Monday, 2 April 2012

Initial frequency range estimate

A couple of days ago I read about the rtl-sdr project. By chance a usb digital tv dongle I purchased ages ago was compatible. Here are my findings from my ezcap 646 with a fc0012 tuner.

To investigate the performance I have been recording samples with the dongle connected to a RF signal generator:

A number of adapters are used to connect the 75ohm antenna port to the 50ohm connector on the signal generator. No impedance matching is done, which may cause issues but I'm no RF expert. To explore the frequency range of the fc0012 tuner I enabled a -80dBm CW at different frequencies on the signal generator and calculated the power spectral density from recorded samples at 3.2MS/s. A power of -80dBm was chosen at it was the highest value before aliasing effects became visible (probably ADC oversaturation caused by the behaviour of the AGC).

The data was recorded using the drivers provided by the librtlsdr branch of the rtlsdr git project. The rtl-sdr program was used to save samples to disk, and this was then processed using MATLAB.

Here are the results for a CW at 50MHz (located at an effective frequency of 3.2MHz) using a pwelch power spectrum estimate using 100ms of samples:

These are nice clean results, looking as good as what I would expect out of something like a USRP. There's a spur located a couple of MHz away, but this is nothing out of the ordinary for a tuner of this price.

With the RF off at 50MHz the histogram of the samples look fine as shown below. This implies that at this input power at-least the AGC is operating nominally:

At other frequencies it is not as clean though, but I've never had trouble decoding channels using it as a digital tv receiver. Here are the results at 191MHz:

At 600MHz the tuner is deaf to outside signals. There is also some spectral leakage visible, possible from a local oscillator. Here are the results at 600MHz:

Interestingly the CW reappears at 700MHz as shown below. The spectral leakage is still there as well as some minor spurs a couple of MHz above.

Through testing with a CW every 100MHz the frequency range of the fc0012 (with the current experimental drivers) appears to be from 45-500MHz and 700-930MHz. The E4000 tuner has a claimed range of 64-1700MHz (and tested for GMR signals). From these results it can be concluded that the E4000 has a larger frequency range than the fc0012. I have ordered a dongle with a E4000 and will do some comparisons when it arrives.

Continuing with the testing of the fc0012, below 45MHz the spectrum becomes quite distorted. Past 30MHz the CW can still be seen, but there are also a number of spurs of strong power. Here's the results for a CW at 30MHz:

Past 930MHz the tuner again becomes deaf to any incoming signals:

From these results it looks like adapters with the fc0012 tuner can be used up to part of the first ISM band (900MHz). If your interests are below 930MHz, the fc0012 may be a viable alternative you can't source a receiver with a E4000 tuner easily or cheaply. Over the next couple of days I plan to write some scripts to automate the testing (the signal generator can be controlled over the ancient gpi-b bus) across the whole frequency band (45-930MHz) to locate and characterise the spurs and frequency response of the tuner in a more robust way.

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