ASSA Radio Astronomy Group

WiNRADiO G305e Operation

The incoming signal is filtered and amplified (switchable), then fed to a mixer. Here it is mixed with the first local oscillator, which is performed by a direct digital synthesiser (DDS) with a phase locked loop (PPL). The resulting 109.65MHz intermediate frequency is filtered using a 4-pole crystal filter with an IF bandwidth of 15kHz, and then amplified. The second mixer again uses a DDS with a PLL to mix the 109.65MHz signal down to the last intermediate frequency, which is 12kHz. The second IF bandwidth is adjusted in software. The Professional Demodulator option allows the second IF bandwidth to be continuously variable from 100Hz to 15kHz. Both DDS circuits derive their reference frequency from an internal 20MHz reference oscillator. Frequency stability is optimised by operating each SDR in small temperature controlled enclosures, heated from underneath by non-inductive resistive elements and cooled from above by peltier thermoelectric modules.

Automatic gain control (AGC) can be performed in the first IF stage, based on the level of the second IF output. As the IF bandwidth of the first IF stage is 15kHz, the AGC action is delayed until the dynamic range of the first IF stage is fully utilised – this is in order not to cause desensitisation of the SDR in the presence of neigbouring strong signals, falling within the 15kHz IF bandwidth. The resulting variation in audio output is then compensated for in software, using Audio AGC in the software demodulator. In normal radio telescope operation, both the AGC modes are disabled and the gain of the IF amplifier is set for optimum sensitivity or signal to noise ratio. The SDR's inbuilt 12dB preamplifier and 18dB attenuator are both switchable to enhance front end performance.

The demodulated output is digitised and sent via USB to the computer for further DSP by either:

the WinRadio Advanced Digital Suite software which incorporates an Audio Oscilloscope for voltage/time displays and an Audio Spectrum Analyser for frequency/time displays.

• A DC-10kHz(max.) fast fourier transform display and several types of waterfall displays are available using the internal DSP configuration.
• A DC-20kHz(max.) fast fourier transform display and several types of waterfall displays are available using the computer's integrated sound card configuration. Recording of data to sound (.wav) files is possible using manually set, conditional signal or squelch controlled recorders.

the WinRadio Digital Bridge Virtual Sound Card software makes it possible to interface the SDR's IF output directly to third party decoding/DSP software such as Winrad or Linrad.

• Interface the SDR's AF output directly to third party DSP software such as Spectran.

• Audio generation on the computer's integrated sound card for additional DSP and/or monitoring.