Monday, August 27, 2012

What is Trueform Waveform Generation Technology?

Trueform waveform generation technology is an exclusive technology found in Agilent’s new 33500B Series waveform generators. Trueform technology provides sizable advantages over direct digital synthesis (DDS), the incumbent technology used in waveform generators. These advantages include significantly lower waveform jitter for less test uncertainty and a true representation of the selected waveform, not an approximation. In this blog post, we will look at Trueform and compare it to DDS.

Conceptually, the simplest way to generate a waveform is to store its points in memory and then read those points out one after another and clock them into a DAC. After the last point has been read, the generator jumps back to the first point again to begin the next cycle. This is sometimes called “point per clock” (PPC) generation. Even though this method seems like the most intuitive way to create waveforms, it has two big drawbacks. First, to change the waveform’s frequency or sample rate, the clock frequency has to change, and making a good low-noise variable-frequency clock adds cost and complexity to the instrument. Second, since the stepwise output of the DAC is undesirable in most applications, complex analog filtering is needed to smooth the steps out. Because of its complexity and cost, this technology is used mainly in high-end waveform generators.

DDS uses a fixed-frequency clock and a simpler filtering scheme, so it’s less expensive than the PPC method. In DDS, a phase accumulator adds an increment to its output in every clock cycle, and the accumulator’s output represents the phase of the waveform. The output frequency is proportional to the increment, so it’s easy to change frequency even though the clock frequency is fixed. The output of the accumulator is converted from phase data into amplitude data typically by passing it through some type of look-up table. The phase accumulator design allows DDS to use a fixed clock, but still execute waveforms at a perceived faster sample rate than the clock. So with DDS, not every individual point is being expressed in the resulting output waveform. In other words, DDS is not using every point in waveform memory, but it creates a really good approximation. But since it is an approximation, waveform data is changed in some way. DDS can skip and/or repeat aspects of the waveform in an unpredictable way. In best-case scenarios, this leads to added jitter; in worst-case scenarios, severe distortion can result. Small features in the waveform can be partly or completely skipped over.

Agilent’s new Trueform technology represents the next leap in waveform generation technology. Trueform provides the best of both worlds. It gives you a predictable low-noise waveform with no skipped waveform points like PPC technology, but at the price point of DDS technology. Trueform works by employing a exclusive virtual variable clock with advanced filtering techniques that track the sample rate of the waveform. In the following sections, we will look at some of the waveform generation advantages Trueform provides over DDS.

Improved signal quality
One of the key advantages Trueform provides over DDS is better overall signal quality. One of the best ways to show this is by doing a jitter measurement comparison with DDS. The following figures show a jitter measurement made on a 10-MHz pulse signal using a high-performance oscilloscope. The scope view is zoomed in on the rising edge of the pulse signal with the persistence setting of the scope turned on. The histogram function of the scope is used to measure the period jitter of the signals. The standard deviation measurement in each figure is circled in red and represents the signal’s RMS jitter. The Trueform pulse signal jitter measurement is shown in the below figure and the DDS pulse signal jitter measurement is shown in the next figure.

Trueform signal with < 5ps of RMS jitter
DDS signal with > 50ps of RMS jitter
In the above figures both the amplitude and time scales for the scope are the same. The Trueform pulse waveform has more than 10 times less jitter compared to the DDS pulse waveform. The substantially lower jitter that Trueform offers over DDS means less uncertainty in your tests. This is especially true when you consider edge-based timing applications like generating a clock signal, trigger signal or communication signal.

