Sunday, 16 March 2014

BladeRF, the transverter and HF TRX; it's been there all the time!

BladeRF, the XB-200 transverter and HF TRX; it's been there all the time!

The BladeRF and now its XB-200 transverter are very neat pieces of gear. Now the big "but" or "however".

Many people have been wanting to use the BladeRF on HF and were waiting for the transverter to be delivered in anticipation of it covering HF. However, the original range of the transverter was 30 to 300 MHz, with no apparent coverage. When the final design and photos were released, coverage was 60 kHz to 300 MHz, which no doubt pleased many people.

I went though the schematics trying to find the modifications to cover HF, but they were not apparent, a point I raised in the Nuand forum. I received a reply from Nuand to say that the HF access was not very obvious and went to the ADC/DAC. This mystified me for a few days until it finally clicked on how HF is done, which is exactly what they way they said. However HF TRX is virtually independent of the transverter and could have been used with just the BladeRF with some extra circuitry, that is still probably needed.

HF TRX is possible with the BladeRF because of the design of the main TRX chip, the LMS6002D. Its block diagram is shown below:



The chip is a superhet TRX that uses a local oscillator to mix 300 -3000 MHz down to baseband then through the ADC/DAC. The main signal path for the receiver is in at pins RXIN1-3, mix down to baseband, through the ADC and out via RX_IQ_SEL and RXD(11:0) 12 bit bus for digital processing. The transverter adds to this by up-mixing 300 - 300 MHz to a 948-1218 MHZ and through the normal signal path. All reasonable, but what about HF?

HF TRX is achieved by bypassing the most of the LMS6002D circuits and going straight to the ADC or DAC via pins RXOUTI/Q and TXINI/Q. As such the RX samples directly via the ADC at for HF. These pins are accessible on the BladeRF. The transverter just adds a balun and connector, shown on the last page of the transverter schematics and not mentioned in the block diagram.


The PWR_HDR6/PWR HDR6  connectors J5 and J6 on the transverter board connect to PWR_HDR6/PWR HDR6 connectors J60 and J61 on the BladeRF.




There is nothing wrong with this approach as many SDRs are based on inputs/outputs directly to ADC/DACs. It how the new Red Papaya (http://redpitaya.com/ and see my other posts)(14 bit 125 Msps ADC/DAC) could be used as a very capable HF TRX. However, for the BladeRF on HF there are a few consequences.
First, the software to drive the HF part of the BladeRF will be different to that for the 30 - 3000 MHz, assuming the transverter can be essentially invisible via firmware.

Second, the HF parts may need extra circuitry such as a low-pass filter and a RF amplifier.

Third, the actual coverage of the ADC/DAC would seem to be limited a maximum of 20 MHz, half the sampling rate of the ADC/DAC of 40 Msps. It is possible for the transverter to go down to 20 MHz but there is a high-pass filter and the local oscillator/images may cause issues.

In conclusion, none of this is intended to be a criticism of the BladeRF and its transverter; I look forward to receiving my transverter and am already impressed with BladeRF's capabilities. It is intended to point out that HF coverage is done quite differently to the main 30 - 3000 MHz coverage, necessitating a few changes in hardware and software.
However, while the Red Pitaya is primarily for software-defined instrumentation, it may be a better hardware platform than the BladeRF for a HF SDR TRX, particularly with its much higher sampling rate, 125 vs 40 Msps? Each to their own?

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