Designing a 20/40 band CW rig – Part 1

Hi all,

I have been spending some time working on the design of a ultra portable CW rig. I’ve been spending time looking at various kit designs from Steve KD1JV, Ramsey Kits, You Kits, the Elecraft KX1 and others. Some rigs are quite simple, others have a more sophisticated approach.

Goals

In setting down to design a rig, I was wanting something that would work well in a SOTA activation context. I would want the rig to do the following:

  • I did not want to be limited to a narrow band of frequencies around a crystal, but wanted the whole band to be available.
  • The power output needed to be around 5W – full QRP. 500mW CW can be tough going in VK. One attraction with SOTA activations is working the DX chasers, so 5W is nice, while keeping the QRP endorsement intact.
  • I’d like to keep open the option of some digital modes, even if the rig would not do these directly itself.
  • The rig would need RIT.
  • If possible, to switch bands without needing to use external switches.
  • Deliver near 5W from a 12V source, safe for 12.6V – a three series LiPo cell, usable down to 9V.
  • Do all of this in a kit that would cost less than US$200, good, $150 better, $100 best – but no promises!

Overall rig design

The rig could basically be broken down into a number of sub-sections.

  1. Power – a cut-off for low input voltage, and regulators for 5V and 3.3V
  2. Audio frequency – RX audio and sidetone
  3. TX – SSB rigs mix up, but this being a CW rig, I only need to switch an oscillator output and amplify up to 5W. I also need to impedance match for 50ohms at the RF connector
  4. Oscillator – Given my tune the whole band requirement – I’m going to need a PLL and VCO or equivalent circuit, such as a DDS.
  5. RX – Taking the RF connector, I need to either use a relay to switch away the TX, or a transistor switch. I then feed a mixer to an intermediate frequency for filtering, and then can feed a second mixer to audio frequency
  6. Control – Need to use an embedded controller – I am familiar with the PIC line, so I’ll use something there. This will need to set dividers for the PLL or DDS, take button input from the user for tuning, band selection, RIT, provide a keyer facility (I won’t be using a straight key) – with the ability to configure at least the wpm send rate

I’ll go through these in greater detail over a number of posts. I have been influenced by some of Steve KD1JV’s ideas, but also want to do some of my own things. I have already mentioned that I want internal band selection. I also want a volume control, with the rig able to drive at least a 16 ohm speaker, along with low impedance head/ear phones – like the ones supplied with mobile/cell phones. I also want it to be capable of driving 200 ohm headphones.

Above, I floated ideas of how to switch TX/RX. A relay could be used, but if full break-in is going to be supported, relays are a pain. Relays are good for things like band switching – ’cause you don’t switch band 100 times a minute, but to keep the power down, they will need to be latching.

At time of writing this post, I have most of the sub-section circuits designed and thrashed through simulations and a bit of bread boarding. I’ll hopefully put up some posts over the next few days or so looking at TX, the audio section and some of the RX.

This design is going to be largely SMT, but to keep things sane, I’ll use 0805 or larger size components. I could make it thru-hole, but the physical size would be much larger, and it would be significantly more expensive.

73 de Wayne VK3WAM

Continued in Designing a 20/40 band CW rig – Part 2

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One comment on “Designing a 20/40 band CW rig – Part 1

  1. […] This is a continuation of Designing a 20/40 band CW rig – Part 1. […]

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