Adding a Low Pass Filter to an External Local Oscillator Source for the RTL-SDR

At the end of last month we posted about Milan’s work on using an Si535A voltage controlled oscillator with breakout board as a stable replacement for the default RTL-SDR’s low quality 28.8 MHz local oscillator.

Now in order to reduce the number of spurious signals produced at multiples of 28.8 MHz by the external local oscillator, Milan has added a simple low pass filter to the oscillator output. He used a 3-pole Butterworth filter with a 30 MHz cut off point. This filter acts to the convert the Si535A’s square wave output into a sine wave, which should help reduce the number of spurious signals produced.

Square wave before filtering.
Square wave before filtering.
Closer to a sine wave after filtering.
Closer to a sine wave after filtering.

Milan tested his filtering by creating a wideband comparison sweep with rtl_power. With the low pass oscillator inserted, the spurious signals were clearly reduced a significant amount. Then by reducing the drive level from 8mA to 2mA, the spurious signals were reduced even more. By using a filter with more poles it’s possible that the spurious signals could be reduced even further.

Spurious signals with no low pass filter added.
Spurious signals with no low pass filter added.
Spurious signals are reduced after adding the low pass filter.
Spurious signals are reduced after adding the low pass filter.

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