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Old 4th Mar 2005, 2:07 pm   #42
David_Robinson
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Join Date: Feb 2004
Location: Nottingham, UK
Posts: 94
Default Re: Multi-Channel VHF Modulator Design

Quote:
Originally Posted by flyback
David's KB Majestic ... can we see it on 405 lines!
Trust me Malcolm, that photo does show 405 lines! Thanks for the unwitting compliment to my standards converter / modulator.

Darius, you make some good points. I do like a real technical discussion!

You could add another emitter follower between the filter and the modulator chip, but as Malcolm has already pointed out, there is 500Ω in series with the video inside the MC44 chip anyway so I don't think it would make much difference.

So how significant are these 'high-ish pulse currents'? The video input current is quoted as 1µA maximum. To supply this current on average, the current during syncs (just taking line syncs for a ball-park figure) will be

1 * 98.8 / 9 = 11µA approx.

The drop across (180 + 68 + 500) Ω will be 11µ * 748 = 8.2mV which is 2.7% of the sync pulse amplitude, worst case (0.6% typical). There is no cause for concern!

As for the ability of my emitter follower to supply this current, the drop across its emitter resistor caused by this pulse current will be

11µ * 470 = 5.2mV

This will not come anywhere near to cutting the transistor off so the difference from a PNP emitter follower will be negligible.

Malcolm's suggestion of NPN on Tuesdays and Thursdays is therefore vindicated.

Phew - I think I need a lie down!
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