Originally posted by mp20748
I don't think 600mhz is really needed (for 1080P). I think we'll need far less. But when using a 300mhz chip, we're no where near the best bandwidth performance for the signal. And because it's so hard to get these chips to perform at specs, you'll need a higher bandwidth chip to get the best headroom for the usable signal. And that's what puzzles me about these designs that use, say a 30mhz chip for a 30mhz signal.
For this circuit (30mhz), I would use a 100mhz chip. A 100mhz chip is very easy to tame (parasitics), and it'll provide excellent headroom for the usable signal.
Only 30MHz? How do you arrive at that? I calculated 66MHz. also, I thought your ATSC was about 50MHz or thereabouts anyway?
Our 1080i DVB is only 50Hz, so it's a bit less in MHz, but ATSC is about 55MHz , I thought. Correct me if I'm wrong.
Yes, I think a 100MHz chip(g=1) would be adequate but getting a 280MHz chip(g=1) is not hard.
eg. my favourite OPA3691 chip is 500MHz but at g=1 it is 280MHz with a flatness and -3dB point that are good enough for this puppy. It is dual supply but can be made to work with a single +5V supply. In fact the circuit has unused positions for resistors and caps that were most likely to be used for a dual supply chip version. Fitting a daughterboard with a 16pin SOP chip would be a pain though.
Actually, I'm not using a chip at all there. I removed the BA7623, and I'm by-passing that circuit with a direct path to the RCA's. I thought of using a replacement for the BA7623, but after seeing that once it was removed from the circuit, the roll-off problem decreased. And with 6' of good quality RCA cables, I had a perfect HF test pattern at the other end of the cables. So I could not see a reason for a line driver. And I would never run component beyond 6', with 3' being the best way to go.
When I started out with this, I did not have a diagram. So I used the scope along the signal path. That chip was the first thing I changed, then I arrived at the low pass filters. [/b]
With good quality 75ohm cable there is no reason that you can't go past 2m in length. My projector is 12m in length away, so a short length is not an option for me if I were to drive the projector directly. If the initial driving circuit has a 75ohm output then all will be ok but I doubt that an emitter driver with 150ohm is an accurate 75ohm drive.
Yes, for your short lengths, I guess you could live without it but I would check your pulse reflections on screen or by using a good cro.
HF rolloff will occur with long cable lengths, yes, but I intend to use 5 runs of RG11 to my 1292 (RGBHV) .
This leads me to the next sentence by the previous circuitous route.
The other option of course is that if you bypass the opamp as you suggest and have a transcoder as close as possible to the RCA output pins then that would suffice I believe. I, in fact, have to use my transcoder that I built that is dedicated for this machine so this might well be the easiest option. I can put the transcoder with it's inbult buffereing and good 75ohm drive only about 50mm away from the back of the 30k. Voila. I'm starting to like this idea. I just need to load the 30k with the right resistor for 1Vpp out of the 30k(or change my transcoder to have a g=1). You say it looks great with just 1m so with only 50mm it should also look great.
For others that only use component(YPrPb) then simply putting an external buffer might be the most practical method if you need cable lengths longer than about 1m. It needs to be able to handle 2Vpp and have g=1. Not too difficult.
I look forward to seeing some photos.
To help with the previous conversation I've now also put a schematic bitmap of the back panel pcb containg this opamp.www.mogford.com/audio/ntsc-panel.png
It's all sounding rather good. I'm glad someone told me about this thread, I'd never thought that the JVC was lacking. I can't wait to see the final results.