SM API
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I/Q sweep list measurements perform frequency hopping I/Q captures at a list of preconfigured frequencies and capture sizes. Captures can be queued to sustain > 8000 frequency hops per second.
For a list of all examples, please see the examples/ folder in the SDK.
I/Q sweep lists are finite length I/Q acquisitions across a series of frequencies. Lists of up to 1200 frequencies can be provided. At each frequency, the reference level and number of samples to be collected must be configured. One measurement/list is referred to as a “sweep” and iterates through all configured frequency steps. Several sweeps can be queued to maintain maximum throughput.
I/Q sweep lists are advantageous when needing to acquire a discrete number of I/Q samples at several different frequencies. I/Q samples are collected at the devices native sample rate, 50MS/s for the USB SM devices and 200MS/s for the networked SM devices. The absolute fastest the SM device can switch frequencies is 120us. When I/Q capture amounts are small at each frequency, 120us frequency switch times can be achieved for a maximum of 8333.33 frequencies per second.
At each frequency, a timestamp is provided indicating the nanoseconds since epoch for the fist I/Q sample at that frequency. If the internal GPS is locked, this time is GPS time, If GPS is not locked, system time is provided. Regardless of GPS lock, relative timings between timestamps are highly accurate through use of internal device counters.
A list of 3 frequencies is provided, 1GHz, 2GHz, and 3GHz. At each frequency 1000 I/Q samples are configured to be collected. Once configured a sweep can be performed which captures I/Q samples at the 3 frequencies, for a total of 3000 samples. If desired, N sweeps can be queued to be performed back-to-back, resulting in N * 3000 samples to be collected. By queuing the sweeps, blind time between sweeps is reduced or eliminated, improving probability of intercept and overall measurement speed.
While the user can specify an arbitrary number of samples at each frequency, the SM device is internally limited to multiples of 2048 samples. For this reason, it is optimum to round up to the next multiple of 2048, which will not affect acquisition speed and reduce the number samples discarded.
Maximum sweep speed occurs when at most N
samples are requested at each frequency. For the USB SM devices, N is 2048 samples, and for the networked devices, N is 6144 samples. When <= N samples are requested, the device will step at the maximum rate of 8333.33 frequencies per second. This equates to ~333GHz of spectrum coverage per second for the USB SM devices and ~1.333THz of spectrum coverage per second for the networked SM devices.
One use case where I/Q sweep lists are preferred to I/Q streaming for single frequency measurements is when you know in advance how many I/Q samples you want to collect at that frequency. Using I/Q sweep lists to acquire these samples has less overhead than using I/Q streaming. Starting and stopping the I/Q stream can take ~30ms, where as the overhead associated with performing a single I/Q sweep list acquisition is 1-5ms.
See I/Q Acquisiton for more information.