Not so DC waveform of minus 5 volt rail at the ne5534 - could this be cause of noise problems

[Polymer]

I have a very jumpy threshold and am trying to hunt down where all the noise is coming from.

I found this rather odd waveform on Pin 4 of the ne5534. It should be -5 volts DC.
This funny signal only gets to the bypass caps (47uF+100nF) and 27Ohm resistor.
The -5v DC on the other parts of the board are just fine!

From what I have learnt here, opamps are pretty sensitive critters.
So could this funny DC be causing noise problems and what could be done about it?

Has anyone else seen this on their Geotech Baracuda?

Oh, here's the osc pic, nearly forgot:

[Davor]

Your scope is set to 50mV AC for that rail, so it would be very surprising if the voltage there was any different.

[Qiaozhi]

I have a very jumpy threshold and am trying to hunt down where all the noise is coming from.

I found this rather odd waveform on Pin 4 of the ne5534. It should be -5 volts DC.
This funny signal only gets to the bypass caps (47uF+100nF) and 27Ohm resistor.
The -5v DC on the other parts of the board are just fine!

From what I have learnt here, opamps are pretty sensitive critters.
So could this funny DC be causing noise problems and what could be done about it?

Has anyone else seen this on their Geotech Baracuda?

Oh, here's the osc pic, nearly forgot: [ATTACH]42686[/ATTACH]

That's perfectly normal. The "wobble" is caused by the fact that the LT1054 output is paused during the TX-on period. If you like, you can kill off the "wobble" by disabling the LT1054 sync circuit, but this will leave the voltage converter in free-running mode. Whether one solution is better than the other can only be found by experimentation.

[kt315]

Has anyone else seen this on their Geotech Baracuda?
---
this is called 'synchronization of LT of outer signal', in your case this is circuit C21 R25 D8. you can always disconnect it doing LT working 'as-is'. seems you need to read datasheet on LT or 7660...

[kt315]

att

[SVEN1]

Recently rec'd some knock off Texas Instruments NE5534's, they give off a rather wonky wave pattern, not even close to what it normally looks like. The detector still functioned.
As soon as I changed out to a real TI NE5534, the wave form was normal.

[Qiaozhi]

If you're experiencing a noise problem with the Geotech Baracuda, then it's more than likely from an outside source. The detector will run quieter if connected to batteries, rather than a bench power supply, and housed in a shielded enclosure. I've also noticed on PI designs that the coil connector shell must be connected to GND.

[kt315]

they can use a microcoil for test but do not want. they want experiencing with huge coil into huge EMI influency environment
waiting best result. microcoil ->> microinfluence->> micronoise.
big coil ->> big EMI ->> big problem.

[Polymer]

Recently rec'd some knock off Texas Instruments NE5534's, they give off a rather wonky wave pattern, not even close to what it normally looks like. The detector still functioned.
As soon as I changed out to a real TI NE5534, the wave form was normal.

Thanks Sven!
I will look into that as I have some TI NE5534's as you've mentioned in use.
I didn't get them from the bay or China, but even some of the more reputable dealers sometimes seem to have wonky parts in their supply chain

[Polymer]

If you're experiencing a noise problem with the Geotech Baracuda, then it's more than likely from an outside source. The detector will run quieter if connected to batteries, rather than a bench power supply, and housed in a shielded enclosure. I've also noticed on PI designs that the coil connector shell must be connected to GND.

Thanks George!
I am running this on batteries, NO coil connected, but Rdamp in place.
Went to an area away from civilisation, could hear the low frequency pulsing of the speaker anyway.
Don't need an oscilloscope for that. What I'm saying is all possible EMI that I can control (PSU's, Oscilloscopes, notebooks, clocks, mobiles, etc)
are not in the vicinity. Just the PCB, speaker and batteries.
Either it is something PCB inherent OR it might be some signal from aviation radar or maritime radar/signals, doppler weather radar?
I have read that radar is pulsed, could this be a possibility?

Is it possible to build a power supply JUST out of batteries for the plus/minus of this Geotec Baracuda, and hook it up without having to use the LT1054.
This I want to do, so I can see if I can rule out the inverting LT1054 setup as part of the noise problem.

Another noisy pic - the very high amplitudes are when passing metal by the coil.

Sorry if I'm ranting - this is really frustrating, and rendering all builds not too usable - be it the old baracuda, surf PI or the new Geotech Baracuda.
That's why I am leaning towards general radar crap in the whole wide area where I am located.
On my next travels, which is a few months away, I will be taking the PCBs and power packs along and see if I have the same problem in another country.

Thanks for listening!

[Polymer]

That's perfectly normal. The "wobble" is caused by the fact that the LT1054 output is paused during the TX-on period. If you like, you can kill off the "wobble" by disabling the LT1054 sync circuit, but this will leave the voltage converter in free-running mode. Whether one solution is better than the other can only be found by experimentation.

I think there's a misunderstanding here.
The -5v output at the regulator after the lt1054 is just fine - a nice flat DC signal as far as I can see.
The -5v rail on pin 7 of the 40106 is just fine too. Flat as a tack DC as well.

It's the -5v on pin4 of the ne5534 which has this "noise" on it. It's synchronous to the PPS obviously, but how is it getting there
and does this "jumpiness" of the -5v influence the function of the ne5534?

I know I can be a stickler for details - more often than not I've seen the devil hiding in them

[waltr]

When the -5V drooping is Synchronous with the TX pulsing then it should NOT effect RX noise. Since the -5V out of the regulator has no droop and there is a series resistor to the neg rail pin of the op-amp means that op-amp is drawing a good amount of current. The Vdrop times the series resistor value (27 Ohm) equals the current draw. This is when the TX is ON which is then OVER VOLTAGING the Op-amp (driving its output to the power rails which is drawing more current. This is what you see.

