Sunday, July 29, 2007
The phasing version receiver is a little more complex, and introduces phase shifting circuitry. Since we will be using one of the 500kHz resonators for L.O. frequency control it is not possible to run the L.O. at 4X the desired injection frequency and then use a pair of divide-by-2 Flip-Flops to generate the quadrature phase. While not a show-stopper issue, this does introduce some interesting problems. 500 kHz is way too low a frequency to allow use of tuned coaxial stubs to get a 90 degree phase shift. But, since this is a fixed-frequency receiver we can probably use LC networks for the phasers. EMRFD, Page 9.14, shows how this might be accomplished by using over-coupled LC transformers. This provides a rather simple method for deriving the needed phase shift at the L.O. operating frequency.
This blog starts a new Ham Radio project, and also initiates a new experiment for the integration of Web Pages and Blog Site content. The intent of this integration is to make a web page that supports interaction by readers, using a blog as the vehicle for reader correspondence and comments. Hopefully we can use this vehicle to work together in the design of yet another electronics project for Amateur Radio use.
This particular DC Receiver Building Block project started when I included a couple of the "Grab Bag" items on my order to Dans Small Parts and Kits. As usual Dan sent me more than my money's worth of electronic components, including a large number of 500 kHz ceramic resonators. After using one of these to build the requisite Ceramic Resonator VXO Frequency Standard (ceramic resonator and 74HC4066 binary divider chain) I still have a bunch of 500 kHz resonators and feel a need to do something more with them.
After some consideration, and inputs from Hans G0UPL, Ian K3IMW, and Loman W6NM, it seems possible to design and build a 500 kHz fixed frequency DC receiver that uses these resonators for front-end bandwidth control and as the L.O. for an efficient Switched Capacitor Detector (SCD). The result should be a fixed-frequency Input Filter, Detector, and Audio section for use with tunable converters to cover any LF, MF, HF, or VHF band. However, this Web/Blog is only about the fixed frequency DC receiver part. You will have to come up with your own tunable converters.
There are several directions that design of this DC receiver block can take. The simplest is probably just a single-ended SCD conversion to audio, but it also seems possible and potentially very interesting to make this into a phasing receiver using either the Dan Tayloe active phasing method or Hans Summers passive polyphase method, or the phasing receive techniques shown in EMRFD (Experimental Methods for Radio Frequency Design).
At present I do have block diagrams drawn up for both simple and phasing methods, but am having problems with insertion of images at specific locations in this Google Blog posting. Either I will solve this problem, or the blog part of this web page will be moved to the livejournal.com blog engine.
Arv - K7HKL