AMX Digital

[Carl-NC]

This discussion should really be at the concept level.

Regardless of hardware or software demodulation, 12 bits ain't gonna cut it if we want cutting-edge performance. For hardware demods I would suggest 16-18 bits and it really needs to be simultaneous sampling in order to keep the data time-aligned, especially if GB is achieved after the demods. Simultaneous sampling ADCs are mostly limited to 8 channels unless you consider ultrasound chips, which I would not recommend. Besides, 8 analog channels is about the most you can practically pack on a PCB. At FTP I'm mostly using the ADS131E08 24b/8ch ADC @ 1kHz. However, it's a little rigid in it's sample rate requirement and as you sample higher it loses resolution.

For direct sampling, it needs to be 24 bits. Nothing less. A fast SAR like the AD7766.

Either method results in a very high serial data rate. In both cases you probably want to DMA the data into memory and even do some real-time data summation before presenting the readings to the DSP task. This will slow down the task loop to a reasonable rate and minimize overhead. Speaking of task loops, I strongly recommend using FreeRTOS to manage the firmware.

So is it analog demods or direct sampling?

Whether you choose analog demods of direct sampling may depend on how the TX is designed. Tony suggested a chirp method. This can be done using a linear sweep (what chirp really means) or using a few discrete steps. For the linear sweep, I don't know what the RX scheme would look like, Tony would have to elaborate. Discrete steps could use either analog demods of direct sampling. It should be noted that analog demods have the advantage of doing real-time integration, whereby you are integrating the RX signal as long as the demod switch is closed. This helps with SNR and also with implementing GB. Direct sampling cannot do this, you get an instantaneous point and that's all (think about this for GB). All-in-all, analog demods will probably always outperform direct sampling but uses more board space, consumes more power, costs more (maybe), requires more timers, is limited to 8 channels, and is less flexible.

Since I can't envision off-hand how to accomplish a linear chirp system, I would propose a pulse train with discrete selected pulse widths. The more different pulse widths that are used, the less depth you get via processing gain. So I would limit it to 2 or 3 different widths at the most. Probably start with 2. For maximum flexibility and probably faster prototyping I would use direct sampling and only consider changing to analog demods if performance demands it.

[Willy Bayot]

Direct Sampling

Advantages:

• Simpler receiver circuits
• more flexible signal windows organisation

ADC calculations
integrated 12-bit ADC

• 0-3.3V --> resolution 0.8mV

External 16-bit ADC

• 0-5V --> resolution 76µV
• -5/+5V --> resolution 152µV

External 24-bit ADC

• 0-5V --> resolution 0.3µV
• -5/+5V --> resolution 0.6µV

Our own experiences with Direct Sampling:

• Integrated 1.5Mbps 12-bit ADC under DMA support
• Digital integration (x1500)
• One net sample result every milliseconds

did not give acceptable detection sensitivities with the maximum analog gain we could apply on the signal before saturation.

The fastest External ADC's required for direct sampling have parallel interfaces.
Taking 16 or 24 digital pins out of a micro-processor is usually excluded.
Thus, we are back to the serial interface (fast SPI with DMA support)

External ADC's come with or without integrated multiplexer.
Since we certainly need to capture signal data from several analog sources, we have the choice between:

• Mutiplexed ADC or
• one Single input ADC with external multiplexer or
• Several single input ADC.

Fast external ADC's are expensive, so, we are left with integrated or external Multiplexers feeding a single ADC.

Analog Integrated ADC sampling

Requires less speed but still requires a resolution of at least 16 bits with a supplementary digital integration and filtering
More ADC's of that type are available at an acceptable price.

[Willy Bayot]

This is where digital modulles considerations and discussions could be started.