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Tinkerings with DDS VFOs

DDS VFOs have revolutionised homebrew and QRP construction. Amateurs or yore typically had the choice of a frequency agile VFO (controlled by hard to find variable capacitors and reduction drives) or a basically fixed frequency crystal oscillator, which unless you were lucky, relied on having a crystal specially cut. DDS (or direct digital synthesis) has changed all that, with frequency agility and stability both readily possible. And as a bonus you get variable tuning rates and an accurate digital frequency display.

These videos show my experiences with DDS VFOs. Whether you buy, build from a kit or design from scratch I can highly recommend DDS as a worthwhile and versatile amateur radio project.


Arduino for Amateur Radio: DDS VFO using the AD9850

Here I use an Arduino to drive an AD9850 Direct Digital Synthesiser Module as an HF variable frequency oscillator. The advantage of this arrangement is you can customise it to what you want. For example if you are building a phasing direct conversion receiver that requires the local oscillator to be four times that of the received frequency (due to divide by 4) then you can accommodate that in the Arduino programming. Can be frustrating, rewarding and definitely educational!

Links referred to in the video are as follows:

* Arduino website
* Build own Arduino for $5
* Digital voltmeter
* Arduino single frequency generator with code
* VK5TM website
* AD7C website
* VK8BN website


Obtaining parts for DDS VFOs

Here's the good news. 30 years ago homebrew transceiver construction slowed to a trickle. Variable capacitors and reduction drives became expensive and difficult to obtain and we thought that surface mount would spell the death of hobby electronics. As it turns out technology has made RF construction easier and allowed the construction of projects we wouldn't have attempted back then. DDS VFOs are a prime example. The links below are examples of modern parts that are cheaply and readily obtainable.


The Arduino DDS used in a phasing direct conversion receiver


Reviewing the OzQRP CDV DDS VFO

A DDS VFO kit from OZQRP. Recommended if you want a fairly small easy to build DDS VFO that you can drop in to a converted or homebrew transceiver to give it improved frequency coverage or stability.


Removing DDS generated noise from QRP transceiver

The main disadvantages of DDS VFOs are their higher current consumption (compared to minimalist crystal or VXO controlled oscillators) and the risk of noise generated by the logic circuitry or display. This can require some hunting to find it. Video describes how to do it and a simple cure that worked for my noise problem.



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Items were chosen for likely usefulness and a satisfaction rating of 4/5 or better.


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