EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: ELS122 on April 14, 2021, 10:42:53 pm
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I got this TV Line Amplifier for some reason, and don't have any use for it, so I thought of experimenting with it with audio, it says there that it only goes from 47MHz which is miles over audio, but I'll hope that it was an error on the sticker and they mean 47Hz ;D
Also it could just have a high pass filter inside which I could bypass perhaps...
but I have no clue how it's supposed to get the 12V (it seems) power? do I just connect like a 1k resistor to 12V from the output, and then get my signal trough a coupling capacitor?
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No idea if it works at all down to that but the bottom of the VHF TV range is starts around 47 MHz and the UHF TV range extends up to 890 MHz or so (this is fairly country dependent I believe) so I think the labeling is correct.
Good luck with the experiments.
TonyG
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You're looking for, drumroll... a bias tee. :)
Basically, little more than an inductor from DC to line, and a series cap to block DC going out to the receiver. Evidently you'd want somewhat more than 75R / (2 pi 47MHz) = 0.25uH, and the capacitor can be way bigger than needed, ceramic chip caps are no problem here.
High frequency response depends on quality of both components. The capacitor can be laid out in a microstrip transmission line of the same dimension, that'll automatically be good to many GHz.
The inductor may have impedance dips due to internal resonances, particularly if it's a complicated winding (as you'd have for an oversized value), so there is some wisdom in using only what you need, not going gonzo. Shop for one with impedance curves and verify it's free of dips in the range. (The first (parallel) resonance is fine, by the way. Peaks are good. What matters is impedance dips.)
Also it needs to be rated for whatever current draw the line amp needs, including saturation current if it's a cored inductor.
Tim
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A resistor will work in place of the inductor for the bias-t with some loss of gain.