New Year; new project

This past year I have finally come very close to completing my Elecraft K2 (and most of its options), which has been a learning experience. While soldering the multitude of parts to the PC boards can be mind-numbing, the steps involving check-out of the rig help teach a bit of theory. Earning an Extra Class license requires an understanding of the theory involved; however, actually applying it to a project helps cement the concepts in the mind.

In 2010 I will be traveling a bit for my job and may find myself in lonely hotel rooms at night. Rather than spend the evenings flipping through channels on the television, I would prefer to get on the air and make some contacts. Given the fact that I travel with a laptop computer and often travel light, I thought it would make the most sense to try and bring along a transceiver which would allow me to get on the air and operate some of the digital modes. Dave Benson’s PSK-xx kits fit the bill.

I chose the PSK-20 because of the high activity on 20m PSK31 and because the other kits are for bands which would require an antenna of greater size than I would prefer to manage on business travel (PSK-30 on 30m, PSK-40 on 40m, The Warbler on 80m).

I’ve previously operated PSK31 on my Yaesu FT-817D with a Tigertronics SignaLink interface and been quite successful, earning the QRP-ARCI 1000 Miles Per Watt award (see post here about those adventures). Using a transceiver with a built-in interface would save space and weight when they are at a premium.

I ordered a PSK-20 rig in the summer of 2009 and finally received it in the fall, but have yet to begin the project. With increased confidence in my homebrewing abilities and greater understanding of theory, I have set out to modify the PSK-20 kit to tune the entire 20m band and allow me to reach the calling frequencies of digital sound card modes other than PSK31.

The PSK-20 rig itself is an interesting rig. It has a ~4KHz bandwidth centered around the PSK31 frequency for 20m (14.070MHz) and operates on the non-traditional lower sideband (LSB). The sideband is of no consequence for some variants of PSK31 (e.g., BPSK) but matters more for others (e.g., QPSK).

According to Dave’s documentation about the receiver…

U1 is fed with a 5.07 Mhz Local Oscillator (LO) signal and converts the incoming 14.07 Mhz received signal to the 9.00 MHz Intermediate Frequency (IF).

and

Product detector U2 receives the 9 Mhz IF filter output and multiplies (mixes) it with a ~ 9 Mhz LO signal.

Also (for the transmitter side of things)…

Single-sideband filtering is performed by a second filter comprising Y7-Y10 and related capacitors. After passing through the transmit IF filter comprising Y7-Y10 and related components, the signal has been reduced to a 9 MHz SSB signal. Its output drives 2nd mixer U7. This mixer is also driven by a signal from the 5.07 MHz LO, Colpitts oscillator Q9 and associated components.

The output of this mixer consists primarily of both the sum (14.07 MHz) and difference (3.93 Mhz) frequencies applied to the mixer. Q7 is an emitter follower used to buffer the high-impedance output of the mixer. U8 is a Monolithic Microwave IC (MMIC) and provides approximately 12 dB of gain.

My plan, as it currently stands:

1. Pull the 9.0MHz local oscillator LSB signal to USB by adding a level of inductance in line with the trimmer capacitor already on the board
2. Replace the 5.068MHz crystal and its supporting circuit with a frequency-agile signal generator
3. Mount the entire rig, including a digital frequency readout, in a Ten-Tec enclosure (the enclosure that Small Wonder Labs provides for the PSK-xx rigs is not large enough to accommodate the size of the digital frequency readout and dial)

In order to make this a fully portable digital station, I’ll need to bring along an antenna (I’m thinking either a 1/2 wave wire antenna cut for 20m) and perhaps an antenna tuner. The tuner, should I need it, will be filled by my Elecraft T1. It offers small size and excellent performance that will be appreciated when packing my bags for my business trips.

Should be an exciting few weeks. My first trip will occur in the third week of January, and I hope to take the rig along with me. First, to construct the signal generator of choice–the Si570 Controller and Frequency Generator Kit #2 by K5BCQ and K5JHF.