MPP Coil And Preamp Speed

[KRinAZ]

In my post titled "Getting Ready To Build The MPP" a discussion between baum7154 and I revealed a surprising and unexpected delay in the Rx signal - very important to those of us building fast/low TC coils:

In short, the MPP preamp section itself adds a total of 4uS delay, more than [my] the coil itself.

'Scope shots attached

For all 3 screen shots
Timebase = 5uS/Div
CH1 = 2V/Div Probe x1 - on TP1


For the Coil Flyback pic
CH2 = 5V/Div Probe x100
Note the coil has recovered at 3uS


Output Stage 1 pic
CH2 = 5V/Div Probe x1
There is an almost 2uS [propagation?] delay in the output of the first preamp stage (U5 pin 1) which isn't present at the input (U5 pin 2).


Output Stage 2 pic
CH2 = 5V/Div Probe x1
There is a more than 2uS delay in the output of the second preamp stage (U5 pin 7 / TP3).

Two things become apparent:

1) It would benefit the MPP to use preamps with far less delay from input to output.

2) The use of two preamp stages will unfortunately double the minimum main sample delay. Is it possible to keep the MPP sensitivity while using one stage with the required gain (I am guessing the original design attempted this)?

[Old cart]

As long as you position the early sample to begin where the signal starts to recover the delay should have no negative impact. You should be able to see sample feed though on the preamp output. Alternately you can just look at the gate drive voltage.

[Old cart]

It is also useful to trigger on the falling edge of the preamp output stage. On an analog scope like yours that will move the edge to the left side of the screen. This allows you to speed up the sweep speed so you can more accurately measure time. You should try to spread the signal out to use as much of the screen as possible, vertical and horizontally. This will maximize accuracy.

[baum7154]

A single stage amp will only go deeper into saturation than 2 seperate stages of less gain. The high gain single stage amp is the weak point of most kit detectors when optimizing for small gold.

Dan

[green]

In my post titled "Getting Ready To Build The MPP" a discussion between baum7154 and I revealed a surprising and unexpected delay in the Rx signal - very important to those of us building fast/low TC coils:

In short, the MPP preamp section itself adds a total of 4uS delay, more than [my] the coil itself.

'Scope shots attached

For all 3 screen shots
Timebase = 5uS/Div
CH1 = 2V/Div Probe x1 - on TP1

[ATTACH]33575[/ATTACH]
For the Coil Flyback pic
CH2 = 5V/Div Probe x100
Note the coil has recovered at 3uS

[ATTACH]33572[/ATTACH]
Output Stage 1 pic
CH2 = 5V/Div Probe x1
There is an almost 2uS [propagation?] delay in the output of the first preamp stage (U5 pin 1) which isn't present at the input (U5 pin 2).

[ATTACH]33573[/ATTACH]
Output Stage 2 pic
CH2 = 5V/Div Probe x1
There is a more than 2uS delay in the output of the second preamp stage (U5 pin 7 / TP3).

Two things become apparent:

1) It would benefit the MPP to use preamps with far less delay from input to output.

2) The use of two preamp stages will unfortunately double the minimum main sample delay. Is it possible to keep the MPP sensitivity while using one stage with the required gain (I am guessing the original design attempted this)?

The coil volts is decaying exponentially. The first op amp can't come out of saturation until the coil volts has dropped to about .1 volt. Can't see .1 volt looking at coil volts. Try looking at - input(across back to back diodes). The - input has to go to zero before the amp can come out of saturation. The output of the first amp has to drop to about .1 volt before the second amp comes out of saturation. Don't think it's all amplifier delay.

[Qiaozhi]

A single stage amp will only go deeper into saturation than 2 seperate stages of less gain. The high gain single stage amp is the weak point of most kit detectors when optimizing for small gold.

