# Can the QA401 measure FM Tuner distortion? **URL:** \ **Category:** QA401\ **Created:** [January 17, 2021, 9:35pm UTC](https://forum.quantasylum.com/t/can-the-qa401-measure-fm-tuner-distortion/237 "2021-01-17T21:35:47Z")\ **Posts on this page:** 1\ **Page:** 2
**Author:** ![SABristol](https://yyz2.discourse-cdn.com/flex032/user_avatar/forum.quantasylum.com/sabristol/32/380_2.png) [@SABristol](https://forum.quantasylum.com/u/SABristol)\ **Post date:** [August 30, 2021, 12:33pm UTC](https://forum.quantasylum.com/t/can-the-qa401-measure-fm-tuner-distortion/237/21 "2021-08-30T12:33:16Z")
Rather than continue with the rather mediocre home-brew stereo MPX modulator I posted earlier, I’ve instead bought a used Rohde & Schwarz SML-01 with the B5 stereo modulation option. Using a 1970s Technics SA-200L as the DUT, I connected the QA-401 left and right outputs into the SML-01 analogue inputs and the QA-401 input to measure the tape output of the DUT. Here are the resultant spectral and Bode plots: ![FM Stereo Radio spectrum - THD 20Hz-18kHz](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/1X/fed15e09aaeb5e89916a3acd30cb1ffbf2c6e07b.png) Due to the worryingly high 19kHz pilot tone in the output signal, I set the measurement of THD to stop at 18kHz. This old tuner gives a respectable THD of ~0.1% when fudged that way. It’s no wonder cassette decks need MPX filters! ![FM Stereo Radio Frequency Response](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/1X/dd6d12ba8555992bf68fac7aedb1c8ebe6b8137e.png) The SML-01 has switchable pre-emphasis for 0, 50uS and 75uS allowing a region corrected response to be measured. Ignoring the 19kHz pilot again, the frequency response of this DUT is just about ±1dB 20Hz-15kHz. Once I’ve hooked up the SPDIF between PC and SML-01 I’ll see if the digital approach is any better. [Previous page](https://forum.quantasylum.com/t/can-the-qa401-measure-fm-tuner-distortion/237.md?page=1)