Let's made a PC-base metal detector with usb interface !!!

[Aziz]

Noise of Transistor-Pairs

Hi all,

I have a first result of the noise performance of the transistor-pairs now. For this measurement, I have modified the test circuit into a high gain single-ended DC amplifier. The gain is set to 60 dB (x1000). This makes noise comparisons of the transistors more accurate. The input is grounded, the output is observed in the FFT spectrum (up to 45 kHz) and the noise floor level is measured around 10-20 kHz.

Sound card noise floor level (signel ended): -116 dB (left channel, input plug not connected)

Transistor-pair, Noise floor level
========================
BC550C, -85.5 dB
2SC1815-Y, -89.5 dB
BC337-40 (from new batch), -94.5 dB
BC337-40 (from old batch), -97 dB
LM394-CH, -97 dB

The highest noise has BC550C pair, the lowest noise the LM394. Interestingly, the BC337 worked as well. The new batch has slightly higher noise. It probably depends on the manufacturer.
But the LM394 worked outstanding stable. The hand matched pairs had offset voltage drifts due to thermal grandients. For an AC amplifier, this should not matter much. But for a high gain DC amplifier, it is a K.O. criteria.

I was dissappointed from the 2SC1815. The hFE classification Y (yellow) has the lowest hFE (around 200) and this was not specified from the supplier. To have a high gain transistor is necessary to lower the bias current. My LM394 has hFE well above 1000 (around 1200). The LM394 is extremely temperature stable.

What's the next? I will buy more super-matched transistor pairs in the coming month.

The voltage controlled amplifier (VCA) for the PI Controller project is getting more realizeable now.

Aziz

[Aziz]

Not amused about the VCA performance

Hi all,

I am really not very amused about the VCA performance. Spice simulations predict high noise contribution to the signal chain at low gains (x1 .. x20). If I compare it to the common products (VCA610, VCA810, ..), they also behave similar. No a real benefit. I even have not built it yet.
Ok, it's time to kill the VCA part.

PGA? Don't know yet. Very likely trashing the idea too.
I have a much better solution with the lowest noise performance.

I am looking into some MUX/analog switch chips now. I don't like their huge non-linear behaviour at +/-5V. I have to rise the supply voltage. Fortunately, they need not much power. Yet another charge pump is beeing designed. This time much better, symmetric output, synchronized, low noise, different voltage levels.
The CD4051..4053 can be powered from +7.5/-7.5V
The MAX312..MAX314 can be powered from +15/-15V.
Now it was a good idea to have a 8.5V system voltage. All relevant supply voltage levels can be generated from the 8.5V system voltage with less circuit overhead.

Aziz

[Aziz]

Never Trust a Software!!!

Fnckn!

Sorry, but LTSpice had bad noise calculations on their own op-amp model (LT1007). Something got really wrong. I have recognized this with a simple closed-loop differential-pair amplifier simulation.

I know, do not use their mosfet models.
Do not use their diode models.
Now, do not use their op-amp models too.

Now, this seems to be very interesting. Noise contribution has been dramatically reduced... Something, I can start optimizing the VCA.

VCA is not died yet.

Aziz

[Aziz]

Low Noise VCA

Hi all,

now the VCA is getting very very attractive. Two MAT02 or LM394 are forming a good simple VCA with 1nV/sqrt(Hz) voltage noise density at maximum gain. And 3nV/sqrt(Hz) at minimum gain, in which the output stage is more dominating here. The results were obtained making a spice circuit simulation.

To get the low noise operation, I had to remove all base resistances (input voltage divider). This reduces the input voltage range of the VCA (max 50 mV) for linear operation and fortunately increases the input impedance. Anyway, we do have very low signals and we need not to attenuate the input signal. For very early signal sampling, the pre-amp output should be used. The VCA is versatile configurable for variable gain and linear operation region.

And I was coming near to trash the VCA. But the circuit needs to be optimized further.

Aziz

[simonbaker]

We appreciate your hard work Aziz -- it may come in handy in many different ways.

-SB

[WM6]

The software part will do all the things required for the operation:
- pulse timing, pulse width, synchronisation, pulse operation mode (all with stereo line output: left/right output channel)
- frequency domain analysis (FFT/Lock-in amplifier/Goertzel algorithm)
- signal analysis and feature extraction (discrimination, filtering, ground balancing, ..)
- detection signalling (sound output in the internal sound card of the netbook)



Aziz

Hi Aziz, what do you mean about use Anisotropic filtering in deeep signal extraction?

http://en.wikipedia.org/wiki/Anisotropic_filtering

[Aziz]

Hi WM6,

Hi Aziz, what do you mean about use Anisotropic filtering in deeep signal extraction?

http://en.wikipedia.org/wiki/Anisotropic_filtering

this technique is totally different from our application. We don't really need it.

There are lots of other digital signal processing algorithms to meet our requirements. For deep signal extraction, digital lock-in amp or Goertzel algorithm is the best. Particularly for decoding our signal channels coming from the sound-card line.

