19-05-2019, 01:49 PM
Dear All,
After getting interested in old non 625 TV standards like the 405 line system in the UK I soon thought about making suitable modulators that can deliver accurate signals for these systems. My first attempt was a channel 1 System A modulator using the SA612 balanced modulator as an AM modulator for both vision and sound. While the original design worked well I will build a new revision of it because I found both a 41.5MC and a 45MC crystal oscillator at Mouser. These deliver a digital signal but with a simple LPF this can be turned into a nice sine wave.
In this thread, however, I would like to introduce a new project that will cover more channels and deliver signals in various formats. Here is a list of planned features:
- Programmable vision carrier in 250KC increments between 40-70MC. Other increment values like 200KC are also possible by choosing a different reference crystal.
- supports any sound vision carrier spacing including but not limited to 3.5, 4.5, 5.5, 6.5 MC
- Positive and negative vision modulation can be selected.
- AM of FM sound modulation support.
- Powered from a single 5V USB source such as a power bank or charger.
This a “work in progress” project that I’m working on off an on. Here you can find a block diagram. It will also include a video distribution amp (VDA) to split existing signals and a “PICGEN” so as to provide a test signal for 405 line equipment.
Both vision and sound frequencies will be synthesized by means of PLL circuits. No exotic crystals will be used in this project nor will there be a need for a microprocessor, except for the PIC used in the PICGEN but this is an optional item.
Most of the concept is already working as an experimental circuit:
The two boards on the left are VCOs the switch in the front selects positive or negative vision modulation.
I had to overcome quite a few obstacles when building this project. One of the setbacks was when I first tried to add FM modulation. I found out that the NE612 internal oscillator would impose a varying load on the tuned circuit that would lead to FM even in AM mode. I overcame this issue by designing a separate VCO with a buffer stage that you can see here:
This VCO can be tuned to 40-70MC with a varicap diode this needs between 1V and 22V to deliver this range of frequencies. The circuit of the VCO board also includes the prescaler MB501 that divides the generated frequency by a factor of 64.
This divided frequency will be compared by the PLL to an adjustable reference generated from a 4MC crystal in order to generate the desired carrier frequency. In the VCO I used a Harley oscillator circuit with two SMD inductors. Diode D3 acts as an AGC device by rectifying the generated sine wave and applying a negative voltage to the gate of the oscillator FET proportional to the signal amplitude. This leads to an equilibrium eventually with a stable amplitude with low harmonic distortion.
Since my plan is to run everything on 5V, but the varicaps in the tuning circuit required up to 22V I had to make a voltage converter board that would boost the 5V VCC to 25V. I used a MIC2288 from Microchip for this:
In order to prevent interference due to the high switching frequency of this boost converter I added a lot of filtering to the design of this item. Here you can see the finished boost converter board.
As a PLL chip I used a Motorola MC145106 which can be programmed by a parallel port.
I will use a diode matrix for programming the desired frequencies which will allow me to select one out of 6 channels with a rotary switch. Channels will be coded by simple diodes on pluggable small PCB boards, so altering the channel scheme can be done by exchanging these coded PCB boards.
Each board is double sided and will carry the code for two channels.
To be continued…
Cheers
Semir
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After getting interested in old non 625 TV standards like the 405 line system in the UK I soon thought about making suitable modulators that can deliver accurate signals for these systems. My first attempt was a channel 1 System A modulator using the SA612 balanced modulator as an AM modulator for both vision and sound. While the original design worked well I will build a new revision of it because I found both a 41.5MC and a 45MC crystal oscillator at Mouser. These deliver a digital signal but with a simple LPF this can be turned into a nice sine wave.
In this thread, however, I would like to introduce a new project that will cover more channels and deliver signals in various formats. Here is a list of planned features:
- Programmable vision carrier in 250KC increments between 40-70MC. Other increment values like 200KC are also possible by choosing a different reference crystal.
- supports any sound vision carrier spacing including but not limited to 3.5, 4.5, 5.5, 6.5 MC
- Positive and negative vision modulation can be selected.
- AM of FM sound modulation support.
- Powered from a single 5V USB source such as a power bank or charger.
This a “work in progress” project that I’m working on off an on. Here you can find a block diagram. It will also include a video distribution amp (VDA) to split existing signals and a “PICGEN” so as to provide a test signal for 405 line equipment.
Both vision and sound frequencies will be synthesized by means of PLL circuits. No exotic crystals will be used in this project nor will there be a need for a microprocessor, except for the PIC used in the PICGEN but this is an optional item.
Most of the concept is already working as an experimental circuit:
The two boards on the left are VCOs the switch in the front selects positive or negative vision modulation.
I had to overcome quite a few obstacles when building this project. One of the setbacks was when I first tried to add FM modulation. I found out that the NE612 internal oscillator would impose a varying load on the tuned circuit that would lead to FM even in AM mode. I overcame this issue by designing a separate VCO with a buffer stage that you can see here:
This VCO can be tuned to 40-70MC with a varicap diode this needs between 1V and 22V to deliver this range of frequencies. The circuit of the VCO board also includes the prescaler MB501 that divides the generated frequency by a factor of 64.
This divided frequency will be compared by the PLL to an adjustable reference generated from a 4MC crystal in order to generate the desired carrier frequency. In the VCO I used a Harley oscillator circuit with two SMD inductors. Diode D3 acts as an AGC device by rectifying the generated sine wave and applying a negative voltage to the gate of the oscillator FET proportional to the signal amplitude. This leads to an equilibrium eventually with a stable amplitude with low harmonic distortion.
Since my plan is to run everything on 5V, but the varicaps in the tuning circuit required up to 22V I had to make a voltage converter board that would boost the 5V VCC to 25V. I used a MIC2288 from Microchip for this:
In order to prevent interference due to the high switching frequency of this boost converter I added a lot of filtering to the design of this item. Here you can see the finished boost converter board.
As a PLL chip I used a Motorola MC145106 which can be programmed by a parallel port.
I will use a diode matrix for programming the desired frequencies which will allow me to select one out of 6 channels with a rotary switch. Channels will be coded by simple diodes on pluggable small PCB boards, so altering the channel scheme can be done by exchanging these coded PCB boards.
Each board is double sided and will carry the code for two channels.
To be continued…
Cheers
Semir
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