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sinantasdoken
Joined: 31 Oct 2010
Posts: 4
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 Posted: Tue Nov 02, 2010 4:40 am Post subject: Framed-mode Triggering |
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Hi there,
I tried to acquire data in framed-mode with software triggers but i observe that samples are buffered somewhere and next trigger gets those stored samples not respecting the current real-time input data. What is the mechanism here in framed mode and how can i acquire this discontinuous set of data ignoring all others?
Morely, in the snap code i observe module.framed(ch, some_size / 256). Where does this 256 come from?
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jhenderson
Site Admin
Joined: 07 Mar 2006
Posts: 2267
Location: So. Cal. USA
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| Posted: Tue Nov 02, 2010 9:07 am Post subject: |
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From the LTC2255 data sheet, the A/D uses a 6-stage pipeline. So, I believe you're seeing the residual pipeline effects at the beginning of each frame. Unfortunately, the firmware does not handle this anomoly, so your software will have to ignore the first six samples of each frame.
The firmware is designed such that frames must consist of an integer number of 256 samples. That is why the example code divides the frame count by 256. The implied restriction is that frames must contain an integer multiple of 256 samples. |
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sinantasdoken
Joined: 31 Oct 2010
Posts: 4
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| Posted: Tue Nov 02, 2010 12:08 pm Post subject: |
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Actually it is not six samples in each frame. When I trigger for a new frame, the new frame does not respond to current state of the input but seems like i read some buffered input. Maybe it has something to do with my selection of frame size, packet size?
I'm making use of 4 DDC channels (by enabling channels 4-7) and acquire IQ data from each channel (all channels provide interleaved IQ data data in sequential blocks of 1I, IQ, 2I, 2Q, 3I, 3Q, 4I, 4Q, maybe this has something to do with 5016 configuration, so packets from different channels - numbers of which are obtained by examining the packet headers- provide IQ data of all channels - or physical ddc outputs). 5016s downsample by a factor of say 32 (if it has something to do with read size selection). Does triggering correspond to adc outputs and or ddc outputs? Rather than frame size do i need to make use of ddcframesize, if so how? Could you please provide me with some example set of parameters for frame_size in module.framed(ch, frame_size) calls assuming at each software trigger i want interleaved 1024 I,Q samples for each input (a total of 4096 complex numbers). |
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jhenderson
Site Admin
Joined: 07 Mar 2006
Posts: 2267
Location: So. Cal. USA
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| Posted: Tue Nov 02, 2010 12:33 pm Post subject: |
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| Quote: | | Actually it is not six samples in each frame. When I trigger for a new frame, the new frame does not respond to current state of the input but seems like i read some buffered input. Maybe it has something to do with my selection of frame size, packet size? |
The frame size specifies the number of samples to be acquired from each A/D per trigger event. Normally, the packet size should be set to be an integer multiple of the frame size, so that an even number of frames will fit into each packet.
| Quote: | | I'm making use of 4 DDC channels (by enabling channels 4-7) and acquire IQ data from each channel (all channels provide interleaved IQ data data in sequential blocks of 1I, IQ, 2I, 2Q, 3I, 3Q, 4I, 4Q, maybe this has something to do with 5016 configuration, so packets from different channels - numbers of which are obtained by examining the packet headers- provide IQ data of all channels - or physical ddc outputs). |
Each enabled DDC channel can process data from any or all A/Ds. The configuration of the 5016 created using the GC5016.exe applet and loaded into each DDC at runtime controls the actual format of the data produced at the output of each DDC.
| Quote: | | 5016s downsample by a factor of say 32 (if it has something to do with read size selection). Does triggering correspond to adc outputs and or ddc outputs? |
Triggering controls only whether raw A/D samples are preserved and processed by the downstream logic/DDCs. If triggering is false, A/D samples are discarded. If true, they are processed. Any DDC processing is independent of the trigger logic.
| Quote: | | Rather than frame size do i need to make use of ddcframesize, if so how? Could you please provide me with some example set of parameters for frame_size in module.framed(ch, frame_size) calls assuming at each software trigger i want interleaved 1024 I,Q samples for each input (a total of 4096 complex numbers). |
Unfortunately, we do not have any examples beyond those already present in the kit. But, to produce 1024, 16-bit interleaved IQ samples, you would presumably need to capture 1024 * decimation rate samples from each A/D, where decimation rate is a function of the specific DDC configuration you create using GC5016.exe. |
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