Measuring receiver distortion - Page 1
Forum section: electronics / rf-microwave

author: mojoe
author_rank: Frequent Contributor
timestamp: October 05, 2025, 09:37:26 pm
content: I don't have a distortion meter. I do have a Sinadder Linear 5. It must be an early model, as the SINAD scale only goes up to 24 dB. The later ones go to 32 dB, and have another scale for distortion.If I set the signal generator level so that the receiver is putting out its rated audio power (into a resistive load), the needle on the meter is pegged to the left, more than 24 dB. Using the numbers from the manual for the later model, can I assume that the audio distortion is probably less than 5%, perhaps much lower? On the later model, 26 dB = 5%, 32 dB = 2.5%.Am I using the correct procedure? The manual glosses over measuring distortion. Otherwise, is there a way to measure the audio distortion with the equipment I have?

author: radiolistener
author_rank: Super Contributor
timestamp: October 05, 2025, 10:08:49 pm
content: If your goal is mainly to estimate distortion rather than SINAD itself, you can do that quite well using a spectrum analyzer and a low-distortion two-tone signal source, that’s all you really need. Set the generator to two non-harmonically related tones of equal level, close in frequency, feed them into the receiver or amplifier, and observe the output spectrum. The levels of the intermodulation products relative to the main tones (in dB) directly indicate the amount of nonlinear distortion — for example, spurs -26 dB below the main tones correspond roughly to 5 % distortion.

author: rf-fil
author_rank: Regular Contributor
timestamp: October 07, 2025, 01:23:58 am
content: I have created a little Python app for this. It's a proper SINAD as used in radio comms, based on the Psophometric filter curve. I've just put it up on github. You'll need to get the code off of there and pre-install some Python libraries. I'll turn it into a proper exe file, when I have a bit of time.It uses any sound input device. A microphone next to a speaker works OK as well.Link:https://github.com/VK2ZJ/VK2ZJ_SINAD_Meter

author: mojoe
author_rank: Frequent Contributor
timestamp: October 07, 2025, 05:30:11 am
content: That looks useful. Thanks for posting. I have a Sinadder, though.

author: radiolistener
author_rank: Super Contributor
timestamp: October 08, 2025, 12:44:08 am
content: Quote from: rf-fil on October 07, 2025, 01:23:58 amCould you please share some test WAV files with a precisely measured SINAD — verified using your calibrated measurement equipment — so that others could use them for validation and comparison?

author: rf-fil
author_rank: Regular Contributor
timestamp: October 08, 2025, 01:40:44 am
content: Quote from: radiolistener on October 08, 2025, 12:44:08 amGood idea. Currently, the app only takes in audio input in real time, so I'll need to add wav file i/o.

author: G0HZU
author_rank: Super Contributor
timestamp: October 08, 2025, 09:54:39 pm
content: I don't know if this helps, but a while back I posted up some wav files that demonstrate signal, noise and signal+noise.https://www.qsl.net/g/g0hzu//RF%20Transistor%20data/SINAD/12dB_SINAD_OOK2.wavhttps://www.qsl.net/g/g0hzu//RF%20Transistor%20data/SINAD/15dB_SINAD_OOK2.wavhttps://www.qsl.net/g/g0hzu//RF%20Transistor%20data/SINAD/20dB_SINAD_OOK2.wavThere should be 12dB SINAD, 15dB SINAD and 20dB SINAD in each link above. Note that the wav file sometimes plays signal+noise and sometimes it is just noise and sometimes it is just signal. So watch out for this.In other words, each wav file contains all three states below:I can't guarantee the SINAD will be correct from your soundcard, but it might be worth trying these files on your meter?

author: rf-fil
author_rank: Regular Contributor
timestamp: October 09, 2025, 10:55:06 am
content: Quote from: G0HZU on October 08, 2025, 09:54:39 pmThis is great, thanks. I get within a fraction of a dB when I run the wav files through the underlying algorithm. No need to involve the sound card.I explained what I did at my repo:https://github.com/VK2ZJ/VK2ZJ_SINAD_Meter.My app has limitations, of course, and they are listed there as well.