On 10/28/2014 5:24 PM, Lostgallifreyan wrote:
rickman wrote in :

I have a project in mind that would need a very good antenna in the
frequency range of 60 kHz. Originally I looked at loop antennas and
liked the idea of a large shielded loop made of coax tuned with a
capacitor. My goal is to get as large a signal as possible from the
antenna and matching circuit to allow the use of a receiver with very
low sensitivity... in fact an all digital receiver.


MSF time signals? Just a thought... If you're interfacing an analog signal to
digital, one trick I used (for audio but it ought to help here too) is a
CA3140 with a bit of positive feedback through a few Mohms for hysteresis to
clean the signal a bit. The resulting Schmitt trigger, powered by about 5 or
6V, could be sensitive to take a lot of strain off your antenna. Whether this
alone gives you enough gain I don't know, but it is cheap to try.

Thanks for the suggestion. I'm not sure this would be any better than
feeding it directly into my digital input. That is a differential input
and I expect to use feedback to overcome the residual input offset. So
the input will be pretty sensitive, the question is whether I need mV
level signals or maybe just uV signals which might not require an amp.
By using positive feedback the threshold would be shifting and the
amount of level shift would set the floor for the signal level from the
antenna I think.

--

Rick

rickman wrote in :

MSF time signals? Just a thought... If you're interfacing an analog
signal to digital, one trick I used (for audio but it ought to help
here too) is a CA3140 with a bit of positive feedback through a few
Mohms for hysteresis to clean the signal a bit. The resulting Schmitt
trigger, powered by about 5 or 6V, could be sensitive to take a lot of
strain off your antenna. Whether this alone gives you enough gain I
don't know, but it is cheap to try.

Thanks for the suggestion. I'm not sure this would be any better than
feeding it directly into my digital input. That is a differential input
and I expect to use feedback to overcome the residual input offset. So
the input will be pretty sensitive

Well, try it. If it works then inputs are better these days. Or at least,
more sensitive to small changes. As far as I know, digital inputs are usually
specified with a wide dead band for levels, amounting to HUGE hysteresis and
a need for a lot of gain first sp you already ned an op-amp stage no matter
what unless your digital inputs have hair triggers at exactly the threshold
you wanr.

The thing about the CA3140 is that with just three passive parts: M-ohmage of
positive feedback, input series capacitance, and input ground resistor after
the cap, you can empirically set some very nice signal preconditioning as
well as raw gain, all on a very convenient single rail supply at 5V.

On 10/29/2014 6:53 AM, Lostgallifreyan wrote:
rickman wrote in :

MSF time signals? Just a thought... If you're interfacing an analog
signal to digital, one trick I used (for audio but it ought to help
here too) is a CA3140 with a bit of positive feedback through a few
Mohms for hysteresis to clean the signal a bit. The resulting Schmitt
trigger, powered by about 5 or 6V, could be sensitive to take a lot of
strain off your antenna. Whether this alone gives you enough gain I
don't know, but it is cheap to try.

Thanks for the suggestion. I'm not sure this would be any better than
feeding it directly into my digital input. That is a differential input
and I expect to use feedback to overcome the residual input offset. So
the input will be pretty sensitive

Well, try it.

Yes, easier said than done. The receiver isn't built yet, I am
currently looking at the antenna design again and wish to improve my
simulation by adding the radiation resistance. If the antenna will only
put out microvolts even after tuning I will need to figure out how to
add the amp without having to double or quadruple the power budget.


If it works then inputs are better these days. Or at least,
more sensitive to small changes. As far as I know, digital inputs are usually
specified with a wide dead band for levels, amounting to HUGE hysteresis and
a need for a lot of gain first sp you already ned an op-amp stage no matter
what unless your digital inputs have hair triggers at exactly the threshold
you wanr.

This is a differential input which is not far from an analog input.
Actually even single ended digital inputs don't have much hysteresis
unless they are designed for that. But there is always some because of
the parasitic capacitance between the input and output of the buffer.


