Motivation
This PCB is part of my Bark Scanning Filter project. You can find more information on the Bark Scanning Filter project page
This PCB holds the first and second Bark filter. The low pass with 80Hz cut off frequency and the first band pass with 150Hz center frequency. It holds the associated VCA's and the associated precision full wave rectifiers as well. For understanding how everything works together please have a look at the block diagram on the project page.
Specs and features
- Low pass filter 80Hz cutoff frequency
- Bandpass filter 150Hz center frequency
- Log VCA for each filter
- Envelope follower for each filter
- Quiescent current around 30mA each rail
Implementation
Schematic
Bark Scanning Filter: PCB 110-LF schematic 01
Bark Scanning Filter: PCB 110-LF schematic 02
Description:
On page one you can see Bark Filter one, the low pass with 80Hz cut off frequency, the precision full wave rectifier for the envelope and the VCA. On page two you can see Bark Filter two the band pass with 150Hz center frequency, the precision full wave rectifier for the envelope and the VCA.
Calibration
I highly recommend to calibrate all filters before putting everything together. It is much easier this way.
Bark Scanning Filter: Filter calibration
First power the filter up. It should draw a quiescent current around 30mA each rail. You can see how I made the power connection on the lower right corner in the picture above.
- Filter output amplitude
- Apply a sine of the nominal filter frequency with 0.6Vpp to the input (odd or even).
- Measure the filter output with an oscilloscope (odd or even).
- Adjust trimmer TR_1(TR_4) for the output to 5Vpp.
- Envelope follower output voltage
- Set the jumper to pre VCA position (odd or even).
- Measure the ENV output with voltage meter or scope (odd or even).
- Adjust trimmer TR_2(TR_5) for the output to 5V DC.
- OTA (VCA) offset)
- Connect a low current LED with a 5k6 resistor from GND to Iabc input (odd or even). Watch the polarity.
- The LED should lit up brightly.
- Watch the signal from the VCA output with an oscilloscope (odd or even).
- The signal should be the same as at the filter output (odd or even).
- Remove the input signal.
- Adjust trimmer TR_3(TR_6) for the output to 0V offset.
- Modulation test
- Apply a sine of the nominal filter frequency with 0.6VPP to the input (odd or even).
- Connect a low current LED with a 5k6 resistor from Iabc input to Iabc output (odd or even). Watch the polarity.
- The LED should dimly lit.
- Watch the signal from the VCA output with an oscilloscope (odd or even).
- Apply a slowly moving voltage at the scanner input (odd or even).
- The amplitude of the output voltage on the VCA should follow the slow moving signal (odd or even).
Building hints
- None
Special parts
- Replace the CA3280 with the now available AS3280.
Download
Bark Scanning Filter PCB LF documentation downloadBark Scanning Filter PCB LF Gerber files download
