Tektronix RSA306 IF Frequency Verification

After posting the teardown video of the Tektronix RSA306 USB 3.0 Spectrum Analyzer on my YouTube channel, someone asked me to verify that the IF sampled by the Linear Technology 14-Bit / 125 Msps ADC is really at 140 MHz. Using the Tektronix MDO4104B-6 and a passive RF probe, I gladly did so.

With a 100 MHz, 0 dBm signal applied to the spectrum analyzers input and the SignalVu-PC settings set to 100 MHz center frequency, 40 MHz span, the signal at the ADC input looked like this:

Spectrum at the LTC2261-14 input with a 100 MHz, 0 dBm signal

Spectrum at the LTC2261-14 input with a 100 MHz, 0 dBm signal

So indeed, 140 MHz is our strongest peak. 20 dBc lower is a 28 MHz peak. Before taking the picture I of course tried a large span to make sure that there aren’t any relevant signals at a lower frequency. So the next test uses a 105 MHz signal still at 0 dBm.

Spectrum at the LTC2261-14 input with a 105 MHz, 0 dBm signal

Spectrum at the LTC2261-14 input with a 105 MHz, 0 dBm signal

So indeed, it seems that the LTC2261-14 samples the 140 MHz IF directly. How you ask? The answer is undersampling. The ADC is actually sampling at 112 Msamples/s. As a result, it sees an alias of the 140 MHz signal at 28 MHz. That’s all the magic happening here.

This probing expedition also yielded some pretty pictures that I would like to share. Enjoy!

Probing around inside the Tektronix RSA306 USB 3.0 Spectrum Analyzer

Probing around inside the Tektronix RSA306 USB 3.0 Spectrum Analyzer

Probing around inside the Tektronix RSA306 USB 3.0 Spectrum Analyzer

Probing around inside the Tektronix RSA306 USB 3.0 Spectrum Analyzer

Probing around inside the Tektronix RSA306 USB 3.0 Spectrum Analyzer

Probing around inside the Tektronix RSA306 USB 3.0 Spectrum Analyzer

4 thoughts on “Tektronix RSA306 IF Frequency Verification

  1. Could you post a high resolution picture of the backside of the board. You video did not show circuits leading into the 1st mixer’s (LTC5510) RF and IF paths.

    • Snap! Already put it back together and the amounts of super glue I had to use make another teardown impractical. I have a whole fault full of pictures of the board on my phone though. Not certain if they show the desired areas but if you eMail me (KF5OBS@gmail.com), I’ll package the images up for you and eMail / Dropbox them to you.

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