Preamp

[Sean_Goddard]

Point taken on mixing threads.

The micro will have some fancy filtering afound it and careful ground plane placement. Can I suggest you design with an Analogue ground and a Digital ground then I can star the analogue ground an bring the main feed back to the battery connection point.

[Teleno]

If you only take one sample will there not be a lot of noise? Typically sample windows represent maybe 10Us of integrated data. A single sample would not have this advantage unless you average many of the samples, one target taken after each transmit pulse, together.
to form the equivalent integration effect. I do not know how to calculate how many samples would have to be averaged to get equivant noise reduction.

I have designed for speed as a means to improve the Signal/Noise ratio, for early samples increase the amplitude exponentially (the best noise filter). Surfmaster, for example, waits 15us before acquiring a sample and no amout of integration will recover the signal decayed in the delay.

We'll see how it pans out.

[Old cart]

I'll be taking 2 samples per cycle of 1ms, an early sample at 5us and a late sample (EF) just before the Tx pulse. For this it is fast enough.

10 bit at 5V provides a resolution of 1.2 mV which is OK because the noise output level will be about 2mV with 700 gain.

Control of sampling time can be made accurate to one clock cycle (62ns at 16MHz) as follows:

- Write Tx-on pulse to port,
- start Timer,
- enable Timer interrupt.

When the timer interrupts:

- write Tx-off to port,
- start counting loop (set your delay here),
- sample ADC
- wait till conversion is ready.

If you only take one sample will there not be a lot of noise? Typically sample windows represent maybe 10Us of integrated data. A single sample would not have this advantage unless you average many of the samples, one target taken after each transmit pulse, together to form the equivalent integration effect. I do not know how to calculate how many samples would have to be averaged to get equivalent noise reduction.

[Teleno]

Teleno
Wow!
That is quite involved and ingenious.
Thank you for the explanation and quick reply,
Chet

Hey, no problem! Electronics is my passion.

More explanations:

- D2 prevents the reverse Vbe voltage to go beyond -0.7 V. Higher values can degrade the transistor mddle term, values above -5V will pemanently damage it.
- R15/D1 limit the current in Q2 during flyback to 20mA.
- After flyback, D1 is closed and the coil current (target current) flows through Q2 and J4 into R2, causing direct amplification.

[Chet]

Teleno
Wow!
That is quite involved and ingenious.
Thank you for the explanation and quick reply,
Chet

[Teleno]

teleno, can you control when the ADC samples accurately. Also the model I looked at had only a 10 bit 16 kS/S ADC. Theat may not be fast enough.

I'll be taking 2 samples per cycle of 1ms, an early sample at 5us and a late sample (EF) just before the Tx pulse. For this it is fast enough.

10 bit at 5V provides a resolution of 1.2 mV which is OK because the noise output level will be about 2mV with 700 gain.

Control of sampling time can be made accurate to one clock cycle (62ns at 16MHz) as follows:

- Write Tx-on pulse to port,
- start Timer,
- enable Timer interrupt.

When the timer interrupts:

- write Tx-off to port,
- start counting loop (set your delay here),
- sample ADC
- wait till conversion is ready.

[Old cart]

teleno, can you control when the ADC samples accurately. Also the model I looked at had only a 10 bit 16 kS/S ADC. Theat may not be fast enough.

[Teleno]

Teleno, I agree. As long as the design can be reproduced AND we can keep the digital noise form the MCU out of the works it is a better and more flexible way to go.
I was mainly concerned about the desires of others:<)
The question then is you going to willing to either provide either preprogrammed MCU's OR to share the code, and instructions on how to program it?

I'm willing to write the code with all of you, perhaps set up a separate thread for that. But for now I'm concentrating on the amplifier alone.

Hi Teleno
I like the way you set up the -5v regulator for the ATtiny84 and the ADC. I lack the knowledge to understand all of the workings of the receiver circuits. Maybe later when get your final design you could give us less talented a simplified explanation?
Thank you,
Chet

Q2 is set up as a very low impedance, common-base amplifier. p-FET J4 maintains the collector of Q2 a fixed, controlled voltage relative to Vdd. Together they form a cascode configuration. Because there's no voltage swing at the collector of Q2, its output capacitance has no effect on the bandwidth. Instead, it's the output capacitance of J4 (which much lower - 2pF) and R2 what limits the speed of the response.

n-FET J1 is a constant current sink. It fixes the base voltage of Q2 at a steady value relative to Vdd.

Together with p-FET J4 it has the effect of attenuating any spurious variation of Vdd. With the values shown the attenuation is 52dB, or 1/400. For example, a 100mV ripple is translated into 250uV at the preamp's output.

