IB Metal Detector Project, Part 5

[Robert Hoolko]

I got an ABC Maxi Board made by Investment Technologies with an 8 MHz
AT90S8535. Since it comes with a Basic compiler I thought it would be easier
to run the A/D tests in Basic. I set up a 6600 Hz software delay loop (no
interrupts) to drive a coil and read the A/D.

First I used a voltage divider to bias the A/D in the center of its range
and took many of samples of the DC voltage. There is no more than one LSB of
noise. Then I drove the coil with a 6600 Hz pulse with enough current to
develop a 1 volt peak to peak signal across the coil. I adjusted the
sampling point to the steepest part of the waveform. I also tried a 2.5 volt
peak to peak signal. In all cases the noise is no more than one LSB. So it
looks like this A/D is going to be ok for directly reading a 6600 Hz signal.

But I did run into some problems before I started getting good results. On
my first try at reading the AC signal I was getting about 5 bits of noise on
the 10 bit sample. The main problem turned out to be that the Basic compiler
sets up one or more timer interrupts for a real time clock and a timer
function. Those interrupts were causing a couple of microseconds of jitter
in my software timing loop. So I was seeing almost a hundred millivolts of
noise because the sampling time was not consistent. I had to disable the
Basic timer interrupts while I was making measurements.

While I was tracking down that problem I also noticed that the A/D does not
start when you tell it to. It does not start until the next tick of the A/D
clock. If you use a 4 microsecond A/D clock then there can be a delay of
from 0 to 4 micro sec before the A/D actually starts. This means that my
high speed interrupt will have to be synchronized with the A/D clock, which
means that the interrupt period will have to be a multiple of 4
microseconds. That means that the period of the transmit signal will have to
be a multiple of 16 microseconds, because there are 4 interrupts per cycle.
Unfortunately there are no multiples of 16 micro sec that give 6600 Hz. I
have not decided what to do about that yet.

But the good news is that the on board A/D seems to do a good job of reading
a rapidly changing signal (tested at about 30,000 volts/sec).

Robert Hoolko

[Carl]

Re: IB Metal Detector Project, Part 5

Robert,

I am intensely interested in your project, but have not had the time to look closely at what you are doing. Do you think a 10-bit ADC is sufficient to get the phase accuaracy? I have not done the calculations.

On the ADC, what do you mean when you say it does not start until the next tick? Are you running the ADC continuously or in a burst mode? Most ADCs have some amount of latency making burst operation difficult, like you pointed out. Can you adjust the ADC clock (and thus all timing) to be closer to the desired TX frequency?

Interesting project...

- Carl

[Robert Hoolko]

Re: IB Metal Detector Project, Part 5

Carl

10 bits should give enough phase accuracy on strong signals but not on weak signals. I think it will be enough for some useful tests. I expect the phase accuracy to degrade with depth. If I get everything else working and the only problem is not enough A/D bits I can always add an external A/D. But I am a long way from knowing if I can even get the project working at all.

The A/D has a free running mode but the timing would not work out. I need to drive the coil at 6600 Hz and I need to sample the A/D at 8800 Hz or some other odd divider of 26400. The A/D takes 13 clocks per sample and the A/D clock prescaler only divides by powers of 2, so the master clock would have to be a power of 2 times 8800 * 13, such as 7.23 MHz. But the timer interrupt to drive the coil has to occur at some multiple of 6600 Hz and it also uses a power of 2 prescaler. There is no frequency in the 4 to 8 MHz range that would meet both requirements. The transmit frequency does not have to be exact. it can be off by a couple of percent. But the ratio of transmit to sampling frequencies must be exact, not close.

I have tested the interrupt routine that drives the coil and samples the signal. I will write about it within a day or so.

Robert

[Robert Hoolko]

+/- .01 degree

Carl

Over a period of two days I have been letting the circuit run and have been looking at the output from time to time. I am reading a phase angle of 35.46 +/- .01 degrees. The signal amplitude drifts up and down more than that, but both channels are drifting together and the measured phase angle stays about the same.

I think that answers the question of whether 10 bits of A/D is enough for accurate phase measurements on strong signals (3.5 v P to P). To get some idea of how it might do on weaker signals I dropped the signal strength by a factor of 100. With a 35 mv p-p signal I am reading 36.3 to 37.9 degrees over a period of a few minutes. Then I dropped the signal to 10 mv p-p. This is about +/- 1 LSB. At this point I can barely recognize the signal on the scope, I can see almost 10 mv of noise on top of the 10 mv signal. The phase angle reads 35.7 to 39.4. It's amazing what you can do with enough data samples.

These measurements were done with the system described in Part 9 plus a low pass filter on the back end with a time constant of 4/60 sec.

Robert