It may be possible that the NE5534's Bypass caps are not being effective. Maybe try a different 47uf (or other value) & 100nF caps.

What does the output of the Integrator stage (U7a) look like on the scope?


You can wire in a another battery to supply all the Voltages without the LT1054. Just ensure you get the battery polarities correct and also still have the 5V regulators.

As for the 'wobbling' of the final output & audio- I can have this issue in the Hammer Head I build (see the thread "my take on the HH2" in the HH II sub-forum). I did two things that helped decrease this:
1- adjusted the PPS rate so remove AC Mains beat.
2- decreased the Gain of the SAT stage (U7b on the Barra).
There is still a little wobbling but not enough to cause real issues.
If I ever find a better solution I will be posting that in my thread.

[Qiaozhi]

I think there's a misunderstanding here.
The -5v output at the regulator after the lt1054 is just fine - a nice flat DC signal as far as I can see.
The -5v rail on pin 7 of the 40106 is just fine too. Flat as a tack DC as well.

It's the -5v on pin4 of the ne5534 which has this "noise" on it. It's synchronous to the PPS obviously, but how is it getting there
and does this "jumpiness" of the -5v influence the function of the ne5534?

I know I can be a stickler for details - more often than not I've seen the devil hiding in them

OK - I've got my Geotech Baracuda on the bench to take a look at what's going on. Firstly, I can see a flat -5V out of the regulator, and at the various opamps. It is also flat on the side of R22 that's connected to the -5V power rail. However, as in your pic, pin 4 of the NE5534 shows a small amount of droop when the TX is on. As waltr correctly says, this is being caused by the preamp being overloaded by the TX pulse. However, on my board, the ac signal at pin 4 only has an amplitude swing of approximately 4mV, whereas your pic shows a swing of 75mV. Can you check the value of R22 that you have installed? It should be 27 ohms (27R). Also, check that C18 is inserted the correct way round.

By the way, as an experiment I did try shorting out R22, but (although it removed the droop at pin 4) there was no change in signal quality at the preamp output.

[Polymer]

When the -5V drooping is Synchronous with the TX pulsing then it should NOT effect RX noise. Since the -5V out of the regulator has no droop and there is a series resistor to the neg rail pin of the op-amp means that op-amp is drawing a good amount of current. The Vdrop times the series resistor value (27 Ohm) equals the current draw. This is when the TX is ON which is then OVER VOLTAGING the Op-amp (driving its output to the power rails which is drawing more current. This is what you see.

It may be possible that the NE5534's Bypass caps are not being effective. Maybe try a different 47uf (or other value) & 100nF caps.

What does the output of the Integrator stage (U7a) look like on the scope?


You can wire in a another battery to supply all the Voltages without the LT1054. Just ensure you get the battery polarities correct and also still have the 5V regulators.

As for the 'wobbling' of the final output & audio- I can have this issue in the Hammer Head I build (see the thread "my take on the HH2" in the HH II sub-forum). I did two things that helped decrease this:
1- adjusted the PPS rate so remove AC Mains beat.
2- decreased the Gain of the SAT stage (U7b on the Barra).
There is still a little wobbling but not enough to cause real issues.
If I ever find a better solution I will be posting that in my thread.

Thank You waltr! It's a consolation to know I am not the only one with this problem.
It has been dogging me for quite some time, nearly having me throw all this stuff to the dogs ...

I have put another 100nF ceramic cap directly from pin 4 of the 5534 to the ground plane of the 1k resister next to it.
That didn't change anything. I then popped in a 100uF cap in the same position.
Didn't want to unsolder the 47uF first to find out it's not the problem.
The DC signal at pin 4 of the 5534 is for what I can see, quiet now , so that is solved.

The wonkiness of the output is still the same though .... aaarghhhh.

Here's the output of pin 1 of the LF412 like you asked (This is the output of the integrator stage U7a I believe)

The first pic is the signal in normal display mode.
The second pic is the signal in persistance display mode (5 seconds) to easily see the amplitude of the wonkiness here.
The jumpiness is already in that stage, with the threshold loudspeaker output dancing to this signal.




here's another on from pin 7 of the LF412



I have not gotten my head around connecting two battery supplies to this, as I get all mixed up with which potential should be where.
What has me puzzled the most is where to connect the ground plane on the pcb.
If I say connect that to the centre of two 7.5v batteries (-7.5v -- Ov -- +7.5v),
then I would only have 7.5v across the coil. Would that be right?
Hope I haven't lost you, because I am, my heads in knots now ...

To be continued ...

I have been reading up on the 5534 and its apparent quirks, most is written by Douglas Self
and I hope to find a solution (or least a partial one) there ... may this not be a wild goose chase ... again ...

[waltr]

Sounds like either R22 (27 Ohm) value is not correct as has been suggested (did you check?) or the 47uF cap is NOT a Cap. Adding a 100uF seems to have fixed this so that should be good enough.

Your scope capture of U7a Output (pin 1) looks like the Offset from the Pre-amp is not adjusted correctly- but the pre-amp is AC coupled (cap) so can not be the case here.
OR - the second Sampling switch (Q2) is not working correctly.
Re-check the second sample pulse and Q2 is switching. Scope probe on Q2-R26 connection should show a change of Voltage level when Q2 switches on.
Look at Q3 output and the RevD to see what this point should look like.

We were suggesting using a second battery instead of the LT1054 Voltage inverter. To do this remove the LT1054 and connect a 7-15V battery into pin 5 (B-) and pin 3 (B+) to power the 79L05 (negative regulator). This second battery's Positive goes to 0V (Ground).
The 12V batter still connects to +VB & 0V to power the 78L05 (positive regulator).