ANTENTOP- 01- 2010, # 012

Some Thoughts on Regenerative Receivers

Plug-in coils -- the classic solution, but clumsy to do, and you have to have some place to conveniently store the un-used coils. My own early sets used these, mostly because I could buy a nice set of short wave and broadcast band coils, versus winding my own coils. If you note pictures of early commercial sets for marine use, there was a coil box near the receiver. The "Sierra" QRP transceiver suffers from this problem.

A design that uses only a single coil in the tuned circuit such as the Colpitts design is one such solution that greatly reduces the complexity of band switching. Of course, a Hartley oscillator design uses a single tap on the coil, but again, this is an additional source of complexity due to the tap on the coil. The Colpitts design does require a DC return for the cathode in the case of vacuum tubes or source in the case of FET designs.

Plug- In Coils at Radio "Sierra"

Credit Line: http://www.w0ch.net/sierra/sierra.htm

By the way, FETs have an advantage as they are high input impedance devices like vacuum tubes, and do not load the tuned circuit to any degree. Both of the two commercial regenerative receivers I have use FETs in the detector and isolation amplifier. The MFJ-8100 has a series of inductors in series with a simple single pole selector switch. The Ten-Tec 1253 uses nine separate inductors, but switches them with a clever PIN diode and counter circuit. Both of these designs use an isolation "front end," which eases the complexity of coupling into the tuned detector circuit.

It is generally agreed that designing a receiver for a single or small range of frequencies is much easier than developing a general coverage receiver. If you can get a 10:1 range of capacitance change with a variable capacitor, for example, then you can expect about a 3:1 tuning range (square root of the ratio of the maximum to minimum capacitance). This problem rears its ugly head especially in the case of varactor tuning, where the capacitance change range is more modest. More about this later, with a couple of "real life" receiver designs.

The usual approach to fine tuning, when using a variable capacitor is to have a "band set" capacitor and then a "band spread" capacitor of about 10% of the value of the main capacitor.

To save money, an alternate approach is to use some sort of mechanical vernier reduction drive, such as the MFJ-8100. With varactor tuning, a secondary variable voltage source with a smaller voltage variation range serves the same purpose. Kitchin used a combination of varactor (fine) and variable capacitor (coarse) tuning in one of his designs.

If we are willing to accept a modest degree of increase in complexity, the so-called "Regenodyne" receiver is a good choice as a solution to the difficulties of developing a wide coverage receiver. A Google search will turn up some extensive discussions of this concept.

The idea here is to have a regenerative receiver of modest tuning range as a "back end" to a superhetrodyne front end, which converts a series of band segments with a crystal oscillator with a series of switched crystal frequencies. A somewhat similar line of attack was used with the "Q Multiplier" tuned/regenerative IF system on early vacuum tube general coverage receivers. This was available from Heathkit. I had one on a Hallicrafters S-20R, which was quite effective. The front end can be pretty broad, and only need remove the image response.

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