Radar Altimeter
Customer Case
In this RADAR application the customer has pulses coming in at 1660 Hz. For every pulse they need to capture 500 samples which, at that rate, will amount to 1.7 Megs of data every second. The experiment will be running for at least a few minutes. They would then like to log all the captured data to the PC host memory.
The final requirement is for the system to be on the PCI bus. By having the system on the PCI bus, the customer can stream the data directly to the PC’s memory, which is limited only by the amount of memory on the motherboard.
GaGe Case Solution
The application requires that they sample I and Q signals from their Radar. The customer would therefore need a system comprised of 2 simultaneous channels – one for the I signal and one for the Q signal. The product which meets their requirements is GaGe’s CompuScope 8500, as they must sample at 166 MSPS and be on the PCI bus. CompuScope 8500 is a single channel card, therefore, to be able to capture both I and Q signals they will need a multi-card system comprised of 2 cards providing 2 simultaneous channels.
They will also require an external clock modification to be able to capture at the specific sampling rate of 166 MSPS. Since their sample rate is greater than 100 MSPS (which is the limit of the PCI bus direct stream throughput), they will have to capture the data to the CompuScope 8500’s on-board buffer and then use the master move routine to burst the data across the bus at 100 MSPS.
The Pulse Repeat Frequency (PRF) is 1660 Hz. Each channel will have to capture 500 samples for each trigger pulse. We have to make sure that we have enough time to transfer each data set before the next pulse comes in so we will not miss any triggers.
We have calculated the maximum PRF that can be achieved with 2 channels of 500 samples each, using a conservative PCI 8-bit transfer rate of 100 MSPS:
MAX PRF= 1/ (channels * samples per channel * (1/transfer rate)
+ (1/( sample rate in samples per second * number of simultaneous samples))
= 1/ ( 2 * 500 * (1/100,000,000) + ((1/166,000,000)*500) )
= 75050.64 Hz
The customer’s requirement is well within this calculated maximum PRF.
Another way of looking at this problem is to see how long it would take to transfer the data in order to make sure that all the timing is right. The following is the sequence of events:
Trigger is received.
Specified number if points are captured in the on-board memory. This number can only be a multiple of 64, e.g. 512. At a sample rate of 166 MSPS, each sample is 6.01 ns apart. As such, 512 points will take: 512 * 6.01 uS = 3 uS
When capture is finished, read trigger address and do other housekeeping functions. This should normally take no more than a few I/O read commands but, in order to be conservative, let us assume 10 uS of housekeeping time.
Transfer data from Board 1 (I channel). CompuScope 8500 features a transfer rate of 100 MB/s to the PC’s DRAM. The time taken to transfer 500 points will be: Transfer Time = 500 / 100 MB = 5 uS
Transfer data from Board 2 (Q channel).
Re-arm the two boards. Again, this should take no more than a few I/O write commands. However, to keep this analysis very conservative let us assume 20 uS of housekeeping time.
Graphically, this is represented as follows:
It is fairly clear that the customer will have more than enough time to transfer the data and be ready for the next trigger – guaranteed!
GaGe Case Recommended Products
- 2 x CompuScope 8500, Master/Slave
- External Clock Option
Research & Development Application Request
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