field test unit no 001 "model T"

[Tinkerer]

If you do not make the right through hole PCB, to solder red marked soldering points on top side on belonging parts contacts are required.

Congratulations for the excellent work.

Tinkerer

[moodz]

On second thoughts .....

Dont fit that diode ... this needs more investigation.

[moodz]

IMPROVEMENT No 0001

By adding a diode to "balance" the circuit across the differential damping input a few microseconds are shaved off the flyback .... well I reckon its a good improvement. Sensitivity seems to remain unchanged. The diode has to go to VPP and not ground or a constant current would flow.

Below is the change note D13.



Below the proof of the pudding ... purple trace is a reference trace saved with diode fitted.
White trace is diode NOT fiitted.

[WM6]

If you do not make the right through hole PCB, to solder red marked soldering points on top side on belonging parts contacts are required.

[moodz]

Hi moodz,

thanks for suggestions and very usable tips. More would be needed.

1. As I understand all diodes from D2 to D7 (on my redraw) are 1N914 (or equiv.)? Not too weak?

2. What type of diodes you suggest for D8 to D11 (my redraw)?

3. I set value of C5 to C8 (my redraw) to 10 uF. Is this correct? Have value to be exact paired (say C5 and C6)?

4. What are suggested voltage for C1?

1. Ahhh ... on my proto they are 1n4148 ... according to data sheet they can handle 0.5A 1ms repetitive. I have not blown any up despite some vigorous workouts.

2. These are all 1N4148 also ... have used this configuration many times for low currents with no issues.

3. Correct all 10 uF. Pairing not so important. C5 and C6 ensure stability of regulators ... ie should be close to regulator. Note the bias generator does depend on the switching waveform frequency and duty cycle... if the waveform is not fast enough there will be impact on bias and ripple. I have tested from 100 us to 60us pulses at 4 - 6 Khz and works OK.

4. C1 should be rated to handle the supply battery voltage. I used a number of caps in parallel mainly because it was cheaper and ensure good filtering on supply as the analogue ground is connected to VPP.

moodz.

[WM6]

Hi moodz,

thanks for suggestions and very usable tips. More would be needed.

1. As I understand all diodes from D2 to D7 (on my redraw) are 1N914 (or equiv.)? Not too weak?

2. What type of diodes you suggest for D8 to D11 (my redraw)?

3. I set value of C5 to C8 (my redraw) to 10 uF. Is this correct? Have value to be exact paired (say C5 and C6)?

4. What are suggested voltage for C1?

[moodz]

PCB notes

Excellent job !!!!

Some minor improvements / suggestions.


The amplifier gain resistor could have linking arrangement or provision for switched resistor or POT to set gain. See THAT1510/12 data sheet for values.

D1 on the WM6 schematic is an SF18G ....not critical ... it does provide some improvement .... the circuit will work without the diode ( ie replace with shorting link ).

R1 ( on WM6 schematic ) the main damping resistor needs to be on posts or have arrangement for easy change as this value will need adjusting for different coils etc. High Q coils easily burn out this resistor. See 1K 2Watt resistor burnout pic below Flyback voltage across resistor was 1100 volts with NO mosfet breakdown ( mosfet only sees half of this voltage or 550 volts .... breakdown of mosfet in this case was approx 630 volts ). Maybe a resistor network is required. Note also that damping currents flow through the two differential resistors R3 and R2 via the protective diodes to analogue ground ( VPP ) however these resistors only experience half of the peak differential voltage on R1.



moodz

[WM6]

For those willing to experiment with MOODZ PI FrontEnd in connection with clasical PI like HH, SurfPI, GarryPI etc. here Moodz PI FrontEnd Schematic and (double sided) PCB:

Not checked yet! Checking welcome!

[moodz]

Hi moodz

Questions regarding jumpers j2, j4 and j5, X1-1 and X1-2:

Is this "puppy" working as complete PI detector if we need only audio signal over audio line on j5 (without LCD)?

