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Spread Spectrum Resources Directory

Be sure to visit our Spread Spectrum Resources Directory for information on a broad range of spread spectrum topics, products, and services. There you will find tutorials, white papers, schematic diagrams, development tools, OEM products, and more!

We also invite you to check out our line of Spread Spectrum Generators.


What are Spread Spectrum Generators?

"What exactly is a Spread Spectrum Generator, and why have I never heard of such an instrument?"

These questions aren't new to New Wave Instruments. After all, we invented the spread spectrum generator! Well, OK, we weren't the first company to actually build one, but we were the first to introduce them as standard off-the-shelf instruments.

A Brief History of DSSS Spread Spectrum Generators

One of our founders, Sanford Larsen, was one of the first to design spread spectrum generators. He did so in the early 1980s while working for Signal Science Inc., which was a spin-off of ESL Inc., which was a spin-off of TRW. He designed both the DS-1 and DS-2 direct-sequence spread spectrum (DSSS) generators, which were used by the National Security Agency, and other shadowy organizations, in their communications intelligence (COMINT) and reconnaissance operations.

The government paid a handsome price for those two generators -- over half a million dollars, when adjusted for inflation! Larsen, being an industrious young man, decided to go out on his own to "mass produce" these instruments and sell them at a fraction of that cost. This caused quite a stir in the defense electronics circle, as contractors quickly lost business to the fledgling New Wave Instruments.

OK, so much for their history. But what exactly does a spread spectrum generator do, what is it used for, and how is it used? To answer these questions, we'll assume you already know what direct sequence and spread spectrum are. If you don't, you might want to take a glance at our Resources page before continuing. You'll find some handy tutorials there, and a spread spectrum glossary that's ideal for brushing up.

DSSS Generators in Radio Transmitters and Receivers

Conceptually, spread spectrum generators are rather simple devices, having been aptly referred to as "glorified shift registers." In short, they generate linear recursive sequences (LRS) using a linear-feedback shift register (LFSR). These sequences are often referred to as pseudonoise, or PN, codes because of their noise-like properties. But, while the sequences do appear random for a while, they will eventually repeat in a periodic fashion.

In a direct-sequence spread spectrum transmitter, the digital data stream to be transmitted is multiplied by a PN sequence running at a much higher clock rate, or chip rate, than the baud rate of the data. This results in the frequency spectrum of the data being spread by a factor equivalent to the chip-rate/baud-rate ratio. This ratio is called the processing gain, and is a measure of the interference rejection provided by the system.

Spread spectrum generators generally drive a double-balanced modulator, which upconverts the sequence's frequency spectrum from baseband to some RF carrier frequency. In general, the RF carrier itself may or may not contain data information. But in direct sequence applications, the carrier is a pure sine wave containing no data. Rather, the data stream is modulo-2 added, or exclusive ORed, directly with the PN sequence prior to upconversion. For this very reason, spread spectrum generators usually provide a data input port, so the XOR operation can be performed internal to the instrument. The use of this port is optional, and is left open in non-direct-sequence applications.

So, in direct sequence applications, the data stream to be transmitted is input to the generator, and the output of the generator drives a double-balance mixer to upconvert the spread signal to some specified RF frequency. The net effect is that the data stream has been multiplied by the generator's PN sequence, and the result has been subsequently multiplied by a sine wave at the carrier frequency. It's as simple as that.

But the role of the spread spectrum generator doesn't stop there. At the receiver, the transmitted signal requires despreading. This is accomplished through a process that is essentially the inverse of what was performed at the transmitter. The received signal is fed to a mixer, whose other input is driven by a second spread spectrum generator, outputting the same PN sequence as that of the transmitter's. Then, a second mixer and local oscillator are used to downconvert the signal back to baseband. So, at the receiver, the spread signal has again been multiplied by the PN sequence, and has again been multiplied by a sine wave at the carrier frequency.

If you study the whole process carefully, you'll discover that multiplying the data stream by the same PN sequence twice, once at the transmitter and once at the receiver, is essentially equivalent to multiplying it by a constant 1. This is because the PN sequences driving the inputs of the modulator and mixer are both either +1 or -1 at any given time, representing logic levels high and low respectively. Of course, either of these values multiplied by itself will result in 1. Therefore, the two PN sequences cancel each other and thus have no net effect on the data stream.

For the sake of clarity we should note that, while multiplying a data stream by a PN sequence twice will result in the original data, the same cannot be said of multiplying the signal by a sine wave twice. This is easy to see, as a sine wave multiplied by itself does not yield 1, or any particular constant. But it can be shown that, through proper low-pass filtering at the receiver to eliminate spectral images, effects of multiplying the data signal by both the carrier and local oscillator sine waves can be eliminated, and the original data stream will be recovered.

Features of Direct-Sequence Spread Spectrum Generators

While spread spectrum generators might be called "glorified shift registers," it is important to explore the "glorified" part, as this is what makes these generators particularly useful. Spread spectrum generators often have a host of features that make them useful in a laboratory setting.

