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Spur Removal

Sometimes mixing of harmonics creates artificial signals in a scan making it difficult to know what the real signals are. Spur reduction uses either wobbling the high IF frequency within the pass band of the high iF bandfilter or swapping the LO between above IF and below IF. During this wobbling or LO switching the desired signals remain at their position but all products of harmonic mixing or aliases or mirrors will move around. Spur reduction works best below 190MHz or using an RBW of 100kHz or less.

Reducing spurs

Here is the bottom 20MHz with ALL spur reduction measures switched off and the IF set to a fixed 433.8MHz

The first measure used is to shift the IF at certain scanning frequencies. The impact is clearly visible when the IF is set back to automatic.

The tinySA has an internal table recording at what frequencies the IF has to be shifted to eliminate certain spurs. This spur avoidance approach has no negative impact on performance.

There are still some spurs left and these can be reduced by spur reduction techniques that do have an impact on the scanning speed and the ability to measure transients. Below scan is done with the SPUR REMOVAL enabled

This results in an almost clean noise floor. To check if there has been no impact on sensitivity the same scan while supplying a -95dBm signal at 10MHz

Measuring such a small signal is an art in itself as can be clearly shown by what is measured when the signal generator and the tinySA where both connected to the PC using their USB interface

Always make sure there are no ground loops, use good quality cables and tighten SMA connectors.

Unavoidable spurs

There are some spurs however that can not be eliminated because these are actually present as input signals. An example is the leakage from harmonics of the MCU clock at 48MHz that can be seen in below scan

These harmonics will become even stronger when connecting the telescopic antenna and, as they are real signals, can not be removed by spur removal.

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Page last modified on December 27, 2021, at 02:09 PM