17-04-2019, 01:15 AM
Hello fellow 405 enthusiasts,
When building the Hedghog converter I stumbled across another one of Franks nice projects, the “PICGEN” pattern generator. I found this to be a very interesting project and went on to build one.
I started with a simple universal board:
Ever since building the Hedghog I had become a fan of SMD technology, so I decided to make an SMD version of the PICGEN generator I will call it “PICGEN-S”. I not only wanted to make it smaller, but I also changed some parts of the circuit to improve usability. In the following image you can see a first PCB layout with SMD components. This will be revised, however, as this version still has some shortcomings. I will post the final version here when is is done.
Here is a list of changes I have made:
a) Added some low pass filtering to the output signal in order to limit its bandwidth. This will result in less steep rise and fall times of the signal pulses thus reducing the chance of vision buzz. Here is an image of the horizontal sync interval as it appears at the output.
b) Powering from a USB port: By design the output level of this generator depends on the supply voltage. In order to allow it to run from a 5V mobile phone charger or even a power bank I changed the circuit to run on a stabilized 3.3V supply thus eliminating any influence of the supply voltage on the output signal level. Adding some variable resistors now also allows for a precise setup of the sync and video amplitude and ratios. This image shows the PICGEN and my channel 1 Modulator running while being powered from a USB power bank.
c) More outputs: Instead of the simple single transistor output I added the THS7314 output chip to the design. This is the same output driver that the Hedghog uses. This IC can run on a mere 3V of supply voltage and has 3 output drivers. I used two of them as video outputs and the third as a “Composite Trigger Pulse” (CTP) output. By altering a solder jumper the third output can also be turned into a video output.
d) Single scope trigger pulse for H and V: Instead of having separate horizontal and vertical sync outputs, I created a sort of composite sync mainly for scope triggering. The trace on the top shows what it looks like:
The pulse has two levels; 0-1V is negative going vertical sync, 1-2V is positive going horizontal sync. When this signal is applied to a scope it allows the user to select vertical or horizontal triggering by merely adjusting the trigger threshold of the scope. The output level of this pulse is 1Vpp into 75Ω.
Another application of this pulse is to set the free running frequency of the flywheel sync in a TV. Due to the negative going vertical sync this signal can be supplied to a modulator just like a pattern signal. As it does not have horizontal sync pulses the the TV can distinguish, only the vertical sync will be locked while the horizontal will be free running. By adjusting the horizontal hold to show the positive going horizontal sync pulse which shows up as a white vertical line the free running frequency can be set precisely on the TV set. I found this useful as the setting of the horizontal hold influences the picture position. By first setting the H-hold to the right frequency the positioning magnets on the CRT can be set for the correct horizontal position.
e) I had read that some users of the original PICGEN had experienced a hum bar rolling across the picture. While this is usually due to bad mains filtering in the TV set in question, the effect is most visible when the frame rate of the signal supplied to such a TV is not exactly 50Hz i.e. 20ms. I noticed that the PICGEN needs a clock frequency of 5.994Mc to deliver the exact 20ms. When using a 6Mc crystal this can not be pulled low enough to oscillate at that frequency but a ceramic resonator can. I added a variable capacitor to the design to allow for precise adjustment of the frame rate in conjunction with a ceramic resonator. This will render all hum bars stationary on the screen which will make them less objectionable.
f) Two versions, generator only and generator with modulator: Finally I am going to combine the PICGEN with a modulator. I already have a Band I modulator working in my workshop which uses PLLs covering all 5 band I channels. My plan is to make the PICGEN board plug into the larger PCB of the modulator. A band III PLL modulator is also in the planning stages.
To be continued…
Semir
When building the Hedghog converter I stumbled across another one of Franks nice projects, the “PICGEN” pattern generator. I found this to be a very interesting project and went on to build one.
I started with a simple universal board:
Ever since building the Hedghog I had become a fan of SMD technology, so I decided to make an SMD version of the PICGEN generator I will call it “PICGEN-S”. I not only wanted to make it smaller, but I also changed some parts of the circuit to improve usability. In the following image you can see a first PCB layout with SMD components. This will be revised, however, as this version still has some shortcomings. I will post the final version here when is is done.
Here is a list of changes I have made:
a) Added some low pass filtering to the output signal in order to limit its bandwidth. This will result in less steep rise and fall times of the signal pulses thus reducing the chance of vision buzz. Here is an image of the horizontal sync interval as it appears at the output.
b) Powering from a USB port: By design the output level of this generator depends on the supply voltage. In order to allow it to run from a 5V mobile phone charger or even a power bank I changed the circuit to run on a stabilized 3.3V supply thus eliminating any influence of the supply voltage on the output signal level. Adding some variable resistors now also allows for a precise setup of the sync and video amplitude and ratios. This image shows the PICGEN and my channel 1 Modulator running while being powered from a USB power bank.
c) More outputs: Instead of the simple single transistor output I added the THS7314 output chip to the design. This is the same output driver that the Hedghog uses. This IC can run on a mere 3V of supply voltage and has 3 output drivers. I used two of them as video outputs and the third as a “Composite Trigger Pulse” (CTP) output. By altering a solder jumper the third output can also be turned into a video output.
d) Single scope trigger pulse for H and V: Instead of having separate horizontal and vertical sync outputs, I created a sort of composite sync mainly for scope triggering. The trace on the top shows what it looks like:
The pulse has two levels; 0-1V is negative going vertical sync, 1-2V is positive going horizontal sync. When this signal is applied to a scope it allows the user to select vertical or horizontal triggering by merely adjusting the trigger threshold of the scope. The output level of this pulse is 1Vpp into 75Ω.
Another application of this pulse is to set the free running frequency of the flywheel sync in a TV. Due to the negative going vertical sync this signal can be supplied to a modulator just like a pattern signal. As it does not have horizontal sync pulses the the TV can distinguish, only the vertical sync will be locked while the horizontal will be free running. By adjusting the horizontal hold to show the positive going horizontal sync pulse which shows up as a white vertical line the free running frequency can be set precisely on the TV set. I found this useful as the setting of the horizontal hold influences the picture position. By first setting the H-hold to the right frequency the positioning magnets on the CRT can be set for the correct horizontal position.
e) I had read that some users of the original PICGEN had experienced a hum bar rolling across the picture. While this is usually due to bad mains filtering in the TV set in question, the effect is most visible when the frame rate of the signal supplied to such a TV is not exactly 50Hz i.e. 20ms. I noticed that the PICGEN needs a clock frequency of 5.994Mc to deliver the exact 20ms. When using a 6Mc crystal this can not be pulled low enough to oscillate at that frequency but a ceramic resonator can. I added a variable capacitor to the design to allow for precise adjustment of the frame rate in conjunction with a ceramic resonator. This will render all hum bars stationary on the screen which will make them less objectionable.
f) Two versions, generator only and generator with modulator: Finally I am going to combine the PICGEN with a modulator. I already have a Band I modulator working in my workshop which uses PLLs covering all 5 band I channels. My plan is to make the PICGEN board plug into the larger PCB of the modulator. A band III PLL modulator is also in the planning stages.
To be continued…
Semir
