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Transformer instead of capacitor in oscillator output

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neazoi

neazoi

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Hello, my oscillator **broken link removed** couples to the next stage through a capacitor.
But I have also found it coupled with a transformer.
https://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/Low_Phase_Noise_X-tal_LO.gif

What are the benefits of using a transformer? One I found is that in the capacitor solution the grounds of the oscillator and the following circuit are connected together. with the transformer they could be completely isolated. Also the output could be balanced or unbalanced (if one end of the output side of the transformer id grounded) am I right?

For a 1-30MHz transformer how many turns ofn a ftXX-43 core do you propose for the primary and secondary?
 

The transformer also allows some degree of impedance matching which a capacitor alone can't do.

1 to 30MHz is a 30:1 frequency range so the optimal number of turns would depend to a large extent on the actual frequency but for experimentation purposes I would suggest 5 turns on the primary and 20 on the secondary.

Brian.
 
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    neazoi

    neazoi

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the coupling factor in a capacitor changes with frequency, very quickly. capacitor impedance is Zc = 1/j2*Pi*f*C. A transformer on the other hand can be very broadband, and the coupling factor as a function of frequency will be very flat.

Also sometimes the transformer can be used to inject DC bias into the circuit...making its use a two-fer
 
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biff44 said:
the coupling factor in a capacitor changes with frequency, very quickly. capacitor impedance is Zc = 1/j2*Pi*f*C. A transformer on the other hand can be very broadband, and the coupling factor as a function of frequency will be very flat.

Also sometimes the transformer can be used to inject DC bias into the circuit...making its use a two-fer
Click to expand...

I worry that the transformer directly connected to the resonator circuit, will induce parasitic resonances, or even load the resonator circuit too much, so that it does not oscillate.
 

Picking up the signal from the resonator gives a clean signal, but may affect the Q of the resonator, so the frequency stability and the phase noise.
But the approach showed in the schematic using the transformer is clever, the influence of the transformer to the resonator is minimal.
Beware that the transformer is 1:4, with lower impedance on the resonator side.
 

vfone said:
Picking up the signal from the resonator gives a clean signal, but may affect the Q of the resonator, so the frequency stability and the phase noise.
But the approach showed in the schematic using the transformer is clever, the influence of the transformer to the resonator is minimal.
Beware that the transformer is 1:4, with lower impedance on the resonator side.
Click to expand...

Thank you, this confirms my experiments in some points. Have you got any idea why the 10 ohm resistor is needed? Or it does not have anything to do with the transformer in place?

I guess a low-Q broadband material like the 43 is suitable for the core there.
 

Transformer coupling with a proper resonant capacitor will also act as a filter to eliminate unwanted harmonics.It's has a impedance matching function at the same time.
Also, Pulling effect so changing over the load impedance due to variant reasons will be minimize.
 

BigBoss said:
Transformer coupling with a proper resonant capacitor will also act as a filter to eliminate unwanted harmonics.It's has a impedance matching function at the same time.
Also, Pulling effect so changing over the load impedance due to variant reasons will be minimize.
Click to expand...

I guess, the resonant capacitor in this case, is the tuned circuit capacitor to ground. However, this is an oscillator where it is supposed to change resonators, and cover different frequencies. So I do not see why transformer resonances are good. It will be good to resonate on the frequency where the oscillator oscillates, but in another frequency (when changing crystals) it won't be resonant.
However, the schematic shows it to be quite broadband.
 

With 10 Ohms across the transformer primary I wouldn't worry too much about resonance. There will still be some but the Q would be so low that it shouldn't cause any problems.

Brian.
 

betwixt said:
With 10 Ohms across the transformer primary I wouldn't worry too much about resonance. There will still be some but the Q would be so low that it shouldn't cause any problems.

Brian.
Click to expand...

I thought that this 10 ohms, would just kill the oscillation by loading the resonator too much.
 

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