[Design View / Design Solution]
Using High-Speed Latched Comparators For Simultaneous Instant Frequency Measurement
An emerging solution called the monobit receiver makes it possible to sense simultaneous frequencies in real time.
DIGITAL PROCESSING A number of different processing techniques can be used to perform the frequency measurement. Two particularly strong techniques are fast Fourier transform (FFT) processing and delta-phase processing.
A significant advantage of the monobit receiver is its ability to process simultaneous signals. An FFT can be used to exploit this feature. Figure 2 shows the results for two tones separated by 90 MHz sampled at 10 Gsamples/s. Each peak is clearly discernible. However, there are some disadvantages.
First, the FFT algorithm is computationally intensive, especially for long data frames. Data must be processed as blocks or frames of data. It may be advantageous to overlap the data frames in time, but this increases the amount of processing required. But because the raw data consists of a single bit, a zero or a one, many operations are trivial multiplications by a 0 or 1.
Second, the frequency resolution of the FFT is determined by the sampling rate and number of samples, or Fsample/ no_of_points. For example, a 100 ns record, sampled at 10 Gsamples/s, will have 1000 points and a frequency bin spacing of 10 MHz.
Delta-phase processing, another technique to measure frequency, is based on a delta-phase calculation. This technique isn’t as computationally intensive, but it doesn’t perform well with simultaneous signals. The frequency is measured by determining df/dt.
Phase values are generated from the amplitude values by a number of steps. For example, bandpass filtering and sampling translates an FS/2 block of spectrum to the first Nyquist zone from 0 to FS/2. The data is multiplied by a complex multiplier to shift the spectrum from – FS/4 to + FS/4.
The data is converted to a complex representation with in-phase and quadrature components. The conventional way to generate in-phase and quadrature components from a real signal is to do a Hilbert transform. The complex multiplication eliminates the need to do it as a separate step.
The phase is calculated by taking the arctangent of the in-phase and quadrature components using a lookup table. Finally, delta phase is calculated by taking the phase difference between successive samples. The delta-phase values can be averaged over a period of time (such as an input pulse width) to improve the accuracy.
PROCESSING RESULTS Data was collected at a variety of sample rates, input frequencies, and signalto- noise ratios, and it was analyzed and processed using both FFT and delta-phase processing techniques.
Both the FFT processing method and the “Freq Estimate” yielded by the deltaphase processing method accurately measure the signal frequency. These results are summarized in Tables 2 and 3.
Monobit receivers are a promising emerging solution for measuring simultaneous frequencies of high-speed pulses in real time. The critical elements of a monobit receiver system are the highspeed latched comparator (single-bit ADC), the demultiplexer to interface the highspeed digital data to commercially available FPGAs, and advanced DSP algorithms to extract the frequency and phase information.
REFERENCES 1Tsui, James B. Y. " Two signal monobit electronic warfare receiver." US Patent 5793323. 11 Aug. 1998.
MICHAEL GRODEN, VP of digital Technology at lnX corp., holds a BSee from cornell University.
FRANCIS HO, senior director of business development at inphi corp, received his Phd in physics from Stanford University and his BS in physics from caltech.
Please refresh the page if you have trouble reading this text.
Search Electronic Design
Email Newsletter
Sponsored By:
Electronic Design UPDATE provides readers with late-breaking news, opinions from industry experts, and timely technology stories. It's a unique opportunity to get your product message in front of engineers, engineering managers, and corporate managers while they're reading about critical information online.
Reprints
