View Single Post
Old 19th Oct 2020, 10:44 pm   #1
Semir_DE's Avatar
Join Date: Jan 2019
Location: Heusenstamm, (Near Frankfurt am Main), Germany.
Posts: 54
Default VGA2RF an all-in-one solution for PC generated 405

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:

Click image for larger version

Name:	41,5MHz_Audio_carrier_PCB.jpg
Views:	105
Size:	21.8 KB
ID:	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:

Click image for larger version

Name:	41,5MHz_PCB_Layout.jpg
Views:	111
Size:	112.8 KB
ID:	218400

and this is the assembled piggy back board

Click image for larger version

Name:	Oscillator_Piggy-Back.jpg
Views:	174
Size:	84.7 KB
ID:	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:

Click image for larger version

Name:	complete_board.jpg
Views:	150
Size:	67.0 KB
ID:	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.

Click image for larger version

Name:	complete_board_Component-Side.jpg
Views:	150
Size:	87.8 KB
ID:	218403

If you find this little circuit useful and would like to build one.... Please PM me if you need the KiCAD project files.
Semir_DE is offline   Reply With Quote