Coherent Radar Measurements: Unlocking X-Band Signal Fidelity with the Gage Sabre
Coherent Radar Measurements Using a Gage Sabre Digitizer
A customer is conducting coherent radar measurements, including Synthetic Aperture Radar (SAR), and is focused on monitoring narrowband radar signals in the X-Band (8 to 12 GHz).
To acquire these radar signals, the customer uses an RF down-converter to mix the X-band signals to lower frequencies. The down-converter mixes (multiplies) the input signal with a Local Oscillator (LO) signal, which is a sinewave at a tunable frequency. The mixed down signal is then passed through a 50 MHz low-pass filter, resulting in a final output signal with 50 MHz analog bandwidth.
The down-converter operates in Zero IF mode, meaning the reduced output carrier frequency—called the Intermediate Frequency (IF)—is 0. Additionally, the signal is mixed with both sine and cosine LO signals, which are 90° out of phase. These unfiltered outputs are referred to as the In-Phase (I) and Quadrature (Q) signals. Together, these two signals allow for the construction of a frequency spectrum ranging from -50 MHz to +50 MHz, effectively doubling the down-converter’s output bandwidth from 50 MHz to 100 MHz.
To digitize the I and Q signals, the customer needs to sample at a rate of at least twice the analog bandwidth (i.e., 100 MS/s). The customer has chosen to sample at 250 MS/s to account for the non-ideal roll-off of the 50 MHz filters within the down-converter.
Synchronization and Coherence Requirements
For coherent radar signal acquisition, all down-converter and digitizer oscillators must be disciplined by a common 10 MHz reference signal. This ensures that the frequency of each oscillator is a fixed multiple of the 10 MHz reference. In some cases, the radar signal itself may be disciplined by the same 10 MHz reference, though this is not possible for unknown hostile radar signals.
In this application, the customer needs to monitor radar signals from two separate radar receivers. To achieve this, both down-converters’ LO frequencies must be disciplined by the same 10 MHz reference signal, which is sourced from the customer’s GPS device.
Choice of Equipment: The Gage Sabre
To acquire the four signals (I and Q from each of the two down-converters), the customer has chosen the Gage Sabre digitizer. The Sabre provides four simultaneous input channels, 16-bit resolution, and a maximum sampling rate of 250 MS/s. High simultaneity is ensured because all four channels are digitized by separate Analog-to-Digital Conversion (ADC) chips that share a common sampling clock, delivered to each ADC via equal-length circuit traces. This architecture results in only 5 picoseconds of jitter between channels, which is negligible compared to the 4 nanosecond sampling interval, easily meeting the customer’s needs.
As required, the Gage Sabre also accepts the common 10 MHz reference signal to discipline its 250 MS/s sampling oscillator. A Phase-Locked Loop (PLL) ensures the oscillator runs exactly at 25 times the 10 MHz reference frequency.
Data Acquisition and Storage
The Gage Sabre features 4 GigaSamples of on-board memory, enabling continuous acquisition of up to 4 seconds of waveform data across all four channels. The 2 GigaBytes/second data acquisition rate can be streamed to the PCIe bus and sent to a high-speed GPU or storage device. The customer opted to stream the data to an 800 GB RAM buffer on their PC, which is fast enough to handle the stream. This effectively extended the storage time to 400 seconds, far exceeding the customer’s requirement.
Stream-to-RAM acquisitions were performed using Gage’s stand-alone software, with no custom programming required. Data were saved in simple binary files, which the customer then accessed from MATLAB for coherent radar signal analysis.
