ASSA Radio Astronomy Group

The original Simple Interferometer relied upon one of the receiver boards feeding its internally generated local oscillator to the other receiver board, thus providing a common local oscillator which is an essential component of this type of interferometer. This arrangement is fine for its intended application but the opportunity to implement a cleaner, more stable though somewhat more complex local oscillator has been made available to us.

Analog Devices produce many direct digital synthesis devices, one of which is the AD9854. While there are many newer and more powerful DDS devices currently available, this choice of device is more than adequate for our purposes.  In a nutshell, the AD9854 is a pair of programmable 12bit digital to analogue converters (DAC) with a maximum clock speed of 300MHz. The two DACs can be phase shifted by 90° to provide both incident and quadrature outputs up to a maximum frequency 150MHz, assuming we maintain the minimum Nyquist ratio of 2:1 . Practically, the device output frequency would be run up to about 120MHz. We have been able to obtain some blank printed wiring boards for the AD9854, based on a simplified version of the Analog Devices Evaluation Board that you can find details of here, including a tutorial and datasheets etc. There is a schematic for the simplified version of the evaluation board here and an image of the simplified board (blank) here. The AD9854 device is still available to buy at a moderate cost of about $40 per chip. Along with another $40 to $50 worth of components, some soldering and programming skills we end up with a frequency tunable, amplitude settable oscillator with a very respectable harmonic output and low phase noise figure.

DDS Output Filtering

The original AD evaluation board and our simplified version were configured with 4 outputs, of which 2 are unfiltered and 2 have 7 element elliptic (cauer) low pass filters with a 3dB rolloff at about 120MHz.

For our application as an incident/quadrature local oscillator operating at an output frequency of 20.1MHz, a lower 3dB frequency of 25MHz has been selected and calculated using Elsie, an excellent Windoze program for filter design. There is a link to Elsie here. Short of leaving off the SMA connectors on the unfiltered outputs for cost reasons, all the headers, jumpers and connectors are fitted and available for testing purposes, if nothing else. There is a pdf parts list here that includes easily obtainable component values suitable for the 25MHz low pass filter.

Driving the DDS

At this stage I have not yet powered up one of these items, but intend to very soon. For those following this page/activity and wish to take it further (before me) please read this information here. There is a very real danger that the levels of your PC digital output will destroy the AD 9854, which runs on a 3.3V rail and even a TTL signal easily exceeds this limit. Apart from the advice mentioned above, Jouni Verronen offers some free software that can be used to set the DDS output frequency, amplitude and other functions. Again, I stress that I have not yet tried this software and can only suggest it to you without any recommendation. For now, you are on your own.