If you go back through lots of amateur radio designs, you’ll find many, many circuits that use the nearly uniquitous 365pf air spaced variale capacitors that were nearly ubiquitous up until about 25 years ago. In the last couple of decades however, they have become like Avatar’s unobtanium, seemingly impossible (or at least expensive) to find. One solution to this problem is to use varactors controlled using variable resistors (which are still relatively easy to find) but another interesting technique is to build your own variable inductors. Hence, was born the PTO (permeability tuned oscillator), a nifty little homebrew circuit and gizmo that can provide a variable frequency oscillator. M1KTA talks about building one of his own:
M1KTA’s QRP ham radio blog: PTO VFO.
I’ve had this in the back of my head as an interesting project, so seeing notes on someone building one is inspiring.
Over on NT7S’s Ripples in the Ether blog, he presents a link to a project by Joachim, DL1GSJ, a very nifty little SDR designed to operate near the 30m QRSS watering hole frequncy. It uses pair of subharmonic mixers, whose operation I admit I don’t completely understand, but I’m bookmarking the circuit for later consumption.
de draaggolf ~~~~~: 30m Subharmonic I/Q-SDR Receiver.
Well, I’m not sure that the Softrock I put together is working entirely well. I’m beginning to believe that the transformer that I wound might be bad. My receiver seems a bit deaf, and also seems to have only about 20db or so of opposite side rejection (I’m getting images of signals on both sides of the center carrier frequency). I’m a bit stymied by the fact that I only have my macbook in a place where it can be used to decode the softrock, and the software for it is, well, to a first approximation, there isn’t any. I did a bit of hacking, and took a recording I made and extracted this Morse code recording, which I’ll archive here as the first that I ever got.
Morse via my Softrock
I’ll have to work on it some more in the future, but for now, I’m back to beaconing with my FT-817.
My dummy load experiment still has a few unanswered questions, but I found that the ARRL Handbook has had a circuit which is basically what I built, minus the one series resistor. It looks like this:
Okay, they use a germanium diode with considerably lower voltage drop, and include a 4.7M ohm resistor in series to the DVM. This circuit also appears on the QRPEDIA posting on RF probes, and apparently is constructed as part of the assembly of the Elecraft K2.
Here’s a posting talking about how the different diodes make for different performance.
Diodes for RF Probes.