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-   -   VGA2RF an all-in-one solution for PC generated 405 (https://www.vintage-radio.net/forum/showthread.php?t=172256)

Semir_DE 19th Oct 2020 10:44 pm

VGA2RF an all-in-one solution for PC generated 405
 
5 Attachment(s)
Dear All,

since I had developed the VGA2RF circuit for my French 819 project the thought crossed my mind to use the same circuit with different crystals for a 405 signal coming out of a PC's VGA output. As I still had 45Mc vision crystals for channel 1 all I needed was a solution to get the 41.5Mc for the audio carrier. Luckily one of my surplus suppliers here in Germany had an offer for an SMD oscillator that output exactly 41.5Mc. These were very cheap so I got a bunch.

Of course such an SMD oscillator outputs a square waveform with a lot of harmonics, which is not good for TV modulators as the generated harmonics would interfere with higher channels. In order to get rid of the harmonics I used a 5th order low pass filter with a cut off frequency just above 42Mc:

Attachment 218399

For this Chebychev filter I used the free program "Elsie" which works very nicely. Attenuation of the first harmonic at 83Mc is over 35dB.

I wanted the whole combination of SMD oscillator and low pass filter to be as small as possible so I could fit it on my VGA2RF board instead of a crystal, so I used SMD components for the low pass filter. This is the layout in KiCAD:

Attachment 218400

and this is the assembled piggy back board

Attachment 218402

This little board needs a 3.3V power supply which I obtained from the 3.3V MCP1700 regulator IC on the main board. This is what the assembled unit looks like:

Attachment 218401

In the front you can see the 45Mc vision carrier crystal and in the back the 41.5Mc SMD crystal replacement. The whole circuit runs on 5V and can be powered from the VGA port of many PCs. I have, however, also added a USB port for external power. This can be a USB cable connected to the same PC supplying the signal or an external power supply.

The circuit of this board is the same as in my 819 line project with the difference that all components on the SA612 for the audio carrier crystal can be omitted as the small SMD board already delivers a 200mV carrier signal.

The output level of this modulator is about 500mV so it can easily feed a signal to many TVs without the need for additional distribution amps.

Attachment 218403

If you find this little circuit useful and would like to build one.... Please PM me if you need the KiCAD project files.

Jac 20th Oct 2020 6:42 am

Re: VGA2RF an all-in-one solution for PC generated 405
 
Impressive work Semir!

Jac

PsychMan 28th Oct 2020 6:06 pm

Re: VGA2RF an all-in-one solution for PC generated 405
 
Wow, this looks great. I'm surprised there's not been more replies to this thread, it could be a wonderful alternative to Aurora / Hedgehogs..

FRANK.C 28th Oct 2020 8:12 pm

Re: VGA2RF an all-in-one solution for PC generated 405
 
I login almost everyday I don't know how I missed this thread until now, but I did!


Great work as always Semir.

A very neat solution especially as no external power supply is needed.
If it could be used with a laptop it would make a very portable unit.

Frank

Semir_DE 29th Oct 2020 1:48 am

Re: VGA2RF an all-in-one solution for PC generated 405
 
Hi Frank,

I have tried to use it with a laptop but the usual internal Intel GMA graphics chips will not support low pixel clocks. I believe 20Mc is the lowest they can handle. For the French version it works very well with my dated HP 8440p as this uses a 29Mc pixel clock.

What I have tried is to create a different 405 modeline with more horizontal pixels and thus a higher pixel clock. This did actually work but makes handling aspect ratio a little difficult, because you get something like 25:9.

In VLC if I use 4:3 video footage and set the AR to full screen it works very nicely, however, because the image will automatically be fit into the 4:3 AR of the TV set. I can post the modified modeline here I just have to find the file...


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