Ceramic Filter Units

Some time ago I ordered a package of 500 KHz ceramic filters from Dans Small Parts and Kits , and at the same time also ordered two of his grab bag kits. When the order arrived I found that among other things, the grab bags contained a great many 500 KHz filters (Dan must have thought I really needed 500 KHz filters!).

A hand full of these filters was sent to Ian K3IMW, and together we have been playing with them to get a better feel for their characteristics as both filters and as crystal replacements in oscillator circuits:

• The resonant impedance of 500 KHz ceramic resonators seems quite low.
  
  
• These devices do not require very much feedback drive when used in oscillator circuits. In fact, if you provide too much drive they tend to oscillate on overtone frequencies...sometimes multiple frequencies at the same time.
  
  
• With minimal drive in oscillator circuits these resonators seem to have stability characteristics that approach that of more traditional crystal resonators.
  
  
• By putting 2 ot 3 of these units in parallel in a VXO arrangement it is possible to pull the operating frequency by 30 or more KHz.
  
  
• With 4 of these resonators in a ladder filter configuration, it required 0.002 mfd of capacitance to make up a respectable 3.2 KHz wide IF filter. By adding even more capacitance we were able to make much narrower filters for CW operation. Values of 0.004 mfd resulted in quite nice sounding filters of around 1.4 KHz bandwidth.

As a result of these tests, it now seems that the 500 KHz ceramic resonators will play a significant role in design of my 500 KHz DC receiver building blocks.

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 www.qrpkits.com, 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.
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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.

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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.
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DC Receiver Building Blocks - - K7HKL

Hello

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.

More soon...

Arv - K7HKL
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