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In radio astronomy, a key limiting factor to observations made is the available bandwidth of the system. This thesis looks at two different approaches to building ultra-broadband correlators for use in radio astronomy. The first was a 2-20GHz double-sideband complex analogue correlator that was constructed before the work of this thesis. Characterisation tests are performed and a basic calibration is attempted. Both these sets of experiments show good results, with the basic calibration successfully being able to compensate for gain difference between the lags over a reduced bandwidth range used in the testing. The second approach was the investigation into different techniques for high-speed digital sampling, capable of providing equivalent bandwidths to the analogue system. The use of FPGA high-speed serial interfaces as direct 1-bit 3.125 GS/s samplers was investigated. Single-frequency sampling showed that a signal-to-noise ratio close to the theoretical maximum across the band was achieved (≈ 0.8 effective bits). Techniques were also identified to use multiple transceivers to generate a single interleaved stream at higher effective sampling rate. Two different methods were also explored for producing greater-than 1-bit sampling. A hysteresis approach was shown not to produce the desired results and a reference based sampler in the end was adopted. Finally, the interleaving and multi-bit techniques were combined to generate a single 1.5-bit 6.25 GS/s sampler. This was seen to have reduced signal-to-noise compared to the expected values. This was believed to be caused by the poor method of RF signal injection causing cross-talk between the channels and large amounts of loss. As a comparison to the direct sampling method, an external 1-bit high-speed Hittite comparator was also examined. The single-frequency experiment was repeated with a slightly higher signal-to-noise ratio found compared to the direct sampling method. This was again believed to be due to the RF environments used. From the sampling setups a four-input, six-baseline lag correlator was constructed using the direct sampling method. The entire correlator, as well as the sampling transceivers, was incorporated into a single Xilinx Virtex 5 FPGA. This was shown to have the expected response to single-frequency, broadband and noise signals. The thesis concludes with a characterisation of the RF devices used throughout the testing procedures. Several new devices were developed through the course of the experiments with the designs being documented. All the necessary components to construct IF chains for both the analogue and digital correlators described are present. This leads to simulations being made of complete IF chains, with the expected responses shown.