Spur Reduction (SR) Explained for PowerSDR 1.x Spur Reduction

Spur Reduction (SR) Explained for PowerSDR 1.x
Spur Reduction (SR) Explained for PowerSDR 1.x
System Dependencies
Minimum PowerSDR Version:
Applicable Hardware:
1.X.0
All transceivers
Content provided by: FlexRadio Systems Engineering
Direct digital synthesis provided by a Direct Digital Synthesizer (DDS) is a digital technique of
frequency synthesis providing the advantages of digital control of frequency and phase, high
frequency resolution and fast settling time. However, this technique, in its present form, exhibits
relatively poor output spectral quality, mainly evidenced by the presence of numerous spurious
signals (commonly referred to as "spurs"). The FLEX line of software defined radios was designed
specifically to significantly minimize the number of DDS spurs in the ham bands, there some
places where spurs exist. There is a software feature of PowerSDR called Spur Reduction (SR)
that will move a DDS spur about 3 KHz to remove it from your operating passband. Although
there may not be any DDS spurs in your passband, or even within the ham band you are using for
that matter, it is always recommended that you have Spur Reduction turned ON while operating
the radio in most circumstances. For some digital modes, you may need to turn off Spur
Reduction.
How Spur Reduction Works in PowerSDR
When you change frequency, with Spur Reduction turned OFF, the radio hardware (DDS) is
changed every time you change frequency. To do this a hardware control message is sent from
the software to the radio hardware. The smaller the frequency step rate, the more hardware
control messages are sent to the radio hardware. Even though this is done very quickly, it does
add some inherent latency to the tuning process. With Spur Reduction turned ON, the radio
hardware is only tuned every ~3.051kHz. PowerSDR does the fine tuning using a software
oscillator which is much faster than tuning using the DDS.
Why Should you Keep Spur Reduction Turned On
As noted previously, the reason it is recommended to keep Spur Reduction turned ON, is that it
minimizes the number of radio control messages sent from PowerSDR to the radio hardware when
you are tuning and for that 3 KHz "window", the radio will change frequency much faster since it is
the software oscillator that is changing the frequency whereby reducing the number of DDS spurs
seen. The DDS spur issue isn't very applicable with the FLEX family of software defined radios,
but it is significant with the SDR-1000 that is more prone to have DDS spurs.
There is a more important reason to operate with Spur Reduction turned on. Undesirable audio
artifacts or freezes in the Panadapter can be induced when Spur Reduction of turned OFF when
tuning across the band, especially when dragging the Panadapter to tune the radio. When Spur

Reduction is turned OFF, the radio hardware has to tune the DDS a lot more often and a DDS
change can happen in the middle of a audio sample causing blips, drop outs or "holes" in the
audio. When the software tuning is used, it only tunes in between audio callbacks, thus ensuring
that audio samples are not dropped/skewed during the frequency change.
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Last Modified By: Administrator, Saturday, March 14, 2009
http://kc.flex-radio.com/KnowledgebaseArticle50380.aspx
Saturday, September 05, 2009