field test unit no 001 "model T"

[moodz]

As a test of sensitivity we measure the noise difference between a 1K resistor and a 100 ohm resistor on the target channel ..... very nice

[moodz]

$50 FPGA

[Aziz]

$50 FPGA

F.. Hell!!! Don't give the market leader a fright!!!!!
Expect being sued now....

MUAHAHAHAHAHAHAHAHAHAHAHA
*LOL*

[chudster]

$50 FPGA

Do you (or how do you) get it into the PIC32?

[moodz]

Do you (or how do you) get it into the PIC32?

This one goes straight into the PC ( thats how I get the screenshots ) ... it will easily interface to the PIC32 . eg ..... ADC -> FPGA -> PIC32 .. the comms to the PIC32 is SPI.

[chudster]

This one goes straight into the PC ( thats how I get the screenshots ) ... it will easily interface to the PIC32 . eg ..... ADC -> FPGA -> PIC32 .. the comms to the PIC32 is SPI.

The FPGA must use 'store and forward' as the SPI, even with a 40MHz clock would have trouble with real time at 2MSPS? I presume there is no antialiasing filter with the direct sampling? Do you see that as a problem or is the bandwidth limited by other means to prevent aliasing problems?

Chudster

[moodz]

The FPGA must use 'store and forward' as the SPI, even with a 40MHz clock would have trouble with real time at 2MSPS? I presume there is no antialiasing filter with the direct sampling? Do you see that as a problem or is the bandwidth limited by other means to prevent aliasing problems?

Chudster

well ... you dont need to sample "all the time" and the pic32 will do DMA if you poke it a bit ....

[Davor]

With a coil on the very frontend of the system - yeah, it is bandwidth limited.

[Aziz]

BTW guys,

everyone interested in direct sampling and decoding can also use the sound card + laptop with appropriate detector controller. It works upto 48 kHz bandwidth. The principles remain the same. On the fast ADC system, you just have more bandwidth for more signal integrity.

The sound card solution could be seen as the poor man's solution.
Aziz

[chudster]

well ... you dont need to sample "all the time" and the pic32 will do DMA if you poke it a bit ....

Sorry always trying to solve stuff too far ahead. I realise you haven't done it yet and of course this is one big experiment as per parts of the patent act we were looking at
The UNO32 doesn't have DMA (on the SPI) not that this can't be managed still.


As to the comment from Davor, obvously the coil you attach will self resonate. 2MSPS gives 1MHz via nyquist but in practice you want your frequency response to start to cutout way before 1MHz. I have been figuring on 250KHz or so as being the needed frequency response of (small gold) targets. More is always better but is a frequency response of 250KHz OK or do we need 1MHz? 1MHz probably encompasses the self resonance of the coil. Is that good? Maybe we don't know so the experimental rig has more than we need?

[Davor]

OK, but practically speaking, you do not need MHz bandwidths unless you are not into discriminating sea water salinity Every practical target is much slower than that.

[Aziz]

The only problem is the CPU/FPU horse power if you have a large amount of signal samples.
Let's consider the following example:
Sampling every 0.5 µs (2 MHz sampling rate, 1 MHz bandwidth).
Sampling for 128 µs duration in the sampling period (after the flyback period) -> total 256 samples.
FFT for 256 samples requires some processing time.

Well, if the processing power isn't high enough, it makes sense to reduce the sampling rate and the total samples count. The demodulation (yep, FFT is a synchronous demodulation function *HEHE*) isn't much critical and will work with larger bin sizes (or shorter FFT array sizes) too.

Aziz

[Aziz]

BTW,

you can imagine, why I like to use my Netbook setup. More than 2 GFLOP/s horse power.... enough for all sophisticated methods and algorithms.

Aziz

[hdphilip]

hello moodz.

below are spectrum pix of the target and descrim channels .... I cant show waveforms with target and descrim infos as that may give away too much at this "sensitive" time .....the software PI is coming .... and its a race to the bottom.

correct me if i'm wrong, is that AM radio stations your picking up?

also, who makes the test equiptment that your usuing for these tests?


Philip

[moodz]

hello moodz.




correct me if i'm wrong, is that AM radio stations your picking up?

also, who makes the test equiptment that your usuing for these tests?


Philip

you are right ... they are AM radio stations ... at this sensitivity with the shields off the circuit, even waving your hand a few inches above the circuit board will change the amplitude of the signals by a few db ..

I am not using test equipment ... this my a "roll your own" experimental rig .... a fully digital signal chain with a target and descrim channel and my ( patent pending ) front end. Because a patent process is currently progressing I cant talk about the method obviously. This is unfortunate but certain parties have forced this situation on hobbyists.