Both setups work by analyzing the IF output of the transceiver and displaying a portion of the band. The P3 displays up to a 192KHz-wide segment of the band centred on the tuned frequency. The Softrock will do the same with a 192KHz audio interface, but mine is only 96KHz so that's all I get. Elecraft says that a wider slice is going to be available with a future firmware upgrade but that was more than 16 months ago. Mind you, there's a little device called a RTL 2832u which is wideband VHF/UHF SDR receiver that costs around $20 and doesn't require a sound card. It doesn't tune the HF bands so it's been used along with an upconverter (or with a radio that has its IF on the VHF frequencies) to provide a 2MHz wide display. Maybe the P3 may have to wait a little longer.
Construction and Setup
Building the Softrock took about 10 hours, not including a case for which I still need parts. I'm a relatively slow builder I suppose but it worked first time. There are only two relatively simple toroid transformers to wire up, which compared to the Softrock RX-TX I built a year ago was trivial. There are a bunch of surface mount parts - 10 tiny capacitors and 3 integrated circuits. These were installed with a regular fine-tipped temperature controlled soldering iron with very thin solder. It is fairly simple to do. Just apply a tiny bit of solder to one of the pads and then tack the part onto the PC board on that pad with the iron. Then run around and solder the rest of the pins (quickly!) one at a time. I had to redo a pin here and there after inspection and/or power-on tests.
Once built, there are 3 connections to make. It requires the same ~12v power as the rig, a (well-shielded) connection to the rig's IF output, and a stereo connection to the audio interface. Stereo is an absolute requirement for software defined radio to provide image rejection around the Softrock's local oscillator (LO) frequency. The LO is tuned 23KHz below the K3's IF so that it doesn't feed back into the transceiver. The kit comes with no connectors or box, so right now that's patched together and those parts are on order.
Once the Softrock's audio is getting into your computer, you need software to interpret it. There are tons of options, like PowerSDR (a free version of the Flex Radio software), Rocky, etc., but right now my favourite from my RX-TX operating days is HDSDR, although I might revisit the others to see how well they work with this setup. All of these programs are free downloads. Installation and setup of HDRSDR is relatively easy. It needs to be told about the audio interface, and there is a special setup screen for the receiver front-end. That is where all the main adjustments take place.
The Softrock's oscillator is not going to be exactly 23KHz from the K3's IF frequency. Mine was just under 1300Hz off. Furthermore, there are different offsets for each mode, so CW is different from SSB and AM. The exact numbers were determined by experiment, lining up the sound of the signal with the display. Once this alignment is done, you can identify signals visually on the program's waterfall display, tune them in with a single mouse click, and then use the mouse wheel or direct frequency entry to fine-tune the signal either by ear or with the waterfall display. This is excellent for zero beating a CW station quickly to an accuracy of a few hertz.
Because both the IF and the Softrock frequencies are fixed, HDSDR tunes by changing the transceiver's frequency. How is this done? Well the easy way to run the rig control portion of Ham Radio Deluxe. HDSDR can control the frequency and mode of HRD. It also supports a free library called Omni-Rig which I haven't tried (yet). So tuning the radio becomes a point and click operation on the programs waterfall display.
The tuned frequency appears around 23KHz to the left of the centre on the display for reasons mentioned earlier. So on my setup I see about 1/4 of the display below the radio frequency and 3/4 above it. With a 48KHz (vs. my 96Khz) audio interface it would on the extreme left of the display, which I'd find very annoying. Even with the 96KHz audio it is still a bit frustrating that there's not more displayed below the tuned frequency.
On December 28th I used it for the RAC Canada Winter Contest on SSB. It was very effective. Instead of just scanning across the band with the tuning knob I could QSY directly to any signal, judging strength by the intensity of the display. Often I could see who was calling CQ by the regular timing of signals on the waterfall. Fine tuning can be done either with the mouse or the tuning knob. The other thing that works well is using it to find a free spot to call CQ.
99% of my operation has been using the audio from the radio and HDSDR as a tuning aid, but HDSDR can also be used to demodulate the signals, which can then be listened to directly or fed into some type of digital program. I didn't find this necessary but for something like Digital Radio Mondiale where the bandwidth required exceeds the capability of the transceiver it would work fine. I used it a bit for the alignment of frequency offsets in the various modes and you can really notice the delay going through the computer.
There are some quirks that take some getting used to. The biggest was the effect of changing the filter settings on the K3. On CW for instance the filter centre frequency is 600Hz. Changing this shifts the entire waterfall on HDSDR by the same amount, as the K3 actually changes its local oscillator frequency rather than moving the filter frequency, which makes sense because it uses fixed-frequency crystal filters. So I've found that it is best just not to change the centre frequency of the filters.
One thing that is going to drive me crazy is the radio control. Don't get me wrong it works great, but the big challenge is getting all my favourite software to work with it. As long as I can go through Ham Radio Deluxe or Omni-Rig everything is fine, but not everything works with HRD or Omni-Rig. In particular, I wanted to try the N1MM logger because the HRD logger just doesn't cut it for my style of operating. The N1MM logger wants to control the transceiver directly but it doesn't share. Some hams are using LP-Bridge to allow multiple controllers for the K3 which looks promising but it has prohibitive licence terms (even though it is free) and several additional layers of complexity. It also crashed my computer when I tried it with Ham Radio Deluxe.
This little project has cost me about $40 so far ($15 was for a cable at the local music store) and I will need to invest another $15 or so in putting it in a case. Naturally it would have cost more if I'd needed to buy a sound card to go with it. For that small investment this is definitely a keeper, but it won't be long before I try the RTL 2832u with an upconverter.