Low Frequency Expansion Board
Posted: Fri Mar 01, 2013 11:47 am
From previous discussions on the Kickstarter site, there is an interest extending the low end of the bladeRF board to lower receive frequencies. A group of the Society of Amateur Radio Astronomers (SARA) has been working on this concept for some time. The SARA team has a project Radio Astronomy Software Defined Radio (RASDR) which is using an Evaluation board from Lime as well as a CPLD/FX2 evaluation board from Lattice to prove the concept. Our objective is to have the capability of receiving signals from near zero (0.015 MHz) to the upper maximum of the Lime Chip (3.8 Ghz).
The RASDR expansion board would be configured as follows:
Low RF in -> Amplifier -> 400 MHz LPF -> Mixer -> 800 MHz BPF -> Lime Chip Input
The LO for the mixer is supplied by the transmit side of the Lime Chip. The LO is set to produce a 800 MHz input frequency to the Lime chip receiver input. The transmit output is optioned to produce a +6 dBM CW frequency for the LO. The frequency of the LO is variable under software control.
The minimum receive frequency is only limited by the passband of the amplifier. Our objective is to pass frequencies as low as 0.015 MHz. However due to limitations of our test equipment, we have only tested it at a minimum of 100 KHz (0.1MHz). One might argue that the lowest input frequency is limited by the minimum LPF on the lime chip. This isnt true if the application uses digital processing to perform further filering on the digital stream. For example, using the lowest value LPF 0.75 MHz on the Lime chip, a spectrum from 0.015 to 1.515 MHz could be sampled. Further processing on the digital stream could provide narrow filters within this spectrum.
The issue needing early attntion is the following:
1. The ability to use Input 3 on the Lime chip for this application. Presently, the schematic for the bladeRF shows Input 3 terminated in a resistive network. This needs to be brought out to allow a balanced input on the expansion connector. Without this change, the user would need to reconfigure coaxial cables for input 1 or 2 to be able to use the low frequency input.
Paul Oxley
The RASDR expansion board would be configured as follows:
Low RF in -> Amplifier -> 400 MHz LPF -> Mixer -> 800 MHz BPF -> Lime Chip Input
The LO for the mixer is supplied by the transmit side of the Lime Chip. The LO is set to produce a 800 MHz input frequency to the Lime chip receiver input. The transmit output is optioned to produce a +6 dBM CW frequency for the LO. The frequency of the LO is variable under software control.
The minimum receive frequency is only limited by the passband of the amplifier. Our objective is to pass frequencies as low as 0.015 MHz. However due to limitations of our test equipment, we have only tested it at a minimum of 100 KHz (0.1MHz). One might argue that the lowest input frequency is limited by the minimum LPF on the lime chip. This isnt true if the application uses digital processing to perform further filering on the digital stream. For example, using the lowest value LPF 0.75 MHz on the Lime chip, a spectrum from 0.015 to 1.515 MHz could be sampled. Further processing on the digital stream could provide narrow filters within this spectrum.
The issue needing early attntion is the following:
1. The ability to use Input 3 on the Lime chip for this application. Presently, the schematic for the bladeRF shows Input 3 terminated in a resistive network. This needs to be brought out to allow a balanced input on the expansion connector. Without this change, the user would need to reconfigure coaxial cables for input 1 or 2 to be able to use the low frequency input.
Paul Oxley