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Saturday, 22 June 2013

It's Keying Time with CMOS

Well,

My chum Vince, G0ORC bought himself an antique recently in the form of a TS-830S by Kenwood. The radio is solid state except the PA which is a valve design.

The radio doesn't contain a keyer so I said I would have a bash at making something simple. My tendency was to dive straight into developing something software based using a PIC, but first I decided to use some simple logic gates. A quick trawl on the internet found me this site belonging to N1HFX:

http://www.rason.org/Projects/cwkeyer/cwkeyer.htm

The schematic at the link above looked to be exactly what I was looking for. The first step was to try the circuit as a prototype:


This seemed to do roughly what I expected so I built the circuit on some veroboard and stuck it in a box:


Now, once built and with a paddle connected I found if I leant on the dot or dash paddle, after a continuous stream of dots or dashes the keyer would "miss a beat" in that there would be an unexpected gap or change in the timing. Trying to investigate this problem I attached a 10x scope probe to the clock output - this resulted in the clock slowing down but the keyer never miss behaving! I figured the only thing this could actually be doing was introducing some capacitance to the clock output which seemed to be making the circuit more inherently stable.  I decided to measure the capacitance that the probe was introducing into the circuit:


This was quite interesting as 20pF is certainly enough to de-tune RF circuitry and was clearly effecting the running of this clock...So all I did was modify the schematic as follows:


I've added 18pF to the output and sped up the clock by reducing the R in the timing components.

Here's the end result in it's new home:



Fun though, egh?

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