Hello, I with this circuit, an RF oscillator

[Baluncore]

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
CL = 20e-12 ' at load capacitance, farads
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.

 

[Tom.G]

@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.)

 

[michael1978]

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

 

[michael1978]

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
CL = 20e-12 ' at load capacitance, farads
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]

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.

 

[michael1978]

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.

Hi Merlin this book is from

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.

You can take the book from library genesis.com only the book but the cd of book i don't know where to find.

 

[Baluncore]

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.

I have a copy here, and I see fig 4.24 shows a series output crystal claimed to reduce harmonics.

But it is a Colpitts oscillator with the inductor replaced by a crystal to make a crystal oscillator. It has no tank, that is done by the crystal and capacitive divider. The output voltage appears across the series capacitor. This shows that the frequency of the oscillator is primarily set by the crystal.
Has anyone found any circuit in any reference with the tank circuit and the series crystal output.

I have looked for, but have not found negative resistance in the tank based circuit.

 

[michael1978]

Thanks to everbody for help, and nice wekend for everybody.

 

[Baluncore]

Starting with the circuit from Fig 4.24 of ARRL (2003) “Experimental Methods in RF Design”.

A 7MHz crystal with a Q of 20k has a BW of about 7MHz / 20k = 350Hz. That should give an oscillation rise-time of about 1 / 350Hz = 2.86ms. That same crystal model is used here in all simulations.
Here is the output rise-time envelope, which can be seen is slower than expected to start.

Now we get to the OP's “QRP Homebuilder circuit” which is really a Colpitts crystal oscillator, where the crystal replaces the tuning inductor. The series combination of crystal and output capacitor are an integral part, in parallel with the capacitive divider of the oscillator. Here it is drawn in a way that shows that arrangement more clearly.

The table of C2 shows half Vpp output amplitude as ±Vp, and the highest harmonic relative to the fundamental. There is a region with good output amplitude and good harmonic suppression.

The simple Colpitts crystal oscillator without an inductor or RFC is slow to start. The addition of the inductance can significantly reduce the rise-time of the oscillation, but that requires the bias current be set separately from the capacitive divider, hence the unusual circuit.

It is important that what we have been calling the 'tank' is NOT made resonant near the crystal frequency. The tank must be tuned on the capacitive, lower frequency side of the crystal frequency. That way it looks like a capacitor at the crystal frequency, which is what reduces the circuit to a crystal oscillator with the Colpitts capacitive divider.

The capacitor output technique can reduce the harmonic content to be 50dB below the fundamental, but don't expect anything more than about 50dB down. Here is the clean output waveform, with C2 = 180 pF.

Risetime with C2 = 470pF is;

Risetime with C2 = 195pF is;

Risetime with C2 = 180pF is; ( The saw artefact is a beat between the simulation timestep and the crystal frequency. )

Risetime with C2 = 130pF is; ( Here the artefact shows up earlier in the rise, but then later as whiskers. )

It can be seen that this rises significantly faster than was expected.

 

[michael1978]

Starting with the circuit from Fig 4.24 of ARRL (2003) “Experimental Methods in RF Design”.

View attachment 227805

A 7MHz crystal with a Q of 20k has a BW of about 7MHz / 20k = 350Hz. That should give an oscillation rise-time of about 1 / 350Hz = 2.86ms. That same crystal model is used here in all simulations.
Here is the output rise-time envelope, which can be seen is slower than expected to start.
View attachment 227806

Now we get to the OP's “QRP Homebuilder circuit” which is really a Colpitts crystal oscillator, where the crystal replaces the tuning inductor. The series combination of crystal and output capacitor are an integral part, in parallel with the capacitive divider of the oscillator. Here it is drawn in a way that shows that arrangement more clearly.
View attachment 227807

The table of C2 shows half Vpp output amplitude as ±Vp, and the highest harmonic relative to the fundamental. There is a region with good output amplitude and good harmonic suppression.

The simple Colpitts crystal oscillator without an inductor or RFC is slow to start. The addition of the inductance can significantly reduce the rise-time of the oscillation, but that requires the bias current be set separately from the capacitive divider, hence the unusual circuit.

It is important that what we have been calling the 'tank' is NOT made resonant near the crystal frequency. The tank must be tuned on the capacitive, lower frequency side of the crystal frequency. That way it looks like a capacitor at the crystal frequency, which is what reduces the circuit to a crystal oscillator with the Colpitts capacitive divider.

The capacitor output technique can reduce the harmonic content to be 50dB below the fundamental, but don't expect anything more than about 50dB down. Here is the clean output waveform, with C2 = 180 pF.
View attachment 227808

Risetime with C2 = 470pF is;
View attachment 227809

Risetime with C2 = 195pF is;
View attachment 227811

Risetime with C2 = 180pF is; ( The saw artefact is a beat between the simulation timestep and the crystal frequency. )
View attachment 227812

Risetime with C2 = 130pF is; ( Here the artefact shows up earlier in the rise, but then later as whiskers. )
View attachment 227815
It can be seen that this rises significantly faster than was expected.

Thnx baluncore you are the best