Fascinating stuff Vadim, thanks!
As expected, there's clearly a double whammy when the hand is near the antenna, where phase offset and hand C / damping both act to lower the voltage swing considerably. Wondering if that might adversely affect the sensing of phase, with the comparators having to operate over such a large dynamic range?
As expected, there's clearly a double whammy when the hand is near the antenna, where phase offset and hand C / damping both act to lower the voltage swing considerably. Wondering if that might adversely affect the sensing of phase, with the comparators having to operate over such a large dynamic range?
Good point.
Even with relatively low Q 120-Ohm R_serial inductor, sensing of hand positions near antenna may cause issues.
Better Q LC tank (e.g. 0.15 or 0.2mm wire) may give nice zoom in for long distance, but make even harder to detect out-of-resonance distance.
To reproduce on simulation, we can try to use C_hand=2pF + R_loss=10..100K Ohm
Simple solution - increasing of R_sense value.
As well, I'm trying to review symmetric rail-side sensing based on BJT current mirrors (catching difference between two VCC and GND rails current of output stage) instead of comparator sensing output current using resistor on output.
Current mirror based sensing on BJTs has a good sensitivity and performance - delay is only 2-3ns. But it's only if there is no saturation / clamping.
If output level exceeds working range, waveform is getting distorted, and delay becomes 100-200ns.
Fast comparators are expensive.
BJT current sensor may reduce sensor cost - only some output buffer is required.
Sipeed announces new FPGA board - Tang Nano 20K.
New #FPGA #TangNano 20K Coming soon~
Only 5.5x2.5cm, with 20k luts, 64Mbit SDRAM, RGB LCD, HDMI, WS2812, onbaord codec&PA, support PlayStation2 JoyStick~
And support #LiteX now!
Will be available on aliexpress in April, under $30.
Looks nice for FPGA Theremin.
Has onboard I2S audio output.
Schematic and other useful information is already available on sipeed site.
Tang Nano 20K becomes available soon - Sipeed twitter
Tang Nano 20K is ready now, waiting for Gowin Education IDE to release, ~5.8
New (third) schematic version is available.
Mono I2S audio output is for driving a speaker.
Still no article on Sipeed wiki.
Checked current state of open source toolchain yosys/apicula - many components like BRAM are not yet supported. 
Sipeed Tang Nano 20K FPGA board is now available on aliexpress
From Sipeed twitter:
20K Luts, 64Mbits SDRAM, breadboard friendly, able to run #RISCV and #Linux, also play Retro Games!
All of those in 5.5x2.5cm, $25!
Aliexpress link
There is an $38 option with two joysticks for retro console
8MB SDRAM is embedded into FPGA chip, and has 32bit interface. Should be fast enough.
Chip is powerful enough to run linux on RiscV softcore.
Interesting project - NESTang, which runs on this board, by Nand2mario - FPGA based NES emulator
Price in EUR is 30. VAT included?
Not sure if available pins are enough for theremin.
Actually, there is on-board chip BL616 - RiscV SoC (320MHz, 532KB SRAM, 128KB ROM) which is used to provide high speed USB interface for JTAG and UART.
Probably, it's possible to implement USB MIDI or even audio interface by changing of BL616 firmware.
In addition to 27MHz oscillator, there is an external PLL chip with 25MHz xtal. Any idea how to utilize it? More flexible clock range?
Embedded WS2812 led strip drive is probably useless, but... What about tuner on LED strips?
On-board SD card has 4-bit interface - which is good...
On-board I2S DAC MAX98357 for direct connection of speaker. 3.2W 4 Ohm 5V, should be about 2W with 3.3V. Is it useful?
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