[Elecraft] Technical question on keying of an SDR radio

[Wayne Burdick]

Hi Phil,

In the K2 the sidetone starts off as a squarewave created by the microcontroller. This is then shaped and injected into the AF amplifier. The sidetone is turned on/off at the same time as the carrier, which is generated by the LO (PLL synth) mixing with a gated TX BFO signal.

In our DSP-based radios (K3/K3S and KX2/KX3), both the audio sidetone and the 15 kHz 2nd IF carrier are created by the DSP. Their rising edges are shaped mathematically using what's called a "raised cosine" or sigmodal waveform. We experimented to find the ideal waveform equation, the result being the cleaned signal possible, with virtually no key clicks. The DSP can of course do other things like apply amplitude or frequency modulation, generate voice and data mode signals, apply ALC and audio EQ, etc. CW is just the simplest case of what can be done.

From the DSP, the digitized audio signal codes to a DAC (part of the audio CODEC IC), which then converts it to analog for injection into the AF amp. 

The 15 kHz 2nd IF carrier in the K3/K3S goes to a transmit mixer on the KREF4 module where it's up-converted to about 8215 kHz. It is then routed to the first IF mixer. The 8215 kHz signal passes through two crystal filters enroute mixer. In the KX2/KX3, the 15 kHz IF signal is converted to a pair of IQ signals (90 degrees out of phase with each other) to directly modulate a quadrature up-converter. The other input to the up-converter is a pair of balanced LO signals, again separated by 90 degrees. A result of IQ modulation is that one sideband is cancelled out, resulting in a single RF carrier at the desired frequency.

In all cases, the rest of the T/R sequencing involves the usual amplifiers, PIN diodes, filter switching, etc.

73,
Wayne
N6KR



> On Apr 7, 2019, at 9:58 PM, Phil Hystad via Elecraft <elecraft at mailman.qth.net> wrote:
> 
> If you look at various old circuit diagrams of ancient radios made from electronic circuits you find that the telegraph keying for the radio involves switching on and off the driving oscillator frequency or something similar.
> 
> However, with an SDR, you have other options and I have no idea how it is typically done.  For example, a digital signal can be produced that represents the Morse coding and this signal need merely be converted to analog and amplified.  Or, is there still an analog circuit being switched on and off for Morse code (telegraphy) keying with a modern SDR.
> 
> Can someone describe how this is done in a radio like the KX2.  And, is it done differently in the different Elecraft radios — I presume that the K2 would have the more traditional sort of circuit but that is really just a wild guess on my part.
> 
> 73, phil, K7PEH
> 
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