Sunday, February 27, 2011

Low Cost Solution for Measuring Frequency Hopping

Frequency hopping is a method of switching or "hopping" a carrier signal among many frequency channels, using a pseudo-random sequence known to both transmitter and receiver. Frequency hopping is more widely known in wireless communication, but it is also used in radar where it is sometimes referred to as an agile signal or agile carrier. In wireless communication frequency hopping is a way to lower signal interference and share a small section of bandwidth. It also serves as a method to protect a signal against ease-droppers. In radar frequency hopping is used to guard against jamming and deception or as a way to reduce signal interference. 
To measure the frequency and modulation content on a comm or radar signal the signal analyzer or similiar instrument is typically the solution of choice. But lets say you wanted to measure or capture a long record of frequency hop data to check long term accuracy or validate an algorithm or verify frequency channel transitions, how would you do it? You could use a high speed digitizer or mixer digitizer combo with a lot of memory. You then would have to post process the digitized data to create an easy to read frequency plot. 
An easier low cost way to do it is with a modern universal counter that has gap-free measurement capabilities. A counter works by making high accuracy and high resolution timing measurements between signal edge events. A counter with gap-free measurement capability can make timing measurements without any rests or gaps in between up to a certain sample rate. For instance the Agilent 53230A counter has a gap-free sample rate of 1 MS/s. What that means is if you have a carrier at 2.4 GHz the 53230A will pre-scale or divide down the signal to fit the sampling rate. You can than use the pre-scale value and the resulting timing measurement to calculate frequency on a continuous bases. Since a counter is just making edge event timing measurements and not digitizing you can make a lot of high resolution frequency measurements without using much memory at all.
As an example, below are two plots of 32,000 frequency measurements made on a frequency hopping carrier signal using the 53230A. The plot has frequency on the Y axis and time on the X axis (click on the figure to enlarge).

The frequency range of the signal is about 2.48 to 2.58 GHz. The top view shows all of the measurements and the bottom plot is a zoomed in picture of about 850 measurement points at one of the carrier hop levels. The bottom plot shows that even though we are capturing a large time slice of frequency hop measurements we still have ample measurement resolution to zoom in and see noise on the carrier. 
The 32,000 measurements made at a 1 MS/s rate only took up about 3% of the 53230A's 1 million measurement reading memory. Imagine the price tag on a digitizing based solution that could capture that much high resolution frequency data! Besides just capturing long periods of frequency hopping data the gap-free measurement capability found in modern universal counters also can be used as a low cost modulation domain analysis and close-in noise analysis tool on carrier signals.