The waveform you create is the waveform you get
As we mentioned earlier, DDS uses a fixed clock and a phase accumulator so it cannot guarantee that every point or feature in a waveform will be played. The higher the frequency, the more gaps you will see in the output waveform compared to the ideal waveform. Trueform, on the other hand, plays every waveform point regardless of the set frequency or sample rate. This becomes critical when you are dealing with a waveform that may have a small detail that is critical to the test you are performing. As an example, we created an arbitrary waveform that consisted of a pulse with seven descending amplitude spikes on top of the pulse. The waveform was then loaded into a Trueform waveform generator and a DDS waveform generator. First the waveform was played at a 50-KHz frequency on each generator. The result was captured on a scope, shown in the below figure. The yellow trace is the Trueform waveform and the green trace is the DDS waveform.
Trueform on top in yellow and DDS in green

At 50 KHz, each generator was able to reproduce the waveform with seven spikes on top of the pulse. You can see that the Trueform spikes reached higher amplitude. In the below figure scope screen shot, the waveforms were played again, but this time at 100 KHz.

Trueform on top in yellow and DDS in green. Note that all the DDS spike points were skipped
At 100 KHz, the Trueform waveform generator played all seven spikes and the DDS generator did not play any of the spikes. In the below figure scope screen shot, the waveforms were played again, but the frequency was doubled again to 200 KHz.

Trueform on top in yellow and DDS in green. Note that DDS only shows 3 of the seven spikes

At 200 KHz, once again, the Trueform waveform generator shows all seven spikes in the waveform. The DDS generator went from playing no spikes at 100 KHz to playing three spikes at 200 KHz. Notice that the three spikes played in the 200 KHz waveform do not match the correct time location of any of the seven spikes that are in the actual waveform points. These waveform examples demonstrate that when working with waveforms that have fine detail, DDS cannot be trusted.

Agilent’s Trueform technology offers a new alternative that blends the best of DDS and PPC architectures, giving you the benefits of both without the limitations of either. Trueform technology uses an exclusive digital sampling technique that delivers unmatched performance at the same low price you are accustomed to with DDS.

For more info on the 33500B series with Trueform technology click here

Tuesday, August 21, 2012

Agilent Announces a New Family of Waveform Generators with Exclusive Trueform Technology

In this blog post we will look at Agilent's new family of waveform generators, the 33500B series. The new family is made up of 8 models that offer bandwidths from 20 to 30 MHz and offer one or two channels. The 33500B family features the exclusive Trueform waveform generation technology. Trueform technology plays every point in an waveform, unlike the incumbent direct digital synthesis (DDS) technology which only provides an approximation of a waveform (not every point is played). Also Trueform technology delivers waveforms with extremely low jitter typically around 4 ps of RMS jitter, which is more than 10x less jitter than a DDS generator. I will have more detailed info on Trueform in a future post. Here are some more 33500B series headlines:

  • Full bandwidth pulse and square waves
  • Sine waves with 5x lower harmonic distortion
  • IQ signal player option
  • Advanced modulation features such as sum modulation and channel to channel modulation
  • 16 bits of resolution with 1 mVpp to 10 Vpp amplitude
  • Noise function with variable bandwidth adjust and no repetition for over 50 years of continuous play
  • The ability to upload arbitrary waveforms and sequences without a remote connection

For the two channel version there is an IQ signal player option available. IQ signal waveforms can be played from the 33500B without the option, but the option adds an easy to use single user interface for setting up and controlling both channels at once. Also the option offers features such as channel skew adjust and balance control for adding non-ideal conditions to the signal to test the limits of your design. 

For an IQ example a 64 QAM baseband signal was created in software. It was then uploaded to and played by the 33522B waveform generator using the IQ signal player option. The resulting IQ baseband signal was captured by a high performance scope using vector signal analysis software, as shown in the below figure.

With Trueform technology the 33522B was able to deliver a 64 QAM baseband signal with only 0.3 % error vector magnitude! Notice too that the phase error is in milli-degrees and the magnitude error is in mV.

In my next post I will go into more detail on what Trueform technology is and do some comparison measurements with DDS. As always if you have any questions related to this post please feel free to email me and if you have something to add use the comments section below.

