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Technical Topics => Equipment => Topic started by: ChrisSmolinski on September 21, 2011, 2133 UTC

Title: DDS-60 Direct digital synthesizer
Post by: ChrisSmolinski on September 21, 2011, 2133 UTC
Recently I put together a DDS-60. DDS stands for Direct Digital Synthesizer. It is a way to generate arbitrary frequencies. Samples are fed to a D/A (Digital to Analog Converter) at a fixed clock rate (in this case 180 MHz derived from a 30 MHz oscillator). These samples are generated by a NCO (Numerically Controlled Oscillator). Think of it as a sine wave being generated point by point, at a fixed (depending on the ratio of the output frequency to the 180 MHz clock) number of degrees per sample. The output frequency can instantly be changed by just altering this degrees per sample value...

Read more: http://www.hfunderpants.com/?p=73 (http://www.hfunderpants.com/?p=73)
Title: Re: DDS-60 Direct digital synthesizer
Post by: jFarley on September 22, 2011, 2131 UTC

A very timely post for me.  My R7 is plagued by 3 annoyances, 2 attributable to old age, and one by design.  I was looking at this a short time ago and decided that the DDS-60 accompanied by an Arduino SBC should be able to solve all three problems.  First, the R7 PTO gearing just reaches the far end of the bathtub curve at some point.  The plastic gears become worn, out of round, and prone to lockup.  Using a DDS to implement a digital VFO would be a nice replacement.  Most likely will just need a simple BPF to get to the spectral purity of a permeability tuned oscillator.  Secondly, the band Up/Down buttons at some point just get tired, and are prone to contact bounce.  No amount of contact cleaner seems to do the trick for very long.  Punch Up, and it can be a guess as to what frequency band you end up on.  Thirdly, the RIT control is difficult to use by design; +/- 2.5kHz in a single turn pot makes it very difficult to use in practice.

An Arduino has a lot of flexibility for $30.  14 IO lines to handle DDS loading, frequency increment/decrement buttons, etc.  One of the on board 10-bit ADCs could be easily used to implement the RIT function and possibly the Passband Tuning function (do I see a joystick here?).  It has EEPROM which can be accessed by the code (written in C) to handle last tuned frequency, memories, etc.  It does have a USB port which is used to download code from the compiler, but which can also be accessed programatically for control by a host program.

All in all, one should be able to put together a great PTO replacement for a lot cheaper than an RV75.

Thanks for the post!