technical description
The objective of this project is to convert the FT-991A to a real SDR, providing I/Q output, centred in the radio's IF frequency, taking advantage of the radio's pre-filtering, amplifiers, attenuators, Band-pass and Low pass filters, along providing a good and steady 2.4 MHz-wide bandwidth which is enough to cover most of HF bands and good coverage of other bands.
Panadapter Block Diagram and radio connections
In my implementation, I choose to actually lodge the SDR inside the radio, and expose it to the computer using the built-in USB Hub. That requires to replace the USB2512BI with a USB2514BI USB hub, in order to get another extra USB port to connect the SDR. Read more in Installation.
USB Enumeration of the Radio containing the Panadapter
The board has provisions to only tap the signal with both a user control (a GPIO signal from the SDR) and when the radio is not transmitting, using the RX9 signal from the Radio. You can work around it and apply a 2 ~ 5 V to the RX_EN line, and 9V to the RX9 line.
The IF signal is tapped on the Scope Unit output (right after the RF switch prior to the roofing filters in non-A model), just before the Roofing filters (which would limit the bandwidth to a few kHz):
Radio Block Diagram and Tap point
The board receives the IF signal from the radio. At the default state, the initial RF switch keeps the signal isolated, not tapping the signal. Upon receiving both RX9 and the GPIO_29 signal from the SDR, the switch flips to open and then conducts the IF signal forward in the board.
In the next step, another RF switch will route the IF signal either straight out to the next step (default state) or route it through a 69.450 MHz 3-pole Chebyshev Band-Pass filter, 3 MHz wide, which will filter out any strong signals off the SDR receiving band (and causing image issues). This is controlled by the SDR's GPIO_30 signal and the default state is to pass through.
3 MHz-wide Band-Pass Filter centred at 69.450 MHz Band-Pass filter
In this last step, the signal will either enter a Low Noise Amplifier (default state) or else forwarded unamplified to the SDR OUT port. This RF switch is commanded by the SDR's GPIO_31 line and the default state is to pass through the LPF/Amplifier.