What of role is j4?

Is j2 for PIC programming?

Where to connect X1-1 and X1-2?



Where to get signals for integrator? From which points on schematic?

Yes this a complete detector ... only need audio. Self balancing and calibrating. The LCD is only drawn in as future expansion option. I have no code for it yet. J4 is for future rotary control / quadrature encoder... also no code yet. J2 is the standard PIC programming port. It also is serial port for debug and infos during runtime.
X1-1 and X1-2 connect to battery or power supply connector etc

To adapt front end to s/h & integrator just take signal from C4 and sample at appropriate times.

moodz

[WM6]

Hi moodz

Questions regarding jumpers j2, j4 and j5, X1-1 and X1-2:

Is this "puppy" working as complete PI detector if we need only audio signal over audio line on j5 (without LCD)?

What of role is j4?

Is j2 for PIC programming?

Where to connect X1-1 and X1-2?

It is conceivable that a conventional sample and hold / differential integrator could be easily adapted for a true "no digital bits" solution.

Where to get signals for integrator? From which points on schematic?

[moodz]

Below is the schematic of the the differential front end test unit called "Model T".

Notes.

1. The bias generator is referenced against VPP ( ie battery +Volts ) .... this means that as far as the differential amplifier is concerned VPP is analogue ground. The bias generator feeds the +/- 5 volt regulators which in turn supply the diff amp. In theory given the ratings of the components the DC battery supply voltage could be as high as 35 volts approx !!! The intput to the bias generator is fed by the same output as the gate drive to the NMOS FET. I did have some concern that this would impact the rise / fall ( turnoff time critical ) of the FET switch. However the impact was less than 4 ns measured on the CRO. This helps simplify and cheapen the circuit whilst synchronising bipolar supply noises.

2. The amplifier is run at its maximum specified voltage gain. ( 1000X or 60dB ) This presumably means that a 1uVolt signal at the input will become a 1 mV signal at the output. There are many types of diff amps available .... I have used THAT1510 and THAT1512 with great success. The bandwidth of these amps is several Mhz even at gains of 1000. Noise is specified at 1 nVroot hz or better .. but I have had no need to consider this critical in this relatively simple circuit. As with conventional PIs a coil with less turns will be faster however it will be less sensitive. A further gain block could be inserted after the diff amp to compensate for loss of sensitivity with a faster coil.
A manual gain control could be introduced by varying or switching R4. The penalty would be added complexity / cost and noise.
A cap inserted in series with R4 will reduce the low frequency / DC gain to 1. This could be useful for removing slow earth field responses and low frequency hum etc.

3. The output of the diff amp is fed to the ADC input by a simple cap coupling. Microchip specify a <500 ohm source impedance for feeding the ADC input at high conversion speeds and I did muck around with some buffer amps etc however this turned out to provide the best and simplest coupling. The important bit of the output waveform is very conveniently located at approx +2 volts which is bang in the midrange of the 0 - 5 volt input of the ADC without any special clamping or level shifting. Waveforms to be posted.

4. The NMOS switch can be your favourite PI FET ( IRF740 etc etc ) however note that the voltage flyback developed across the damping resistor will only be half of that on a conventional single ended PI. This is an advantage as you can use a lower breakdown voltage FET than normal. ( and usually a lower ON resistance thus providing a higher peak switch off / saturation current. )

5. This circuit is designed for a so called differential PI coil .... I am not sure how it will behave with a conventional coil. I am fairly sure that the coil sheilding will actually be a disadavantage and pick up every AM radio station and powerline within a 100 km radius because the the input to the wideband diff amp will be unbalanced. Also conventional coils dont have the common "centre tap" so I guess it wont work anyway.

6. The front end is the important bit here. The CPU is incidental .... bolt on your favourite processing backend. It is conceivable that a conventional sample and hold / differential integrator could be easily adapted for a true "no digital bits" solution.

I guess these notes will grow with time.
moodz
picstic_Model_T.sch.zip