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Elektor video modulator simplification discussion

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neazoi

neazoi

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The well known and tested video modulator by elektor which generates harmonics.
**broken link removed**
powered by a stabilized 5V PSU.
Complete article here: RF MODULATOR D.I.Y.

How can this be simplified even more to avoid the coils if possible?
Some of my thoughts:

1. Remove L4 (what is it needed for in the harmonics circuit. can do without it)?
2. Remove L1 and L2 in the 27MHz oscillator (what will be the effect)?
3. Any alternative for the removal of L3?
4. Use another transistor (you do not need such a fast transistor for a 27MHz oscillator, do you)?
5. Use another pair of diodes for D1, D2 such as 1n4148 (will they do for UHF?)

I would like to listen to your thoughts please.
 

IMHO this is an FCC nightmare. You are likely to interfere with many radio/TV services and you won't even know it.

On to your questions.
1. L4 is to reduce the 27MHz fundamental and other lower harmonics. Without it the chances of interference with other services is greater.
2. L1 and L2 are RF chokes. Their job is to provide a low resistance DC path while providing a high impedance to the RF. Without them the circuit might not oscillate at all or oscillate at a reduced level.
3. L3 plus C3 is the primary tank circuit. Not sure how you could do without that and still have it oscillate.
4. Without a lot of thought I might agree with you on the transistor type, but you do need some good gain at 27MHz.
5. The diodes DO need to be high frequency, low capacitance types. They are responsible for clipping the output wave to produce the harmonics. If they are not high frequency types then the wave will still be clipped but will not contain as many high frequencies as the clipped edges will be softer and therefore contain less harmonics.

Since it's a brute force type of RF generator any of the modifications you suggest might still allow it to work but probably not as well. But since there is no specification then who is to say how well it needs to work.

Ray
 
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    neazoi

    neazoi

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Here is more simple VHF video modulator circuit
only one coil.

**broken link removed**

Audio osc. parts dont needed in your case ( if you like so )

Regards KAK
 
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    neazoi

    neazoi

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kak111 said:
Here is more simple VHF video modulator circuit
only one coil.

**broken link removed**

Audio osc. parts dont needed in your case ( if you like so )

Regards KAK
Click to expand...


Audio modulation is good too but the use of these 5.5MHz transformers makes it not a good choice for the homebrewer.
The elector one has excellent results up to the UHF band, and it is very frequency stable even on the harmonics of the fundamental.. Also it uses no complex to make transformers and the coils eventhough 4 of them, are easy to make.
That is why I prefer this one.

---------- Post added at 20:17 ---------- Previous post was at 20:15 ----------
rhaynes said:
IMHO this is an FCC nightmare. You are likely to interfere with many radio/TV services and you won't even know it.

On to your questions.
1. L4 is to reduce the 27MHz fundamental and other lower harmonics. Without it the chances of interference with other services is greater.
2. L1 and L2 are RF chokes. Their job is to provide a low resistance DC path while providing a high impedance to the RF. Without them the circuit might not oscillate at all or oscillate at a reduced level.
3. L3 plus C3 is the primary tank circuit. Not sure how you could do without that and still have it oscillate.
4. Without a lot of thought I might agree with you on the transistor type, but you do need some good gain at 27MHz.
5. The diodes DO need to be high frequency, low capacitance types. They are responsible for clipping the output wave to produce the harmonics. If they are not high frequency types then the wave will still be clipped but will not contain as many high frequencies as the clipped edges will be softer and therefore contain less harmonics.

Since it's a brute force type of RF generator any of the modifications you suggest might still allow it to work but probably not as well. But since there is no specification then who is to say how well it needs to work.

Ray
Click to expand...

It seems that they have designed well this thing back then, I do not see a reason why I should make these simplifications. Eventhough a rude rf generator it works perfectly well provided that you only transfer the signal through a coaxial to the TV. BTW I have tried 2n2222 and bc109 with no success. Maybe a fast transistor is needed indeed eventhough it operates on 28MHz (parasitic oscillations...?)
 

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