Stand-Alone Receiver

In a typical SDR setup the FiFi-SDR assumes the role of the RF downconverter only. A small spectrum window around the desired center frequency is converted down to a zero IF baseband. This signal is available as a complex analog signal (I/Q) at the stereo jack of the FiFi-SDR. From there it can be connected to the sound card of a PC, where it will be digitized and processed with one of the numerous SDR software packages. The differentiating feature of the FiFi-SDR is its built-in sound card, which consists of a stereo ADC with 48 kHz, 96 kHz or 192 kHz sample rate, and a standard USB audio class implementation on the microcontroller. This replaces the analog connection to the PC, and a single USB cable provides power, a control/status channel, and a sound card for the digitized I/Q signal.

Doing all this puts the microcontroller in the FiFi-SDR nowhere near full load, and so the idea was born to let it do some DSP processing in addition to just sending plain I/Q data to the PC. As it turns out, the FiFi-SDR can run the complete digital signal processing part of a receiver for the common analog modulation schemes AM, FM, and SSB. The demodulated audio signal is available on a second emulated sound card via USB. The PC can use this second USB sound card to run applications like PSK or SSTV decoders, without the need for running an additional SDR software that would do the demodulation.

It is possible to make the demodulated audio signal available locally at the DAC output of the microcontroller. However, the associated pin is not brought out to any connector in the existing versions of the FiFi-SDR hardware. An early prototype exists, which has a headphone amplifier connected to the DAC as a proof of concept.

The following block diagram shows a high-level view of the signal flow:

Receiver block diagram

The USB carries three different functions:

  • Softrock interface is the control/command interface, and is compatible with the PE0FKO firmware/DLL to control Si570 based radios.
  • The first sound card FiFi-SDR Soundcard transports the raw digitized I/Q data coming from the stereo ADC as a stereo signal of 48/96/192 kHz sample rate in either 16 bits or 32 bits format.
  • The second sound card is named FiFi-SDR AM/FM/SSB Receiver, and carries the demodulated audio signal from the DSP routines inside the FiFi-SDR's microcontroller. This is a 16-bit mono signal with 12 kHz sample rate. Audio frequencies up to 6 kHz result in a quite satisfying AM broadcast reception.

This is work in progress, so don't expect too much...

How to set it up?

All you need to do is to update your FiFi-SDR to the latest firmware version (461 or later). The firmware runs on all available hardware versions, in particular it supports both the 1.x version with the 96 kHz ADC, as well as the 2.x version with the 192 kHz ADC.

It is possible to use the digital I/Q interface in parallel to the demodulated signal on the second sound card, but only 48 kHz sample rate for I/Q is supported in this case. If you set the I/Q sample rate to 96 kHz or 192 kHz, the demodulator will stop and mute its output.

Example: FLDIGI on a Linux Computer

Fldigi is a nice digital modem program that has full support for the FiFi-SDR in the Linux version. Fldigi can control a rig via Hamlib (among other methods). Recent versions of Hamlib support the FiFi-SDR, allowing Fldigi to directly control RX frequency and demodulation mode (LSB/USB). The RX audio level can be set via standard audio mixers of Linux, although experience has shown that the QasMixer application works best.

Fldigi setup:

Fldigi setup screen for sound card Fldigi setup screen for RIG control

These are the audio controls available via the QasMixer application:

QasMixer view of FiFi-SDR controls

You must be a great famous soccer player to get your own call sign:

RX example BPSK125

Windows Setup

The standard installation of Fldigi won't allow you to control the FiFi-SDR via the Hamlib rig control under Windows (None of the devices with USB interface is available in the Windows version). However, you can download a version of Fldigi software compiled specifically for the FiFi-SDR here: Fldigi-3.23.06-fifisdr_setup.exe

Last modified 21 months ago Last modified on Dec 31, 2015 4:24:03 PM

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