Wideband testing of satellites

-June 29, 2016

To deliver the next generation of satellite services, spacecraft operators are increasingly using larger bandwidths at higher frequencies. Characterising transponder performance such as SNR, SFDR, and flatness over hundreds of MHz or several GHz, can be very difficult for OEMs and equally challenging for suppliers of test and measurement equipment.

Wide bandwidths are sometimes split into multiple channels and dynamic range problems occur when non-linearities, e.g. amplifiers, ADCs, and DACs, generate intermodulation products between the input frequencies. These new frequencies can appear within the bandwidths of other channels causing distortion.

Noise power ratio (NPR) is a wideband test which measures the 'quietness' of an unused channel accounting for intermodulation distortion products generated by non-linearities within the signal chain.

Traditionally, NPR characterisation of an ADC or a complete payload was an analog process: Gaussian noise from a white source would be bandlimited using a low-pass anti-aliasing filter and then a band-stop filter was used to create an empty notch (channel) at the desired centre frequency as illustrated below (Figure 1a).

For ADC NPR testing, as the amplitude of the analog input noise is increased, there is a linear relationship between its power and the measured NPR. At some value, determined by the comparator's hard-limiting behaviour, the ADC begins to clip, creating intermodulation products which raise the quantisation noise floor, rapidly reducing NPR as shown below (Figure 1b).

Figure 1 Analog wideband signal generation and ADC NPR characterisation. Click to enlarge.

The problem with analog broadband signal generation is that many hardware filters are required to accommodate different notch widths and positions, and complete in-band characterisation becomes a slow and manual process. A quicker, automated, repeatable, and more controllable test procedure is needed, independent of mission frequencies, information bandwidths, and individual notch requirements.
With the advent of high-resolution fast DACs, any arbitrary signal which can be described mathematically can be generated flexibly and accurately. A block diagram of an arbitrary waveform generator (AWG), also affectionately known as an ARB, is shown below (Figure 2), where digital samples stored in a waveform memory are read and converted to analog at a desired frequency. The sampling rate determines the bandwidth and the maximum frequency that can be output while the resolution of the DAC sets the binary representation of the output voltage produced at each time sample - this is the concept of a universal signal source!


Figure 2 Block diagram of an AWG.

AWGs have sophisticated sequencers for storing and playing waveform data as well as markers and triggers, which can interface with the external environment. Memory segmentation allows seamless sequencing without any gaps between the last and first samples of contiguous segments, and complex signal scenarios can be created, comprising a collection of sequences.

Mixed-signal converters are non-linear and generate harmonic distortion as well as quantisation noise. The DACs used within Keysight's M8190A AWG have an SFDR better than 90 dBc which appear transparent when characterising broadband space-grade ADCs (Figure 3).

Figure 3 Single-tone and NPR signal generation using the M8190A. Click to enlarge.

Traditionally, AWGs have been used to create baseband and IF signals with external, analog-quadrature modulators producing RF carriers containing imbalances and linearity issues. The M8190A offers digital I/Q up-conversion allowing precise generation of RF frequencies. This ARB has a sampling rate of 12 GSPS capable of directly outputting in its baseband mode multiple modulated carriers suitable for C-band satellites.

The M8190A offers return-to-zero and doublet modes from its DAC to reduce the impact of the zero-order-hold, sinc roll-off and allow access to the higher-frequency image in the second Nyquist zone. x3, x12, x24 and x48 interpolation is supported to position images further away from the baseband information to improve fidelity and SNR, and ease implementation of the re-construction filter.

You can directly access the DAC's differential outputs or route these through a range of amplifiers and re-construction filters to optimise performance in the frequency and time domains. A range of AWGs is available with different sampling rates, analog bandwidths, and resolutions suitable for testing satellites beyond Ka-band as shown below (Figure 4).

Figure 4 A range of AWGs are available for broadband testing of satellite sub-systems. Click to enlarge.

The AWGs can be controlled using their intuitive GUI-based firmware, which allows single and multi-tone signals as well as many types of modulated carriers to be generated quickly. For production testing, scripting can be used to automate signal generation, measurement, and results capture. For example, I create wideband stimuli using SystemVUE or Matlab, upload the pattern to the waveform memory, replay, and then record the results from a logic analyser for ADC characterisation or a spectrum analyser/VNA for end-to-end system testing.
To provide you with further information on AWGs, an introductory tutorial and an excellent primer are freely available for download. There is also a short video demonstrating the capability of Keysight's M8196A four-channel, 92 GSPS, 32 GHz bandwidth AWG.

This post has briefly introduced wideband testing using DDS-based AWGs and if you would like to learn more, this is a topic I teach on my Mixed-Signal course. I also demonstrate the M8190A AWG, as broadband characterisation of ADCs and satellite transponders have become a major challenge for many satellite OEMs wanting to offer the benefits of high-throughput digital payloads to spacecraft operators

AWGs are a win-win for everyone: semiconductor vendors, the test industry, and satellite manufacturers. Individual component or module-level characterisation is possible, as well as complete end-to-end payload testing. High-fidelity single-tones can be generated along with representative, complex multi-carrier signals such as NPR, chirps, PAM-4, and phase-coherent MIMO. Broadband testing has become quick, repeatable, highly controllable, and mission independent, and can be performed early in the product development cycle to de-risk concepts and prototypes. AWGs allow you to sign-off the development of your satellite sub-systems with confidence and deliver your avionics right-first-time, to cost, and on schedule.

I use the M8190A for ADC/amplifier characterisation as well as system-level testing. Figure 5 is a photograph of my bench. Before connecting to the device to be tested, I always route the generated signal to both outputs and check these on a spectrum analyser and a DSO to verify the expected frequency and time-domain behaviours respectively.

Figure 5 M8190A AWG generating a 2 GHz-wide NPR test signal: channel 1 connected to a FieldFox spectrum analyser and channel 2 to an Infiniium DSO. Click to enlarge.

It's time to end this post and get back to my AWG - speak to Keysight if you wish to evaluate one of their ARBs or our colleagues at RF Interlligent. I first started using the N8241A ARB before it was formally released as a product, and have used it and the M8190A extensively. I'd love to hear about your broadband measurement experiences and until next month, may your PAPR always square your crest factor!

P.S. The first person to email me how the last sentence fits with this post will get a free Courses for Rocket Scientists pen. Congratulations to Akito from Japan for correctly answering the riddle from my previous post. It was great to meet so many of you at AMICSA and chat about many topics - I look forward to seeing you again on my courses!

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