Hi T,

I think you missed the point. With a tuned antenna, what comes out of
the antenna+tuning is NOT broadband! Frequencies not so very far from
the one you are tuned to will be greatly attenuated. With that sort of
input, you should be able to get by with even a 16-bit converter, if
it's linear (such as a decent delta-sigma is).

For example, a square loop antenna one meter on a side, at 150kHz,
tuned to resonance with a capacitor, should have a Q around 300. That
means it's about 0.5kHz wide at the 3dB points, and will be down about
20dB at 2.5kHz away from center, and 40dB down at 25kHz away.

-- I just read your other reply to my other posting in this thread,
where you worried about broadcast band overload. I suspect that if you
have that, it's because, as someone else said, the antenna is acting as
something other than a loop for that frequency. It's important to keep
the loop balanced with respect to ground. I'd strongly recommend
against a "shielded" loop unless you understand just why you are doing
that. The shield becomes the antenna, and as such, it must be
symmetrical... Also, for the antenna you described, about 3 meters on
a side and large wire, expect the Q to be even higher and the bandwidth
narrower. I think you'll find the resonated impedance to be more like
a few kohms for a single such turn. Then, use a good balanced FET
amplifier to get to a low impedance to drive your transmission line.

By the way, I would note that the switching detector/mixer/converter in
the schematic you showed is not as good as the usual current
implementation of the H-mode mixer, because the channels of the FETs
doing the switching in the one you gave a link to operate at a voltage
which depends on the instantaneous signal amplitude, if I read the
schematic right, and since the channel resistance is a non-linear
function of that voltage, the detector will not be strictly linear.
The H-mode mixers operate one end of the switches at a constant
voltage, and of course the other end when the switch is on must be very
close to the same voltage. I expect (and I think the practical
experience is) that the H-mode mixer will be more linear. Do you know
what the third order intercept for your mixer is supposed to be? I'd
be pretty surprised if it was better than about +45dBm.

But even so, even if you DID have a broadband antenna, you can find op
amps, and you can make amplifiers with discrete parts, that have
distortion products more than 120dB below the level of signals in
excess of a volt at the amplifier output, in the LF frequency range.
In other words, the distortion products will be less than a microvolt,
with one volt output signals. You don't need to run that preamp with
any appreciable voltage gain, so you're handling some pretty big input
signals. And the best of the 24-bit delta-sigma ADCs shouldn't be far
behind that. (I wish I could do that well at 50MHz!--we do make a
23-bit ADC that samples at up to 20MHz, but it's a bit pricey for what
you're trying to do.) As others have pointed out in this thread, the
atmospheric noise is so bad at LF that the antenna doesn't have to be
very efficient to capture enough signal to be useful for receiving.
Unless you are practically next door to a transmitter operating on a
frequency near the ones you care about listening to, dynamic range
isn't likely to be a big issue at LF. [Your other posting suggests
that folk DO have troubles with other signals. I'd go looking for
answers about WHY before jumping to conclusions about what to do about
them.] That's a far cry from the case at HF.

Cheers,
Tom