SA612AN 3KHz local oscillator how to?

[neazoi]

Here is an example of a local oscillator for the SA612AN that works on HF, using it's internal transistor.
I want to make a 3KHz local oscillator using this transistor.
How can I do so?

 

[FvM]

Common collector LC colpitts oscillator.

--- Updated Oct 28, 2024 ---

 

[neazoi]

Common collector LC colpitts oscillator.

--- Updated Oct 28, 2024 ---

View attachment 194912

Thank you very much.
With experimentation I ended up in this circuit which is pretty similar to the one you presented with minor values changes.

 

[danadakk]

What frequency accuracy and stability are you looking for ?

 

[neazoi]

What frequency accuracy and stability are you looking for ?

I have tried the circuit I presented, an external phase shift oscillator, as well as the bubba oscillator. I did not try a dual crystal mix-down oscillator, neither a wien bridge, cause I do not like exotic parts..
Frequency stability was adequate, not PLL but adequate.

The bubba had a quite constant level too which is a bonus. It might be seem as a more complex oscillator, but because the opamps are inside a chip, in practice it is not. However it requires a quad pot and those are yet available but not so common.

The application calls for a 3KHz (or so) frequency shift on HF. I do not mind of the sidebands (dual shift up/down).
More important is the input frequencies suppression on the output of the mixer. The 612/602 do a pretty good job but not perfect. On very strong signals I can still hear the input frequency at the output of the mixer.
The other thing is the spectrum purity, so I cannot use switching mixers.

I want to build something simple and cheap. Using an SRA-6 is expensive, requires careful matching and amplifiers, which will make the thing too complex. The ne602/612 has been proven ok, but I would like a bit more of suppression if possible. I think ti also has to do with the purity of the local oscillator (3khz in this instance).

 

[FvM]

I just found in SA612 datasheet that pin 6 input voltage should be 200 mVpp to 300 mVpp maximum. With present oscillator circuit, it's much higher, overloading the mixer massively.

 

[neazoi]

I just found in SA612 datasheet that pin 6 input voltage should be 200 mVpp to 300 mVpp maximum. With present oscillator circuit, it's much higher, overloading the mixer massively.

You mean this circuit?
Note I removed the 18k to the ground.
I measure 150mV on pin 6. Increasing the 47nf increases the level on pin 6

 

[FvM]

Yes this circuit. I don't know what actual pin 6 voltage in hardware is, I saw higher voltage in simulation.

 

[neazoi]

Yes this circuit. I don't know what actual pin 6 voltage in hardware is, I saw higher voltage in simulation.

This is the circuit using the Bubba oscillator. I measure about 70-80mVpp at pin 6. The sine quality is slightly distorted (visually only) but in general an acceptable sine is output.
But the pin 4 output, at the fundamental frequency, is only suppressed by 13dB compared to the output sidebands.
I think that this is too low for the 612 mixer.

Any ideas how to improve this?