The thing about the CA3140 is that with just three passive parts: M-ohmage of
positive feedback, input series capacitance, and input ground resistor after
the cap, you can empirically set some very nice signal preconditioning as
well as raw gain, all on a very convenient single rail supply at 5V.

This design won't have a 5 volt rail. Most of the design will run on
1.2~1.8 volts with some I/O at 3.3 volts to drive an LCD. It's very low
power, remember?

--

Rick

rickman wrote in :

Actually even single ended digital inputs don't have much hysteresis
unless they are designed for that.

Well, as a proportion if they only go high above soem fairly close approach
to V+, then low when close to 0V, then the dead band could be wide, the aim
was to eliminate false states so they ARE usually designed for it. I take
your point on very low volt systems, if the actual difference is small even
though proportionally it may not be.

Anyway, now I know that the supply is so small, your suggestion of discrete
transistors is almost certainly the way to go, unless there is enough similar
demand out there to have cause an off-shelf part to be made.

Normally I'd just look at how others are solving similar problems, so I guess
the question I can ask is: what is the signficant difference in this case
that prevents the nearest off-shelf answer from working?

On 10/29/2014 3:06 PM, Lostgallifreyan wrote:
rickman wrote in :

Actually even single ended digital inputs don't have much hysteresis
unless they are designed for that.

Well, as a proportion if they only go high above soem fairly close approach
to V+, then low when close to 0V, then the dead band could be wide, the aim
was to eliminate false states so they ARE usually designed for it. I take
your point on very low volt systems, if the actual difference is small even
though proportionally it may not be.

Anyway, now I know that the supply is so small, your suggestion of discrete
transistors is almost certainly the way to go, unless there is enough similar
demand out there to have cause an off-shelf part to be made.

Normally I'd just look at how others are solving similar problems, so I guess
the question I can ask is: what is the signficant difference in this case
that prevents the nearest off-shelf answer from working?

What off the shelf answer? I have not seen any all digital receivers
for any frequency. I think it may only be practical for this case and
I"m not sure of that. lol

This signal is very unique in that it has a very low data rate. This
allows integration in the digital domain over a large number of samples.
Theoretically the signal would be detectable with a negative SNR.
There are actually a number of issues I need to solve to get a prototype
working. The big one is being able to get a large enough signal that
even statistically it is noticeable at the receiver input.

--

Rick

rickman wrote in :

What off the shelf answer?

I just meant in terms of interfacing. Never mind, one of my other replies
might be far more useful. While you can integrate digitally, why do so? It
seems to me (if I haven't missed something I shouldn't) that you might get
away with much less gain before analog integration, then you can boost the
resulting slow signals with much less struggle with gand bandwidth products
and slew rates for low power and such. If you can do it this way, the
resulting slow pulses can be boosted with CMOS which at those speeds will be
pretty much nanopower.

On 10/29/2014 4:41 PM, Lostgallifreyan wrote:
rickman wrote in :

What off the shelf answer?

I just meant in terms of interfacing. Never mind, one of my other replies
might be far more useful. While you can integrate digitally, why do so? It
seems to me (if I haven't missed something I shouldn't) that you might get
away with much less gain before analog integration, then you can boost the
resulting slow signals with much less struggle with gand bandwidth products
and slew rates for low power and such. If you can do it this way, the
resulting slow pulses can be boosted with CMOS which at those speeds will be
pretty much nanopower.

Before integration comes demodulation. How would you demodulate and
integrate in the analog domain on a 100 uW power budget? The signal is
PSK. But that is not the real reason. My goal is to show it is
possible to do this entirely in the digital domain.

The devices I have available are not 100% optimized for low power at low
clock rates, but they are pretty good. If I can find devices that have
lower quiescent current the digital design has potential of being lower
power than the analog approach.

--

Rick

rickman wrote in :

By using positive feedback the threshold would be shifting and the
amount of level shift would set the floor for the signal level from the
antenna I think.


Yes, basically like a noise gate. The op-amp trick is nice though, it gives
you fine control of it.