Can a transformer-less Audio Video modulator be built?

[Bob60]

What I mean is that if there is any specific requirement to use a shielded coil. I am trying to make the circuit easy to build, that is why I think about the toroid. If Q is really a big problem, I could use a T37-xx iron powder core for higher Q. But, does this really matter in such a low frequency, in that application...?

PS. I wish I was really a designer, I would not ask so many questions here then ;-)

It looks like power electronics staff, but should be like carbonyl iron. It is best for hi Q and hi frequency.
https://en.wikipedia.org/wiki/Carbonyl_iron

 

[betwixt]

Don't lose sleep over the coil Neazoi.

It doesn't have to be high Q or a toroid. I usually use IF transformers stripped from old Japanese AM radios, remove the can and capacitor, unwind the coil and just leave a few turns behind. You get a nice adjustable cored bobbin with mounting pins for free!

Incidentally, domestic TV is transmitted as VSB, not SSB but for this application that is of no importance.

Brian.

 

[Bob60]

Don't lose sleep over the coil Neazoi.

It doesn't have to be high Q or a toroid. I usually use IF transformers stripped from old Japanese AM radios, remove the can and capacitor, unwind the coil and just leave a few turns behind. You get a nice adjustable cored bobbin with mounting pins for free!

Incidentally, domestic TV is transmitted as VSB, not SSB but for this application that is of no importance.

Brian.

Much better Japanese FM radios. IF 10.7MHz much easer get down to 6.5MHz than get from 455kHz to 6.5MHz and wrong ferrite and big loses.
By the way, my sleeping depends from neighbors. If they allow it happens, if not... C'EST LA VIE.

 

[neazoi]

Don't lose sleep over the coil Neazoi.

It doesn't have to be high Q or a toroid. I usually use IF transformers stripped from old Japanese AM radios, remove the can and capacitor, unwind the coil and just leave a few turns behind. You get a nice adjustable cored bobbin with mounting pins for free!

Incidentally, domestic TV is transmitted as VSB, not SSB but for this application that is of no importance.

Brian.

Yes, I have many of them, I know how to rewind them. I will try a toroid though to see how it works. The author makes this using an "unknown" ferrite rod. It shouldn't be critical, as long as it oscillates on the desired frequency.

I will try to build the circuit and let you know, just for reference.
By the way, do I need to physically connect the 75ohm dotted resistor at the video input or this is to denote that the video input has an impedance of 75R?

 

[Bob60]

Yes, 75 ohm will be better. The picture should be more sharp.

I will try to build the circuit and let you know, just for reference.

Sounds good.

 

[neazoi]

I tried the audio oscillator yesterday with a iron powder Amidon toroid (I do not remember the type now, it is the red-black colour paint). It was a bit tricky to find the correct number of turns (about 20 on this toroid). I coupled it to the oscilloscope with a 91pf capacitor from the collector. The waveform is bad but I think I have to adjust the emitter resistor or the base resistor to obtain a clean sinewave at 9Vdc.
Interestingly, this oscillator can oscillate at 0.7V DC in (and it gives a clean sinewave at this voltage), which opens a window for a low voltage vfo.

 

[Bob60]

I tried the audio oscillator yesterday with a iron powder Amidon toroid (I do not remember the type now, it is the red-black colour paint). It was a bit tricky to find the correct number of turns (about 20 on this toroid). I coupled it to the oscilloscope with a 91pf capacitor from the collector. The waveform is bad but I think I have to adjust the emitter resistor or the base resistor to obtain a clean sinewave at 9Vdc.
Interestingly, this oscillator can oscillate at 0.7V DC in (and it gives a clean sinewave at this voltage), which opens a window for a low voltage vfo.

Much better to receive these signal with some kind of receiver. Otherwise you never be sure concerning frequency. Sin or non it depends from feedback deep.Series resistor at the emitter ( at right place ), low voltage all these staff decrease h21 made more stable stage, increase P1( low voltage doesn't ) and so on.

 

[neazoi]

Thank you very much for the detailed answer Brian!
Too many changes for using the opamp circuit, probably a complete refinement needed as you suggest.
The circuit in post #13 seems more feasible and straight forward. I re-post it with the values for the coils just for reference. Obviously L4 refers to L3.

The whole point of the audio part is to bring an FM modulated carrier of 5.5MHz to the base of the T1.
It shouldn't be that hard to find the rough frequency by winding the primary with a few turns on a FT37-43 toroid or similar. Then use a variable capacitor for C6, to adjust at the exact frequency.

The capacitor coupling is never as good as transformer coupling. Since a transformer cannot be avoided, I think it won't be difficult to make the secondary by winding a low number of turns onto the same toroid.

For the UHF RF oscillator the 2SC9018 seems good. I have seen it in old ASTEC modulators. For the video amplifier and audio section I guess a common 2n2222, bc547 or similar could be used.

- - - Updated - - -

Hello, I made this circuit and the video part worked great. But when I make tha audio part, the picture gets crappy, unreadable. Do you know what the problem might be?

 

[betwixt]

Could be three things:

1. the audio carrier is not at 5.5MHz but 4.433MHz which wuold clash with color subcarrier. The result would be a 'grainy' monochrome picture.
2. the audio injection level is far too high and upsetting video circuits in the receiver.
3. the components injecting the audio carrier into the video are capacitively loading the video signal.

Assuming you are using the last schematic posted, try reducing the number of turns on the subcarrier secondary winding or reducing the value of the coupling capacitor.

Brian.

 

[neazoi]

Could be three things:

1. the audio carrier is not at 5.5MHz but 4.433MHz which wuold clash with color subcarrier. The result would be a 'grainy' monochrome picture.
2. the audio injection level is far too high and upsetting video circuits in the receiver.
3. the components injecting the audio carrier into the video are capacitively loading the video signal.

Assuming you are using the last schematic posted, try reducing the number of turns on the subcarrier secondary winding or reducing the value of the coupling capacitor.

Brian.

I corrected the problem. Yes it was a lower frequency audio subcarrier. I used a 1.5uH molded choke and a 10-60pF variable and this setup is capable of bringing the audio carrier to 5.5MHz. I did not use a 5.5MHz transformer as shown in the schematic, the collector of the audio modulator is connected to the next stage through a 3pF capacitor. I used 3pf in order not to load the next stage too much. The audio volume is very much ok.

I hooked up the audio modulator on a oscilloscope and freq. counter and set the unmodulated audio carrier to 5.5MHz.
But when I measured the frequency of the UHF oscillator it was around 200MHz with the components shown on the schematic. I tried less turns on the oscillator coil and also lower diameter solenoids with two turns, but no matter what I did the oscillator fundamental never went above 340MHz.

The transistor used has an Ft of more than 1GHz.
Any ideas of how to bring the oscillator on about 500MHz?

 

[vfone]

Just increasing the Vcc of the oscillator you can get an increase in frequency (due to lowering the internal parasitic capacitance's into the transistor). Unfortunately in your circuit configuration cannot help too much because in the collector of the oscillator you have the modulator.
The best to get higher frequency, and even more stable than Colpitts type used in this moment, is to use a Clapp oscillator, with a series LC from the base to ground. In this situation the antenna coupling could be done from the emitter.

 

[betwixt]

I've got a similar circuit working here at 1.5GHz but it uses PCB strip lines as inductors and 'tombstone' ceramic capacitors. The main tuning capacitor is two insulated wires each about 10mm long which are twisted to set the capacitance. Mine also has varactor tuning so it can be PLL controlled. It is a video/audio transmitter but pumps out about 20W through several amplifier stages. From memory, I think it used a BFR93A transistor in the oscillator.

Brian.