On Sat, 16 Aug 2008 11:06:52 -0700 (PDT), oeguet@[EMAIL PROTECTED]
wrote:
>Hi Bob,
>
>indeed, this is really an interesting and fascinating project. The
>digital lock-in amplifier is a pure software implementation which is
>applied on the input signal (A/D converted receive signal). The lock-
>in amplifier is a very sensitive phase detector even the signal is
>buried in high noise. Any small changes can be detected with it (=B5V
>measuring). Using a 24 bit sound-card at 96 kHz sample rate increases
>the signal-to-noise ratio and dynamic range. It will also work on 16
>bit and 44.1/48 kHz with reduced SNR and dynamic range.
>
>It is quite time-critical application. You must not loose the
>synchronisation of the transmitted to received signal. The continious
>wave form is buffered to the sound-card to avoid signal gaps due to
>operating system task switches. But this is easy to handle and only
>the DMA is busy and relieves the CPU. Laptop has enough CPU power for
>doing this and much more in real-time (FFT, digital filter, lock-in
>amp, detection, signal generation, synch generation, graphical
>output ..).
>
>The operating frequency for the sensor is between 5 to 24 kHz (VLF
>range). It depends only on the resonant frequency of the search head
>(L,C resonant defining elements) and sampling rate (fmax=3DSR/4). The
>higher the operating frequency, the better the sensitivity of the
>sensor (Faraday's law). So it is mostly defined by the sensor
>specification.
>
>The sensors are typically D shaped coils with same inductivity L for
>transmit and receive coil. This will allow a simple matching of the
>capacitors (same for transmitter and receiver). The coils are in
>overlapped co-planar position and forming a circle (two D's). The
>receive coil should have a minimum of signal level (10-50 mV rms).
>This position must be found by moving one of the coils.
>
>
>What about digital lock-in amplifier for your application? This would
>be a quite useful feature.
>
>Regards,
>Aziz
>
Aziz:
Thanks for the explanation. Yes, I am quite familiar with real-time
issues. Daqarta needs perfect sync to do synchronous waveform
averaging for noise reduction, so I've been there and done that!
A digital lock-in would be a definite possibility for Daqarta. I'll
put that on my "Wish List" for future enhancements. I probably won't
offer the "lock-in" (PLL) part that hardware lock-ins have, since I've
always thought that was pretty silly unless you really do need to sync
to an external signal.
For those who are following this thread and aren't familiar with
lock-in amplifiers, they are essentially a single-frequency Fourier
Transform. You separately multiply the incoming signal by the sine
and cosine of the reference signal, and low-pass filter the results.
From the old high-school formula for the product of sinusoids, you get
only terms at sum and difference frequencies. It's the difference
term we want here. The low-pass removes the sum and produces an
output only if the input is exactly the same frequency as the
reference (difference = 0), or very near.
(See <www.daqarta.com/eex01.htm> for an FFT explanation that
goes into more detail about this.)
The only real difference between a lock-in and the output of an FFT
is that the FFT has a very crude low-pass filter (one for each
spectral line) and the lock-in usually has a better filter (longer
time constant in lock-in terms). That statement assumes that the
FFT has a spectral line just where you wanted the lock-in reference
frequency. This is no big deal if you are generating the output
frequency yourself... just make sure it lands squarely on a spectral
line.
Daqarta can already do this. And it can get the noise reduction by
synchronous waveform averaging before the FFT. The only thing
is that it doesn't display the data in lock-in format, with separate
sine and cosine or magnitude and phase readouts. That would be a good
addition!
Best regards,
Bob Masta
DAQARTA v4.00
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
FREE Signal Generator
Science with your sound card!
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