Temperature compensation of Q2 is provided by D3.

The gain of the stage is a little under R2/R1, where R1 is the damping resistor.

The collector current in Q2 should be chosen for the smallest Noise Figure usually specified for 100uA - 200uA for small signal transistors. Q2 could better be BC559 (low noise version of BC557).

[Chet]

Hi Teleno
I like the way you set up the -5v regulator for the ATtiny84 and the ADC. I lack the knowledge to understand all of the workings of the receiver circuits. Maybe later when get your final design you could give us less talented a simplified explanation?
Thank you,
Chet

[Old cart]

The analog part is so designed that it does not depend on a stable power supply. All voltages are referred to one end (the +) and what the other end does is quite unimportant.

The ADC is contained in the MCU and is 12 bit.

If you want to apply this preamp to conventional integrators all the good characteristics will be lost unless you upgrade the integrators to high-speed op-amps ( around 50 Mhz gain-bandwith). Samplig of the Surfmaster has to be redesigned for earlier sampling and faster rise times.

That's why I go MCU, it's just so much simpler. Once the tough analog part is done, programming is the way to go.

Teleno, I agree. As long as the design can be reproduced AND we can keep the digital noise form the MCU out of the works it is a better and more flexible way to go.
I was mainly concerned about the desires of others:<)
The question then is you going to willing to either provide either preprogrammed MCU's OR to share the code, and instructions on how to program it?

[Teleno]

I agree and noted that before. But specs can be easily passed between threads. Btw you are a very prolific and, I think, good designer. This is a very simple design but a long way from what was originally proposed. I suppose for those that want to stay away from a microprocessor we could do this as two PCBs and just use the output of your preamp to drive a conventional integrator. What are you proposing to use as an ADC for the preamp out? And ten there is the matter of split vs not split supply when it comes to ease of integrating high quality analog.

The analog part is so designed that it does not depend on a stable power supply. All voltages are referred to one end (the +) and what the other end does is quite unimportant.

The ADC is contained in the MCU and is 12 bit.

If you want to apply this preamp to conventional integrators all the good characteristics will be lost unless you upgrade the integrators to high-speed op-amps ( around 50 Mhz gain-bandwith). Samplig of the Surfmaster has to be redesigned for earlier sampling and faster rise times.

That's why I go MCU, it's just so much simpler. Once the tough analog part is done, programming is the way to go.

[Old cart]

The preamp cannot be independently designed from the power supply, since it works with the raw power at the input and the digital power at the output.

This is my provisional (still partly conceptual) design:

I agree and noted that before. But specs can be easily passed between threads. Btw you are a very prolific and, I think, good designer. This is a very simple design but a long way from what was originally proposed. I suppose for those that want to stay away from a microprocessor we could do this as two PCBs and just use the output of your preamp to drive a conventional integrator. What are you proposing to use as an ADC for the preamp out? And ten there is the matter of split vs not split supply when it comes to ease of integrating high quality analog.

[Teleno]

Not to be a policeman but we are posting all these comments in the preamp thread. Our original intent was keep this thread clean. While there is lot of good comments here it is hard to keep the thought process going.
I suggest two things:

1 The power supply comments be posted in the power supply section.
2 Once the power supply noise and current requirements are defined we can get back that but there already is considerable discussions on the welcome thread.
Maybe this should be moved to the power supply thread.

Sorry for being a stickler.

The preamp cannot be independently designed from the power supply, since it works with the raw power at the input and the digital power at the output.

This is my provisional (still partly conceptual) design:

[Old cart]

Not to be a policeman but we are posting all these comments in the preamp thread. Our original intent was keep this thread clean. While there is lot of good comments here it is hard to keep the thought process going.
I suggest two things:

1 The power supply comments be posted in the power supply section.
2 Once the power supply noise and current requirements are defined we can get back that but there already is considerable discussions on the welcome thread.
Maybe this should be moved to the power supply thread.

Sorry for being a stickler.

[green]

[Could it be because your GB sample has a higher gain than the main sample to compensate for the loss of target signal?
Therefore higher gain equates to more noise. (Using a longer sample pulse is the same as increasing the gain.)] Reply #162 Qiaozhi

That is some of it, but I don't remember the noise increasing as much as the gain.

[Green try sampling JUST BEFORE the Tx pulse instead of 150us after. Just a thought. Also what about a very LONG ground/ efe sample pulse to integrate as much of the noise as possible?] Reply #172 Sean_Goddard

The GEB sample is about 100usec long. The EF samples are taken just before Tx on.

I'm working on another circuit. My PS noise is to high plus other things. When I get it working I'll post some better measurements.