Dan

This is correct, and is the reason why the MPP has a 2-stage preamp. If you were to compare the same signals on a Hammerhead (for example), you would find that the MPP can sample at an earlier time, even though it has a slightly higher overall gain, 1089 as opposed to 1000 for the HH. Or 60.7dB versus 60.0dB, if you prefer decibels.

[Achillion]

I've done a Spice simulation on the two Op Amp MPP frontend and compared the NE5532 with the ADA4898.

It takes about 4us for the first stage NE5532 to get out of saturation while the ADA4898 has virtually no delay

[green]

Hi KR, Wondering what effect a 1N4148 diode across R13, the first amp feedback resistor would have. Cathode to pin 1. Should keep the output from going to the - supply.

[green]

I've done a Spice simulation on the two Op Amp MPP frontend and compared the NE5532 with the ADA4898.

It takes about 4us for the first stage NE5532 to get out of saturation while the ADA4898 has virtually no delay

[ATTACH]33578[/ATTACH]

Wondering why it takes so long for V(n000) to come out of saturation. KR's scope traces look similar to V(n006).

[KRinAZ]

The coil volts is decaying exponentially. The first op amp can't come out of saturation until the coil volts has dropped to about .1 volt. Can't see .1 volt looking at coil volts. Try looking at - input(across back to back diodes). The - input has to go to zero before the amp can come out of saturation. The output of the first amp has to drop to about .1 volt before the second amp comes out of saturation. Don't think it's all amplifier delay.

Hmmm, OK, it's just that it seems to me that the coil flyback has reached it's minimum level and "flat lined" at 3us (actually slightly before), so I would think the op amps would also be coming out of saturation also. I looked at U5 Pin 2 - quoting above - "There is an almost 2uS [propagation?] delay in the output of the first preamp stage (U5 pin 1) which isn't present at the input (U5 pin 2)." and the signal was present at just about 3uS. I would have liked to get a screen shot but with my limited work bench I can't keep the scope probe there (without shorting things out) and also use my hands to take a pic. If someone looks at this though there is what looks to me like a clear delay between when the actual signal is present at the input and when it is at the output.
If someone could look at this and post screen shots of the delay between U5 Pin1 and U5 Pin2 I think it would benefit this discussion.
Many thanks,
Kyle

edit - question - does that mean the input needs to be at a far lower voltage than the when the diodes come out of clamping? If so then I'm not looking at the input signal to U5 correctly.
I'm saying the input comes out of clamping at about 3uS, but are the op amps still saturated for a while after this until the voltage drops to below .1 volts? Apologies if this has already been answered/stated, maybe I didn't get it earlier in the discussion...

[KRinAZ]

Wondering why it takes so long for V(n000) to come out of saturation. KR's scope traces look similar to V(n006).

I'm not following that - what does V(n000) and V(N006) mean?

edit - oh, never mind, has to do with the spice screen shot which I can't read on my little display...

[baum7154]

Wondering why it takes so long for V(n000) to come out of saturation. KR's scope traces look similar to V(n006).

----------------------------------------

These two amps have vastly different specs. As follows:

=================NE5534 ------------------------------------AD4898
Slew Rate ===========8V/us -------------------------------------55V/us

Gain Bandwidth =======10mHz -------------------------------------65mHz

I think this may have something to do with this. However the AD4898 is not a Dual Amp and is not a direct replacement for the NE5534. Did not research if AN DEV has this in a dual package.

Dan

[KRinAZ]

Hi KR, Wondering what effect a 1N4148 diode across R13, the first amp feedback resistor would have. Cathode to pin 1. Should keep the output from going to the - supply.

Hmmm, I'll try that when I get a chance, might be a couple days...

[KRinAZ]

...and a fyi - for U5 I'm using the LM4562 instead of the NE5532

[Old cart]

The measurement you are trying to make is not possible without getting a better signal capture-more horizontal resolution is necessary.
i don't think you can come to any kind of conclusion from these waveforms as is. See my other posts for suggestions as to how to best do this.