Can a transformer-less Audio Video modulator be built?

[neazoi]

Hello I have found this audio video modulator.
https://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/A-V_Transmitter_ZP5ZDM.gif
The author states that it works ok without using an audio transformer.
Can it be done or is it hoax?

 

[godfreyl]

No need for a transformer. Just use an input coupling cap (maybe 1uF) at the audio input. A big capacitor (maybe 100uF) between "tierra flotante" and ground would be a good idea too.

 

[neazoi]

No need for a transformer. Just use an input coupling cap (maybe 1uF) at the audio input. A big capacitor (maybe 100uF) between "tierra flotante" and ground would be a good idea too.

Thank you I will try this and later on maybe trying a planar coil ar well.
What is the tierra flotante input? is it the ground of the audio signal?

 

[godfreyl]

What is the tierra flotante input? is it the ground of the audio signal?

Google translate says it means "floating ground". That makes sense.

 

[neazoi]

Google translate says it means "floating ground". That makes sense.

ok so it should be the audio signal ground?

Thanks a lot

 

[godfreyl]

No, that would put a DC voltage between audio ground and transmitter ground.

It's better to connect those two grounds together, and add two capacitors as mentioned in post 2.

 

[neazoi]

No, that would put a DC voltage between audio ground and transmitter ground.

It's better to connect those two grounds together, and add two capacitors as mentioned in post 2.

in post#2 you mention to connect a capacitor between these two grounds and in this post you mention to connect the two grounds together.
which one is correct?

Thank you very much!

 

[godfreyl]

Like this

 

[neazoi]

Like this

I am sorry to bring this old topic back.
How should I connect the input electrolytic? with the positive connected to the opamp, or the other way round?

 

[betwixt]

That's a really horrible circuit! It looks like the 741 is being used as a variable capacitor at many times it's bandwidth and the tuned circuit LC ratio is far from optimal. The video input impedance is variable (it's normally fixed at 75 Ohms) and the video level control carries RF and will vary the tuning. It also has no output buffering so the antenna and it's proximity to other objects will change the output frequency.

Brian.

 

[neazoi]

That's a really horrible circuit! It looks like the 741 is being used as a variable capacitor at many times it's bandwidth and the tuned circuit LC ratio is far from optimal. The video input impedance is variable (it's normally fixed at 75 Ohms) and the video level control carries RF and will vary the tuning. It also has no output buffering so the antenna and it's proximity to other objects will change the output frequency.

Brian.

I made it. It is horrible indeed. Far more unstable than the discrete transistor modulators.

How about making the audio modulator like this https://linearparts.com/images/ceramic_filter_bfo-1b.jpg A 5.5MHz version of course.
After all, what is needed is an FM modulated 5.5MHZ (or similar for other countries) oscillator to be fed on the base of the video modulated RF transistor.

 

[Bob60]

That is common base oscillator and there should be 10nF instead 10p capacitors. Frequency modulation ( plus unwanted AM of course ) by parasitic junction capacitors.

 

[neazoi]

I have looked at many schematics about audio/video modulators, including chips and discrete.
The simplest (and at the same time not very unstable) approach I think is something like the attached schematic.
I believe finally, one cannot avoid the audio modulation transformer.

 

[betwixt]

The ceramic filter isn't a good solution, they typically have a bandwidth of 250KHz which is fine to filter an existing wideband FM signal but when you put them in a feedback loop it will oscillate at any dominant frequency within that bandwidth. You could be +/- 125Khz of center frequency and therefore on the edge of the sound filter response inside the TV.

I agree with Bob60's value assumptions, I think they should be much higher in value and suspect the 10pF capacitors should be 10nF but that doesn't solve all it's problems.

Starting from scratch, but following the same basic design, I would use the 741 (horrible IC!) in a fixed gain configuration and wire the level control like a conventional volume control, that lets you ground one side of it and also gives a more predictable input impedance. In the oscillator stage I would make the variable capacitor 10pF and add a 10pF fixed capacitor across it, this will allow more precise tuning, the inductor I would drop from 10mH to 5nH (2 million times smaller!). The audio modulates the frequency by changing the base current and predominantly B-E capacitance so you still get FM sound. In a normal TV, pre-emphasis is applied to the sound before the modulator so you may find it sounds muffled. You can add it with an RC network around the op-amp or at the input if necessary.

The video is more of a problem, you want it to be AM and not change the carrier frequency. The present design modulates in a mix of AM and FM with the FM coming from junction capacitances in the same way as the sound works. There is no simple fix but the best without more complex circuitry would be to add an additional transistor after the oscillator and apply the modulation there. In a better design you would add a buffer stage between them so the capacitance changes in the modulator stage are not coupled back to the oscillator. One transistor is better than none though, so replace the video level potentiometer with a fixed resistor and change the antenna coupling capacitor to one much smaller to reduce the effect a load can have on frequency, I suggest 2.2pF. The modulator/output stage would be a conventional amplifier with say 1K emitter resistor in parallel with 100pF to ground the RF components. The video input would go to the top of a 100 Ohm potentiometer with a 220Ohm resistor in parallel with it, this will give a better match to the video feeding it. The wiper of the potentiometer can go to the emiter resistor through a 100uF capacitor. The input comes from the old antenna coupling capacitor. Take the new output to the antenna from the collector through a 10pF capacitor.

As with sound, it is normal to use video pre-emphasis at the transmitter. Basically, in both cases it boosts the higher frequencies in the sound and video before transmission and in the TV receiver the reverse is done. The overall effect is a flat response but noise picked up in the transmission path is reduced by the receiver filtering. With AM video you may not need pre-emphasis unless your picture appears blurred but the FM sound might lack high notes unless you use it.

Brian.

- - - Updated - - -

We cross posted, if you use the schematic in post #13 you can omit the transformer secondary and connect the capacitor directly to the collector of T2. It will not work as well but it avoids a transformer. Please bear in mind that you need the inductor to tune the oscillator to 5.5MHz and the secondary only has to be maybe 2 turns or wire around it.

Brian.

 

[Bob60]

I have looked at many schematics about audio/video modulators, including chips and discrete.
The simplest (and at the same time not very unstable) approach I think is something like the attached schematic.
I believe finally, one cannot avoid the audio modulation transformer.

Yeah it looks much better. Ceramics BPF 6.5MHz it is well arrowed solution for clipping. And don`t forget that TV signal is SSB ( one side band signal with 50% carrier rejection) but yours non SSB. But it works, more or less.

 

[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 - - -

 

[Bob60]

"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"

It should be special coil ( adjustable from MURATA, for example, with special ferrite for max Q ) in shield not a tor.

 

[neazoi]

"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"

It should be special coil ( adjustable from MURATA for example special ferrite for max Q ) in shield not a tor.

Why? It is a common FM modulated oscillator at 5.5MHz, why a special coil is needed?
For adjustment of the exact frequency, a variable capacitor can be used for C6 instead of having a variable coil.

 

[Bob60]

It`s up to you. It`s not my business of course.You are designer. Last time when I toy with these staff may be 20 -30 years ago.

 

[neazoi]

It`s up to you. It`s not my business of course.You are designer. Last time when I toy with these staff may be 20 -30 years ago.

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 ;-)