They have excellent small bandwidth (adjustable) in which the noise is reduced almost into its noise spectrum density. Due to this fact, I can decode several signal channels on a single signal line. The channel seperation is really excellent and there is (almost) no crosstalk between them.
The two above mentioned signal decoding algorithms also have a built-in low pass filtering and box-car integration technique given by their time constant parameter.

Just an example:
If I have a 1 V rms dynamic range on my 24 bit sound-card input, I am able to decode 1 µV rms signal levels (even lower with higher time constants). This results in a 20*log10(1V/1µV) = 120 dB dynamic range or ld(1V/1µV) = 20 bit digital resolution.

If my sound-card has a SNR of let's say 110 dB (-110 dB noise floor), the noise level reaches 10^(-110/20) = 3.2 µV rms. So I have to go well beyond -120 dB to detect the 1µV signal savely. Without any signal processing algorithms, I can't do it as the sound-card wouldn't allow it (-110 dB noise floor).

Aziz

[Aziz]

VCA with single MAT02/LM394

Hi all,

it is a pure waste of money to put an expensive super-matched transistor pair for a current mirror used in the VCA. There are much cheaper matched transistors available just for a current mirror application:
BCV 61 (NPN) and BCV 62 (PNP). I have already bought several of them in my last order. Well, it's a SMD part but can be soldered in a precision 8-pin DIP IC socket easily.

So I can start testing the VCA soon. VCA application is very critical as the controlling voltage must be very clean and stable. Otherwise, it would modulate the amplified signal and would result in producing a "modulated noise".

Aziz

[Aziz]

VCA Improvements

Hi all,

I have improved my VCA a bit: Now
- fully differential input
- fully balanced output (symmetric)
- automatic offset voltage compensation (no adjustments necessary)
- lower total noise

Well, I have used total:
- 3 current mirrors (2x PNP type, 1x NPN type)
- 1 differential-pair transistor (MAT02/LM394)
- 3 op-amps (2x transimpedance, 1x servo control)

At the end, the number of op-amps remains same. I would need anyway a buffer and an inverter to make a balanced output signal for further processing. And one for the output stage of the VCA of course.

But this version has lower total noise as these op-amps are used at the output-stage of the VCA. And offset voltage adjustment: gone!

The temperature dependent gain (gm = Ic/VT, gm=Transconductance, Ic=Collector Voltage, VT=Thermal Voltage) will be compensated via the software (temperature sensor).

Aziz

[Aziz]

VCA AC Analysis

Hi all,

this is the frequency response of the amplifier (below). I even had to compensate it to reduce the bandwidth. Collector current of differential-pair transistor is max. 2 mA (high gain). The input voltage noise spectrum density even gets better. Not bad yet.

Aziz

[Aziz]

Alternate Method to VCA

Hi all,

the VCA concept is not convincing me as there is a much simpler, cheaper and better solution. No, it's not a PGA. The PGA-switches in the feedback loop path would contribute much noise to the amplifier. Not good. PGA-idea is dead.

But the concept of having several amplifiers is much better regard to stability, noise and temperature dependency.

The signal path would look like this:

Pre-amp -> Amp1 -> MUX
Pre-amp -> Amp2 -> MUX
Pre-amp -> Amp3 -> MUX
Pre-amp -> Amp4 -> MUX
.. more amplifiers, if needed ..

The amplifier stages should have a fixed gain factor (Gain1, .., Gain4). And log(Gain1/Gain2) ~= log(Gain2/Gain3) ~= log(Gain3/Gain4) behaviour (exponential gain factors). Four amplifier stages should be really enough to have a flexible headroom in the time domain range (early and late sampling timings). No doubt, the VCA would give a finer dynamic range possibility, but it's not really necessary.

After the multiplexer, the signal would be buffered and inverted:
MUX -> buffer (signal+)
buffer -> inverter (signal-)

It is very important, that the input impedance of the buffer should be not low. Otherwise, the MUX itself would give more temperature dependency to the system due to input bias currents. Therefore, the low input impedance inverter is following the buffer.

This concept is making the DAC gain control voltage for the VCA also obsolete. Only 2/3 bit control lines are necessary to switch the MUX. Another plus: very fast switching between different gains possible. A VCA would introduce a settling time between gain changes.

It's just simple and stupid. But it would have the best performance.
KISS principle.

Aziz

[WM6]

Working on relative changes instead of absolute phase angles, gives a better and easier implementation.

Thank you Aziz, but can be those usable, mean relialble enough in repeatability to give us correct info?

[Aziz]

Thank you Aziz, but can be those usable, mean relialble enough in repeatability to give us correct info?

I don't know, which posting number you are referring to.
Can you give me the context please?
Aziz

[WM6]

I don't know, which posting number you are referring to.
Can you give me the context please?
Aziz

Sorry, by mistake from the end of first page, post #35. So crauts live everywhere. But are you sure about FPGA? You ca set multiple amplifiers input inside PGA too?

[Aziz]

Sorry, by mistake from the end of first page, post #35. So crauts live everywhere. But are you sure about FPGA? You ca set multiple amplifiers input inside PGA too?

Sorry, can't find the context. Can you give the referring absolute post number? Then your questions.
Aziz