Replies to Minipulse Plus

[Qiaozhi]

Hi Qiaozhi, can you explain what is purpose of mentioned synchronization?

The internal switching that occurs within the LT1054 can introduce noise into the receive chain. It effectively acts like an external source of EMI. With synchronization, the sampling integrators will remove this noise.

[Davor]

... or better say, the noise happens at times away from timeslots when Rx takes samples, just like it was never there.
George, if you are at it, could you please check the frequency applied to the PSU charge capacitors? They might need beefing up if you use a bit more thirsty op amps in a design.

[Dean Sarelius]

The puzzle of how to synchronize the TX oscillator with the LT1054 is now solved!

The solution is ridiculously simple. I had a brainwave and connected a 100pF capacitor between the collector of Q2 and U2 pin7.
Now it's synchronized. Yippee!

Before you do the modification, connect scope channel 1 to TP1, and channel 2 to U2 pin2. You will see the two signals are asynchronous.
Then fit the 100pF capacitor and you'll discover the two signals are locked together. Simple really.

I tested for any unexpected side effects, such as the +5V supply dropping out at a higher voltage, but could not see any difference. There was also no discernible difference in the detector's sensitivity, but at least everyone can stop mourning the demise of the sync pulse. I will update the latest (REV-C) Build Document tonight, and post the fix in both the REV-B and REV-C threads.

Hi All,
I have just put together a summary of the test point measurements except for TP2 (5V rail) these have been taken without the fix to the TX oscillator synchronization..!

[Qiaozhi]

... or better say, the noise happens at times away from timeslots when Rx takes samples, just like it was never there.
George, if you are at it, could you please check the frequency applied to the PSU charge capacitors? They might need beefing up if you use a bit more thirsty op amps in a design.

The LT1054 is still running at its normal internal clock of 20kHz, but the signal at U2 pin 2 goes high during the time the TX pulse is high (Q2 collector low). Effectively, the TX oscillator is giving the LT1054 a swift kick at regular intervals of about 1ms, with the result that it's not being directly clocked at the slower TX pulse rate, and we have a win-win situation.

[Davor]

Thanks,
it means that transitions happen at approximately every 25us, and apart from being away from audio range it is still contributing noise.
I'll see about a cheap and cheerful variant using a pair of mosfets, and a pair of resistors. I'll let you know If I find a solution this way.

[Qiaozhi]

Thanks,
it means that transitions happen at approximately every 25us, and apart from being away from audio range it is still contributing noise.
I'll see about a cheap and cheerful variant using a pair of mosfets, and a pair of resistors. I'll let you know If I find a solution this way.

If you monitor the TX pulse and the LT1054 oscillator, you will notice that the first transition does not occur until 40us after turn-off. Also any synchronous signals will be removed by the sampling and integration process. So far this seems a simple and robust solution. Personally I'm not in favour of discrete diode pump circuits, as they introduce a lot more switching noise than a dedicated IC, are less reliable, and tend to drop out at a higher voltage.

[Davor]

Fair enough. Every contribution that consistently appears as offset is removed by EF pulse.
I'm glad sync works.

[joop]

Hi All,
I have just put together a summary of the test point measurements except for TP2 (5V rail) these have been taken without the fix to the TX oscillator synchronization..!

Thanks Dean, very useful!

[Qiaozhi]

I have added the REV-C Build Document to the project details, for those who have ordered the latest PCB from Silverdog.
Please also note that the fix for the sync issue is included in the REV-C doc, but not in the REV-B version. So (if you're building a REV-B board) read that as well to understand where to fit the extra 100pF capacitor.

[6666]

Step 9 both build docs says

Adjusting the Audio [threshold] pot will produce a DC level from -5V to +5V at TP10

Where is TP10 ?

[Qiaozhi]

Step 9 both build docs says


Where is TP10 ?

Despite the fact that I've built 2 Minipulse Plus detectors using the Build Document, and Andy (Silverdog) and Dean Sarelius have both completed successful builds, no-one has spotted that one so far.

TP10 doesn't exist in REV-B. That should of course be TP9.
In REV-C, TP10 is the ground probe connection.

I've added a note in the Minipulse thread(s), but will not update either document at this time. Eventually, by REV-D(?) we might get rid of all the errors.

[Davor]

I tried simulating a PSU with only MOSFETs, and it is a no go. Duty cycle is too asymmetric for any meaningful currents so LT1054 that is.

[Qiaozhi]

I tried simulating a PSU with only MOSFETs, and it is a no go. Duty cycle is too asymmetric for any meaningful currents so LT1054 that is.

Thanks Davor. I'm not surprised by your results, but it's very good that you've tried to find a working solution.

I don't know how many MPP boards have been shipped by Silverdog, but I'm surprised that more members are not displaying photos of their working creations. So, to anyone who has built an MPP, please email the photos to me (with any [useful] comments) and I'll add them to the "Completed Minipulse Projects by Geotech Members" thread. At the moment the Aussies are in the lead.

[joop]

I am attempting a REV B build, just finished the RX stage. The signal at TP3 seems a bit noisy, is this normal? This is without the sampling integrator added, which starts with R18. Picture attached. Top trace is the flyback signal.

[Qiaozhi]

I am attempting a REV B build, just finished the RX stage. The signal at TP3 seems a bit noisy, is this normal? This is without the sampling integrator added, which starts with R18. Picture attached. Top trace is the flyback signal.

[ATTACH]30670[/ATTACH]

What are the details of your coil? The timebase on your scope is set to 50us, and the RX signal is still settling. Or perhaps your damping resistor is not the optimum value?

Here is a scope shot of my MPP. Channel 1 is connected to TP1, and channel 2 is connected to TP3. As you can see there is some noise on the signal, but nowhere near as much as your's, and this is running from a bench power supply (which is also noisier than using batteries) and next to two computers, and low-wattage fluorescent lighting. I'm using the original Minipulse (500uH) coil, and you can see that it settles within 25us,