Let me try to give some input to your issue.
I am assuming that you are using dBuV vs. dBmV, have a 750-MHz laser, are ulilizing PAL D/K carriers, the input to the laser with the PAL D/K format at 550-MHz is 80dBuV, your system operates with a 50/70 bandwidth diplexer split and the RF levels at the laser input are flat across the entire bandwidth.
First of all, there must be conversions made from PAL D/K and PAL B/G. 60 PAL D/K channels equal the amount of power of 63 PAL B/G channels. This is due to a PAL B/G format utilizes only 7-MHz of bandwidth below 300-MHz and 8-MHz above 300-MHz. This equates to an extra 32-MHz of bandwidth that equals to another 3-4 PAL D/K channels to be figured into the equation. PAL D/K keeps a constant 8-MHz format throughout the spectrum.
Using this information, your posted information and the laser power delta formula, this is what you should yield as an end result:
60 PAL D/K channels = 63 PAL B/G channels (550-MHz) and both will reflect the same power input to the laser at your 80dBuV (20dBmV).
85 PAL D/K channels = 88 PAL B/G channels in power relationship (750-MHz)
The laser MUST BE a 750-MHz laser. If it is a 550-MHz laser, you will have problems. Since the laser is concerned with overall power input to limit distortions and clip, you will need to find the "point of compression" or "clip point" at maximum channel loaded input level from the specification sheet from the manufacturer. If you exceed the inputs distortions will be created.
Assuming that the specification sheet was 60 PAL D/K channels at +80dBuV, this is what should be noted for 80 PAL B/G channels with the above conversions:
-10log(new channels/63)
-10log(80/63)
-10log(.7875)
-1.034dB
The old input was 80dBuV - 1dB = 79dBuV or 19dBmV at the input to the laser.
Another rule constant is every time the channel loading doubles, there is a 3dB change from reference. Increase RF input when input channels (power) decreases and decrease RF input when channels increase to maintain constant power and C/N.
I am not an expert at PAL conversions as I deal mostly with NTSC, but I hope that this may have been helpful from what calculations I have done.
Good Luck.
Greg
Re: Tx Input RF level at Particular Channel Loadin
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Name:
Greg Tobin
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