Monday, September 3, 2007

Temporary Delay

I have not gone away and I'm not deliberately ignoring this blog. It's just that work on the BITX20A project has taken priority for a few weeks. We are at the point with that Kit Project where Doug - KI6DS will be adding it to his list of items on, probably today. This involves some loose ends that need to be taken care of before I can get back to focusing on the 500 KHz block receiver project.

Thursday, August 2, 2007

Switched Capacitor Detector

Thinking about design of the SCD (Switched Capacitor Detector). My initial idea was to use a 74HC4053 with all switch sections in parallel for the switch. After looking at the datasheet it seems that ON-resistance of these devices is 70 ohms per switch. Thus, with all three sections in parallel the ON-resistance would still be in excess of 20 ohms. That seems quite high for a detector that will probably have it's RF input at 50 ohms impedance. Also, the per-switch capacitance of a 74HC4053 is roughly 3.5 pf per section, so the feed-thru capacitance of all gates in parallel when the gates are OFF would be around 10 to 12 pf. Admittedly this will be operating at 500 kHz, but even 11 pf is significant and could cause some signal degradation.

This brings me to the possible option of using a plain old J-FET or MOSFET as the switch mechanism. Since the Antenna input port will be offset to Vcc/2, I can connect the FET as a voltage follower (drain to charge capacitor, base to ground via a 10K or so resistor with VFO input coupling, and source to input RF port). That should provide a better switch mechanism than the 74HC4053. Some experimentation will prove if I an right or wrong.

Hmmm...wonder if you can use two of these switches and charge-capacitors in series and get the combined roll-off effect from both caps. If workable it might be an easy way to make a better LPF function ahead of the audio amplifier.


Sunday, July 29, 2007

Phasing Receiver Block Diagram

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.

DC Receiver Building Blocks - - K7HKL


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 blog engine.

More soon...

Arv - K7HKL