The ability to easily set the size of the shift register (number of stages), and the desired feedback taps, are probably the two most important features of any spread spectrum generator. The user generally needs to be able to do this, not only to work with the particular code or codes he is interested in, but also to explore the effects of other, competing codes, as would be the case in CDMA and WCDMA applications.

An epoch strobe output is another essential feature of spread spectrum generators. The epoch port outputs a pulse each and every time the sequence begins to repeat. This pulse is useful in many ways, most notably for triggering a scope when monitoring the PN sequence.

Beyond these essential functions, there are often numerous other features provided by spread spectrum generators. These features generally offer either convenience, versatility, or application specific functionality, as will be detailed here:

Convenience Features of DSSS Generators

One useful feature found in many spread spectrum generators is the ability to momentarily turn off either the PN sequence, the data stream, or both with a quick press of a button. With this feature, one can easily explore the effects of spreading compared to not spreading, or compare the spreading code alone to that with the data included.

A less common feature is having a built-in clock. This is not as common simply because different applications require different clock specifications, such as frequency resolution and jitter. So the user often prefers to provide a clock source of his own choosing. Yet, an internal clock can be quite handy in situations where clock specifics don't matter.

Another less common feature is a built-in data source, usually referred to as pseudodata because of its periodic nature (as with the case of pseudonoise). The period of the pseudodata is usually kept short, 15 bits for example, simply to make it easily identifiable on a scope.

Versatility Features of DSSS Generators

In many applications, the most important feature is the ability to synchronize one generator with another. This importance is underscored by the example given above, where one generator is used in the transmitter, and another in the receiver. What wasn't mentioned in the example is the necessity of having the PN sequences from the two generators in perfect alignment. In end-user products, PN synchronization is performed by a dedicated acquisition and tracking circuit. But in the lab, no such circuit is available.

It is for this reason that virtually every spread spectrum generator comes equipped with timing ports (usually two) dedicated to maintaining synchronization between two generators, in what is called a master/slave configuration. It is important to note, too, that the better generators (which includes all of New Wave Instruments') are designed to maintain synchronization regardless of any setting changes you make at the instrument -- the feedback taps for example.

A companion feature is the ability to start, stop, or preset the generator (i.e. load the shift register with a specified initial fill) at the push of a button. Also useful is the ability to single-step the shift register using a manual-clock button, and observe the contents of the shift register on a user display. These are debugging features that make it a easy to confirm that generators connected in a master/slave configuration are operating as expected.

Finally, the versatility of a spread spectrum generator can be vastly broadened by making many or all its various settings remotely controllable. Doing so makes it possible to automate or customize a test setup. Remote control is usually provided through the use of either a GPIB interface, ideal for easy control, or through dedicated TTL inputs, ideal for high-speed precision control.

Application Specific Features of DSSS Generators

A spread spectrum generator may provide additional functionality to support specific applications. For example, a generator may support any combination of the following modes of operation:

  BPSK Modulation:   The default mode supported by all spread spectrum generators.
  QPSK Modulation A:   I & Q sequences derived from alternating chips from one PN sequence.
  QPSK Modulation B:   I & Q sequences from two independent shift registers.
  OQPSK Modulation:   QPSK Modulation, but with one sequence time-delayed by 1/2 chip.
  Gold Codes:   The modulo-2 addition of two PN sequences of equal length, each having programmable initial fill. Useful for CDMA and WCDMA applications.
  JPL Codes:   The modulo-2 addition of two PN sequences of differing lengths. Useful for quick acquisition applications.
  Syncopated Codes:   Gold or JPL codes, but with one PN sequence delayed by 1/2 chip.
  Burst Codes:   Output of a single PN burst, for radar and radio-location applications.
  Truncated Codes:   Any of the above codes, where the PN sequence is truncated short (or long) of its natural repetition length.
  FHSS:   A parallel output of the shift register's taps are provided for operation with a frequency synthesizer in frequency hopping applications.

It should be noted that all spread spectrum generators manufactured by New Wave Instruments are designed to make switching from one of these modes to most any another seamless. Some of our generators even allow switching between BPSK and QPSK without having to re-wire the modulator/mixer. This is a tremendously convenient and time saving feature that represents just one of the many well-conceived and implemented ideas commonly found in New Wave Instruments products.

Applications of Direct-Sequence Spread Spectrum Generators

Spread spectrum generators can be used in a variety of test and development applications, including the following:

  • Personal Communication Services (PCS)
  • Wireless LAN and WAN (WLAN, WWAN)
  • Spread Spectrum Radios and Modems
  • Wireless Smoke, Fire, and Burglar Alarms
  • Police Surveillance Equipment
  • Low-Probability-of-Intercept, Antijam Systems
  • Reconnaissance, Ranging, and Radar
  • WCDMA and CDMA Technology






   
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