Impuse inspred flexible TX and RX-FE platform

[KingJL]

I am about to get my experimentation environment where I want it. Recent discussions by Dave J. and Carl peaked my interest in the TX used in the Fisher impulse and the CZ5. I decided to put together a platform that would be as flexible as possible in experimenting with both PI and VLF configurations. It is comprised of 2 stackable boards (each a about 48mm x 48mm); a TX borad and a RX/ coil config board. The TX voltage (and thus TX power) is configurable by jumpers to be at battery (can be +/- 9V or +/- 7.4V LiPo) voltage or +/- 5V. The Coil configuration (select-able by jumpers) can be PI Mono, PI IB, or VLF. There are four optional capacitor mounting footprints for both the TX VLF and RX VLF sections that can be used for resonant tuning. All components are SMD. I plan to send the gerbers to the board house this week for a batch of 10 boards (10cm x10cm), (which will result in 20 TX boards and 20 RX/Coil selection boards). The design is done with KiCad (kicad files attached).

Thanks to both Carl and Dave J. for the inspiration.

[KingJL]

The RX has a jumper selectable gain of (approximate) 32 or 64. For those interested, I am posting a BOM spreadsheet.

[KingJL]

KiCad's 3D view of Tx and Rx board.

[KingJL]

Back from the Board House

Received these boards today... 20 sets of the 2 boards.

now to populate and test!!

[KingJL]

TX done... RX/COIL CONFIG next.

[KingJL]

RX/COIL CONFIG board done.

[KingJL]

Both boards connected together... Tomorrow or the next day, start testing... DC tests done!

[WM6]

Nice project KingJL.
Wish you good testing results.

[KingJL]

Finally got around to some testing. Thankfully it all works. Currently power is supplied by two 9V batteries. I am using the 5V option option for now. Tx 1 & Tx 2 were driven by positive 3 v pulse 50 usec space 500 usec apart... Tx 2 is offset by 250 usec from Tx 1... all this resulting in a bi-polar 4 KHz TX with TX pulse width of 50 usec.

First test with 300 uH, 1.5 ohm monocoil. Red trace is coil voltage, Blue trace is coil current measured across 0.1 ohm resistor, blue trace is 20 mv/div, peak coil current = 610 mA:


Test 2 is with a 265 uH 5 ohm elliptical figure 8 (RX PI-IB mode). Again red trace is coil voltage and blue trace is coil current, blue trace is 20 mv/div, coil current = 580 mA:


The last scope shot is again with the elliptical figure 8 coil, red trace RX out, Blue trace coil current, RX gain is set to 64 :


The elliptical figure 8 is my goto coil for lab testing as it is "quiet". But, I need to get busy and build some more coils for testing... I want a 10.5" concentric 300uH @ 1-1.5 ohm TX, a 10.5 inch concentric 1 mH @ 3ohm TX.

[Dave J.]

The 300 uH is by far the best of the 3 coils. However you need to add some damping resistance across it to get rid of that long decay tail. Looks like somewhere in the vicinity of 1,000 ohms is probably a good place to start. Make sure your 'scope probe is on x10 in order to avoid throwing everything off with 'scope capacitance loading.

Sorry, I can't spend any time on this project. But did see the waveforms and couldn't resist posting congratulations. Brings back fond memories.

[KingJL]

However you need to add some damping resistance across it to get rid of that long decay tail. Looks like somewhere in the vicinity of 1,000 ohms is probably a good place to start. Make sure your 'scope probe is on x10 in order to avoid throwing everything off with 'scope capacitance loading.

Dang... Totally forgot about that... I usually use a 1pf cap in series with my probe to gaurd against that... totally forgot it... brain must have gone to sleep. Thanks Dave.

[Dave J.]

Also, I misspoke in my previous post: a long decay tail is usually the result of damping resistance that's too low, so you probably need to increase whatever you've got there. No damping resistor at all of course produces vigorous ringing.

[KingJL]

Also, I misspoke in my previous post: a long decay tail is usually the result of damping resistance that's too low, so you probably need to increase whatever you've got there. No damping resistor at all of course produces vigorous ringing.

Well, I could not adjust in enough resistance to correct the over-damped condition when operating in PI-Mono mode. Tracked down the problem... In my endeavor to optimize the noise performance, I kept the input resistance to the pre-amp as low as possible... 220 ohms is too low! I changed R201 to 1K and R202 to 8K2 (to keep the gain the same), and now it is not over-damped in Mono mode. I had already previously changed RV101 to 2K, R206 to 2K2, and R207 to 220 as I suspected, I may need more room for adjustment. Everything worked fine except for PI-Mono mode... Now, all is good! New scopeshot of 300 uH 1.5 ohm TX monocoil. Again, red trace is coil voltage, blue trace is coil current... much better damping charactaristcs!



I will be updating the files to show changes.

[KingJL]

Upated Files

The updated schematic, BOM, and kicad files to reflect changes brought on during testing.

gp-tx-rx_schematic.pdf
gp_tx_rx_BOM.pdf
gp_tx_rx_kicad.zip

[KingJL]

I finally received my selection of standoffs so I could properly mate the 2 boards.





Before I did any more testing, I decided to go ahead and make a 10" concentric co-planer coil as I wanted some comparisons other than my 9" elliptical figure 8. Coils specs (as completed) are: TX 10.25", 300 uH, 1.7 ohm; RX 5.6", 400 uH, 5.8 ohm.


Connected up the TX/RX and observed the following coil voltage and current waveforms. Red: coil voltage via 10X probe, 100 mv/div. Blue: coil current measured across 0.1 ohm resistor, 20 mv/div.


Preamp output of balanced coil: Red coil voltage via 10X probe (for sync). Blue: pre-amp output 50 mv/div.


Now for more testing...