Originally posted by Carl-NC
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Ivica, you're asking all the right questions. I'm a Big Fan of illustrations so let's dive in (again, all this belongs in the Concept discussion, but we'll go with the flow). Here is a multi-pulse TX using 2 rates:
I used 25kHz because Tony suggested it, and 5kHz because it gives me a familiar 100us TX width. If you transmit only 25kHz then you are stuck with trying to squeeze a GB sample in that 20us window which is tough. Plus, your GB has a dreaded target hole. In this example I used (5) pulses of 25k per (1) pulse of 5k; in reality we might instead use 4:1 or 8:1 just to make scaling in DSP a little easier. You can also change the ratio to implement variable frequency weighting.
Let's suppose we use analog demods. Here is my first guess as to what to try:
Boy, I love being able to just <paste> these images! So I have 4 RX channels: (1) for 25kHz and (3) for 5kHz. Maybe we could squeeze in another sample for the 25kHz pulse but I doubt it would do any good. GB is achieved with a combination of RX25, RX5a, and probably RX5b. Discrimination might be achievable with RX5a,RX5b,and RX5c.
The clocks above are demod clocks. The demods are integrating the results so the ADC does not need to run this fast. You can sample the demods every 400us (2.5kHz) or even slower if you like. Let's say we sample at 2.5kHz. The ADC creates a DataReady pulse which triggers a micro interrupt. The micro reads the ADC and DMA's the result. So far this takes nearly zero overhead. But you don't want to run a processing loop at 400us unless you are using a really fast micro. So instead, you pile up the samples (either in a circular buffer or by just summing the data) and say, every 5ms, you send all the data to the DSP task. (Most metal detectors run a processing loop of 5ms.) A processing loop of 5ms on a micro running at 32MHz gives you 160,000 single-cycle instructions to work with.
So now you have a hard example of an analog demod solution: 4 demod channels, a 16-18 bit 4-channel simultaneous sampling ADC running at 2.5kHz, and not-too-aggressive micro.
Let's say we want to use direct sampling on the above TX waveform. I will take a guess and say I want 4 samples for every 20us pulse. This now sets my ADC sample rate at 200kHz, and it needs to be 24 bits. You will be hard-pressed to grab data that fast in the same micro as the one doing the DSP. So maybe you need a data capture micro that simply sums up the data and eventually feeds it to the DSP processor at the 5ms loop rate. Or maybe you need a really fast M7 micro. Or maybe I only get data at 100kHz or, worse, 50kHz. Note that you probably don't need 20 samples per 5kHz pulse, but it's way easier to run the ADC at a constant rate and just toss unwanted samples.
Before I hit the Post button, notice that the TX waveform has a positive 100us pulse every 400us. The TDI/Goldscan has a 100us pulse every 320us. So just the response from the single 100us positive pulse should be somewhat comparable to the TDI. But we also get a second (negative) 100us pulse, and 10 (!) 20us pulses in the same time frame. So this thing should hands-down beat a TDI on small gold.
I used 25kHz because Tony suggested it, and 5kHz because it gives me a familiar 100us TX width. If you transmit only 25kHz then you are stuck with trying to squeeze a GB sample in that 20us window which is tough. Plus, your GB has a dreaded target hole. In this example I used (5) pulses of 25k per (1) pulse of 5k; in reality we might instead use 4:1 or 8:1 just to make scaling in DSP a little easier. You can also change the ratio to implement variable frequency weighting.
Let's suppose we use analog demods. Here is my first guess as to what to try:
Boy, I love being able to just <paste> these images! So I have 4 RX channels: (1) for 25kHz and (3) for 5kHz. Maybe we could squeeze in another sample for the 25kHz pulse but I doubt it would do any good. GB is achieved with a combination of RX25, RX5a, and probably RX5b. Discrimination might be achievable with RX5a,RX5b,and RX5c.
The clocks above are demod clocks. The demods are integrating the results so the ADC does not need to run this fast. You can sample the demods every 400us (2.5kHz) or even slower if you like. Let's say we sample at 2.5kHz. The ADC creates a DataReady pulse which triggers a micro interrupt. The micro reads the ADC and DMA's the result. So far this takes nearly zero overhead. But you don't want to run a processing loop at 400us unless you are using a really fast micro. So instead, you pile up the samples (either in a circular buffer or by just summing the data) and say, every 5ms, you send all the data to the DSP task. (Most metal detectors run a processing loop of 5ms.) A processing loop of 5ms on a micro running at 32MHz gives you 160,000 single-cycle instructions to work with.
So now you have a hard example of an analog demod solution: 4 demod channels, a 16-18 bit 4-channel simultaneous sampling ADC running at 2.5kHz, and not-too-aggressive micro.
Let's say we want to use direct sampling on the above TX waveform. I will take a guess and say I want 4 samples for every 20us pulse. This now sets my ADC sample rate at 200kHz, and it needs to be 24 bits. You will be hard-pressed to grab data that fast in the same micro as the one doing the DSP. So maybe you need a data capture micro that simply sums up the data and eventually feeds it to the DSP processor at the 5ms loop rate. Or maybe you need a really fast M7 micro. Or maybe I only get data at 100kHz or, worse, 50kHz. Note that you probably don't need 20 samples per 5kHz pulse, but it's way easier to run the ADC at a constant rate and just toss unwanted samples.
Before I hit the Post button, notice that the TX waveform has a positive 100us pulse every 400us. The TDI/Goldscan has a 100us pulse every 320us. So just the response from the single 100us positive pulse should be somewhat comparable to the TDI. But we also get a second (negative) 100us pulse, and 10 (!) 20us pulses in the same time frame. So this thing should hands-down beat a TDI on small gold.
I've been re-visiting this post to get a better understanding of the various virtual channels proposed and how to go about realising them with direct sampling.
I'm happy with the locations of the RX25 & RX5a + and - slots, but am not so sure about the RX5b & c slots, let me try explain: -
X5b+ and RX5c+ occur in a period where the previous positive going transition of the TX clock occurred after a 20uSec active period, whereas RX5b- and RX5c- occur in a period where the previous negative going transition occurred after a 100uSec active period. Is it possible in this case that the + and - samples may not be equal and opposite to each other (barring EFE effects) ?
Regards,
Olly
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