I bought myself relatively cheap ~430USD working spectrum analyzer Advantest r4131c. With first inspection everything was excellent. However after little more investigation frequency readout was inaccurate and unstable. Something wrong with PLL i thought. However, with some more digging into service manual it turns out that there is no PLL nor any AFC at all (sic!). The r4131c version of this model does not offer frequency correction unlike r4131d that does. So I thought if same series device can have frequency corrector how hard could it be to upgrade this one? Lets see.