Oscillator

[zxcsemaj]

Good day, everyone! Is it possible to replace Crystal Oscillator with Colpitts Oscillator Circuit? I can't find the exact value of crystal oscillator which is 54.90625MHz in the TV Transmitter circuit (yellow picture) that's why I'm looking for alternative for crystal oscillator. Also, which is best oscillator circuit to be use as alternative for this. Thank you!

 

[BigBoss]

There are three doublers to multiply the crystal oscillator. Why don't you use a Colpitts 440MHz oscillator to drive directly the PA. You can also use simple PLL like MC145151/2 or similar.
The circuit will be more reliable and simpler and much consistent in regard of tuning and adjustment.

 

[zxcsemaj]

There are three doublers to multiply the crystal oscillator. Why don't you use a Colpitts 440MHz oscillator to drive directly the PA. You can also use simple PLL like MC145151/2 or similar.
The circuit will be more reliable and simpler and much consistent in regard of tuning and adjustment.

I replace the three doublers present in the circuit with colpitts oscillator then connect it directly to RF Amp?

 

[BigBoss]

I replace the three doublers present in the circuit with colpitts oscillator then connect it directly to RF Amp?

Of course, why not ?? You don't have to use a crystal oscillator. Instead, you can use a Colpitts VCO and a simple programmable PLL to obtain crystal accurate carrier. Don't worry about the PLL because it does not need a programming with complex circuit. It's a parallel loaded simple but efficient wired logic PLL.
I recommend you to insert a Buffer amplifier between VCO and PA to improve the VCO Pulling issue.
If the oscillator has around 10-15dBm Output Power @ 50 Ohm, the final Output Power will probably 25dBm (and more) at the antenna. This is sufficient for you.

 

[vfone]

A PLL is a good option, but if you still want to use the multipliers, you can use a different crystal.
The frequency of the 54.9MHz crystal is not mandatory to be as it is, because the final 439.25MHz could be slightly different for using ATV.
If you don't have the 54.9MHz crystal, you can use any of the CB channel crystals (very easy to get) with frequency between 27MHz and 27.4MHz. In the collector of Q1 is already a resonant circuit on 109.8MHz, so there are chances (when using a 27MHz crystal) to select without any modifications the 4th harmonic of the signal.
If is not working, have to make a separate oscillator for 27MHz, select 2nd harmonic in the collector, and go further with the existing circuit (Q1 working now as a doubler).

 

[zxcsemaj]

Of course, why not ?? You don't have to use a crystal oscillator. Instead, you can use a Colpitts VCO and a simple programmable PLL to obtain crystal accurate carrier. Don't worry about the PLL because it does not need a programming with complex circuit. It's a parallel loaded simple but efficient wired logic PLL.
I recommend you to insert a Buffer amplifier between VCO and PA to improve the VCO Pulling issue.
If the oscillator has around 10-15dBm Output Power @ 50 Ohm, the final Output Power will probably 25dBm (and more) at the antenna. This is sufficient for you.

The problem is I'm only a student 😅 I don't know how to modify it without the guidance of the expert in this field. This is for Thesis Capstone project.

--- Updated Feb 7, 2023 ---

A PLL is a good option, but if you still want to use the multipliers, you can use a different crystal.
The frequency of the 54.9MHz crystal is not mandatory to be as it is, because the final 439.25MHz could be slightly different for using ATV.
If you don't have the 54.9MHz crystal, you can use any of the CB channel crystals (very easy to get) with frequency between 27MHz and 27.4MHz. In the collector of Q1 is already a resonant circuit on 109.8MHz, so there are chances (when using a 27MHz crystal) to select without any modifications the 4th harmonic of the signal.
If is not working, have to make a separate oscillator for 27MHz, select 2nd harmonic in the collector, and go further with the existing circuit (Q1 working now as a doubler).

Okay thank you, I'll try to use 27MHz oscillator from DVD Player, I don't know how to make PLL. I don't have consultant in this field that's why I'm still learning.

 

[BigBoss]

The problem is I'm only a student 😅 I don't know how to modify it without the guidance of the expert in this field. This is for Thesis Capstone project.

Dear Friend,
The actual circuit is much more difficult to adjust. It's much more complex to implement. There are many coils and if they interact between each other, there will be stability problems, mutual couplings, microphonic effects etc.
Could you see the alignment/adjustment components ? They may be very troublesome.
I suggest you to choose my simplified solution.
Also you will also learn many thing with it.

 

[zxcsemaj]

Dear Friend,
The actual circuit is much more difficult to adjust. It's much more complex to implement. There are many coils and if they interact between each other, there will be stability problems, mutual couplings, microphonic effects etc.
Could you see the alignment/adjustment components ? They may be very troublesome.
I suggest you to choose my simplified solution.
Also you will also learn many thing with it.

Is it possible to see the simplified circuit diagram as well? No idea how to start. 😅