On 12/2/09 12:01 AM, "Poul-Henning Kamp" <p...@phk.freebsd.dk> wrote:
> In message <c793a5fe0912012031k12893065w2682c6e74c920...@mail.gmail.com>, > Josep > h Gray writes: > >> I also found lots of Google hits for the AD8307. Within its >> limitations, this chip seems to be very popular. > > Please notice that most of these chips are intended for AGC purposes > for transmitters, they very seldom have a frequency-flatness spec > worth anything The 8307 and it's ilk are actually quite flat over frequency. Real broadband amplifiers in the log chain. Is it 0.1dB flat? I don't think so, and in an case, layout might give you more ripple than that. But certainly better than 0.5dB practically. The 8307 datasheet says 0.3dB typical for the middle 80dB range for frequencies less than 100MHz, and I'd believe that, based on the ones I've seen hooked up. That's about 1/10th of the -3dB frequency for the logamps in the chain, so they're quite flat. Specs on a logamp are tricky to evaluate against a square law or linear detector because it's tough to know if it's the log function or the basic detection that's at issue. For the thermistor mount, you're lucky to get 30dB dynamic range AND the accuracy will be worse at the bottom end, because a good portion of the measurement uncertainty is fixed, not a fraction of the input. For a diode detector, the newer meters know the cal curve of the diode, so they're not depending on the squarelaw characteristic, and you get maybe 50dB dynamic range, but again, the accuracy at the low end is worse than the high end. I wonder (just haven't looked til now) what they're using inside those USB power heads (e.g. MiniCircuits)... Is it a AD8307 type widget or a diode? _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.