10 January, 2023

Audio delay - the unspecified parameter for ham radio transceivers

Elecraft K3: Upper trace is start of sidetone and
lower trace is start of RF ~15 ms later.
An amateur radio transceiver is not a precision measurement device. This is apparent when it comes to measuring time for your own round-the-world signal or when characterizing Long-Delayed Echos (LDE). One cannot just substitute onset of sidetone for the actual start of transmission of RF. Similarly there may be an unspecified delay between received RF and start of audio output on the receiver side.  

The main specification for ham radio transceivers is that the sidetone is synchronized with the keying. The delay to start of RF is only of secondary importance. All numbers should therefore be judged against the length of a dot which for instance at a morse code speed of 30 WPM is 40 ms. 

Here are some measurements for the Elecraft K3 and K2 that I did in 2021 for a presentation on LDEs at the HamSci 2021 conference. A video of the presentation is also available: Long delayed radio echoes – the illusive secret of the ionosphere

The first image shows that start of the sidetone is about 15 ms before RF is transmitted in the Elecraft K3.

Elecraft K3: Upper trace shows RF and
lower trace start of AF output some 35 ms later.
The second image shows that there is a delay of about 35 ms between RF is received and audio appears on the receiver side. This means that measurement of audio delay overestimates the value for RF delay by about 50 ms. It should be noted that in a transceiver with digital signal processing, especially the receiver delay will vary with settings. In the case measured here, the DSP was set for 50 Hz bandwidth.

Elecraft K2: Upper trace is start of sidetone and
lower trace is start of RF ~4 ms later.
The Elecraft K2 without the DSP audio processor is a 100% analog transceiver and has much less delay. The delay from sidetone to RF is about 4 ms.

Second, the delay from RF to audio appears is about 8 ms giving a total of 12 ms additional delay to take into account.

Elecraft K2: Lower trace shows RF and
upper trace start of AF output some 8 ms later.

I would expect other transceivers to have similar delays, that the delay will vary from one type of transceiver to the other, and even with settings of e.g. filter parameters for a single transceiver. 

The post "Audio delay - the unspecified parameter for ham radio transceivers" first appeared on the LA3ZA Radio & Electronics Blog.

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