Regenerative Receivers


Regenerative Receivers

Regenerative receivers provide a surprising level of performance with only a handful of
components. They excel at receiving amplitude modulated signals from below the AM
broadcast band up to the higher short-wave bands above which the superregenerative
detector becomes the better choice. Many designs for regenerative receivers are available
and most will do a fine job. The regen is basically an oscillator circuit with a gain
control that allows the user to adjust the feedback to a point just below oscillation or,
quite often, just above the critical level such that a small oscillation is present.
  The typical regen uses a tapped coil or additional windings to connect into the
tuning tank and the tuning capacitor provides the total tank capacitance.  The
advantage to this approach is that the tuning range is maximized since there are no fixed
capacitors contributing to the tank. The disadvantage is that special, hand-made coils are
required. The regens shown below use capacitive taps to achieve the required feedback and,
as would be expected, the tuning range is restricted to about two to one. For example, the
AM broadcast band would require two inductors to tune, perhaps from 500kHz to 1MHz and
900kHz  MHz to 1.8MHz.  This disadvantage really isn’t all that important when
one considers the advantage of being able to use factory-made molded inductors! If a
multi-band receiver is constructed, there will be a couple more band switch positions.
Actually, the lower tuning range makes fine-tuning a little easier which is critical for
achieving the maximum performance.

The basic receiver is shown below. The components are not critical and the values were
pretty much the first ones found on the bench that were near the "right" value
so don’t hesitate to experiment. The transistor could be just about any small-signal NPN
including the 2N4401, 2N3904, 2N2222, or others. The audio output is fairly weak and will
need an amplifier to drive headphones or a speaker. See the audio amplifier page for
suitable amps. 

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Schematic

The receiver was constructed on a piece of painted MDF (medium-density
fiberboard). This material is great for little projects like this and it is readily
available from home improvement stores. Look for the lumber with dull white primer and a
very fine "grain".  The solder connections were made with solder lugs
secured with 1/2" wood screws (see close-up). Predrilling the screw holes is
recommended.

The inductor is the highest component in the close-up and it plugs into
tiny sockets for experimentation. This inductor could be selected with a double-pole
multi-position switch for a multi-band receiver. A 220uHy will tune the lower part of the
AM band from about 570kHz to 1.15Mhz,  a 5.6uHy will tune from about 3.5MHz to 7.5MHz
and a 2.2uH will tune from about 5.6MHz to near 11.6MHz. For calculating inductors, the
effective tuning capacitance is from about 85pF to 370pF.

Another receiver was constructed with a single-stage audio amplifier
suitable for driving sensitive headphones or a crystal earphone:

Schematic
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The regen control was mounted using an ordinary right-angle bracket from the same home
improvement store. Just drill out one of the holes to accommodate the pot. The audio amp.
does not show a capacitor on the input because there is already one on the receiver’s
output. This amp is nothing special and many substitutes are fine.

When the regen control is set too high, there will be a lot of squealing and whistling
as the radio is tuned. When it is set too low, there will be no sensitivity. There is no
substitute for experience! After a desired station is found, the regen control can be
carefully advanced along with careful fine tuning to get the best results. The regen is
actually oscillating in this mode but it is synchronized to the signal. In fact, with
careful adjustment, a sinewave may be extracted from the collector of the oscillator
transistor which is locked to the radio transmitter’s frequency. I was able to get about
0.1ppm stability from wwv and from local radio stations! When this poor man’s frequency
standard looses lock, the speaker squeals out a warning!

Here are some enhancements from a reader:

Thanks for your great TechLib webpages!

I’ve built your regen receiver and was pleased with its performance.  I’ve added a
few "bells n’ whistles" which enhance its ease of use.

Mods include a bandspread cap, a fine regen control, a BCB Trap, and a simple multiband
coil/switch arrangement which allows it to  tune most of the SW bands (2.5 – 20 MHz),
using WWV signals at the ends of each band (2.5, 5, 10, 20 MHz) in order to simplify
"calibrating" the tuning cap/coil. An SPDT switch simply selects the proper tap
and shorts out unwanted coil turns for each band.

If you can use any of this info, feel free.

Thanks again, for a fun project.

Mike H.

Schematic

Thanks Mike!


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