pleased if anyone could point me to a detailed analysis of the emitter follower with a complex load, rather than a predominantly real one. Otherwise I guess I'll have to make a pathetic attempt at doing it myself and posting a thread to ask for people to sort it out.

Tom.G and michael1978