Monday, August 13, 2012

Testing DC to DC Converters with a Single Instrument

This post features two videos by a colleague of mine who presents a single instrument solution for testing DC to DC converters targeted for handheld or portable devices where power optimization is critical. Typically when testing a DC to DC converter you need a long list of test equipment including a power supply, eload, multi-channel scope or digitizer, current shunt or probe, and a waveform generator to vary the output of the power supply or eload. Agilent's N6705B DC Power Analyzer provides a single instrument solution. The N6705B is a modular power supply with up to four power outputs, voltage and current digitizers, and power waveform capabilities. There are over 30 power modules to choose from. The SMU modules shown in the video can source and sink power so one is connected to the input of the DC to DC converter and the other sits on the output as a load.

The first video provides an overview and example measurements for using the N6705B for testing DC to DC converters. The second video goes into more detail on a DC to DC converter efficiency measurement and shows an example.

Monday, August 6, 2012

Squeezing More Bandwidth Out of Your Waveform Generator

In this post we will look at how you can get more waveform bandwidth out of your waveform generator, bandwidth that goes beyond it's spec'd bandwidth. Before we get started there are some prerequisites that your waveform generator must meet for this to work. First your waveform generator must allow you to set the sampling rate for the arbitrary waveform generator versus just the frequency. Low cost direct digital synthesis generators typically only let you set the frequency for an arbitrary waveform and not the sample rate since they are not actually playing every point of the waveform just providing a close approximation of the waveform. Second the waveform generator must have a sampling rate that is greater than the 2X the spec'd output bandwidth.

A waveform generator's output bandwidth is typically limited by the analogue bandwidth of its signal conditioning output stages (filtering, output amp, etc), not its sampling rate. The sampling rate of the waveform generator is typically set well above the Nyquist rate, a condition known as oversampling. We can use the higher sample rate to create waveforms above the analogue bandwidth of the waveform generator. As an example Agilent's 33522A has a sampling rate of 250 MS/s and an output bandwidth of 30 MHz. Its built-in sine wave function only goes up to 30 MHz, but by creating an arbitrary waveform sine wave we can take advantage of  the oversampling and go above 30 MHz. As an example a 50 MHz sine wave was generated with a 33522A and can be seen in the scope screen shot below.

At 20 MHz above the spec'd bandwidth some amplitude attenuation will occur on the sine wave signal. For the 50 MHz  signal the output measured amplitude was about half of the set output amplitude (-6 dB attenuation). But the signal fidelity of the 50 MHz signal was still well within spec. As shown in the figure below, the second harmonic was < -55 dBc (33522A spec is < -35 dBc). 

For what I will refer to as sine like waveforms achieving higher bandwidths by taking advantage of oversampling works well. For pulse and square like waveforms you are typically limited to the spec'd bandwidth of the waveform generator and if you try to go higher using an arbitrary waveform the result will  look like a sine wave not a square or pulse waveform.

Since achieving higher bandwidths from your waveform generator using oversampling works best for sine like waveforms, it can be used for waveform applications such as multi-tone signal generation, phase or frequency modulated carrier signals, or simulating baseband digital modulation that uses a raised cosine filter. Let's look at a simple multi-tone signal example. Using the 33522A waveform generator a mult-tone signal was generated that consisted of the following tones: 10 MHz, 20 MHz, 30 MHz, 40 MHz, 50 MHz, and 60 MHz. The resulting mult-tone signal was generated by the 33522A and captured on a signal analyzer as shown in the figure below.

As we would expect the amplitude of each tone drops as frequency is increased. This problem has an easy fix though. By using the marker function, the ratio of each tone can be measured against a reference tone, in this example the reference tone is the 10 MHz. The ratios were used to generate amplitude flatness correction factors in software. The new multi-tone signal with flatness correction was generated by the 33522A and measured with a signal analyzer, screen shot shown below.

Using the flatness correction we were able to generate a quality multi-tone signal with a tone that was twice as much as the 33522A's rated bandwidth! 

The bottom line is if multi-tone signal generation or another sine wave like application is what you intend to use your waveform generator for you could save a lot of money by choosing the waveform generator based more on its sample rate instead of its rated bandwidth. And then obtaining an evaluation unit to test how much more quality bandwidth you can squeeze out of it for your particular application, before making your purchase. If you have anything to add to this post please use the "Comments" section below. If you have any questions feel free to email me.