CWTD Sept 18, 2012

September 18, 2012

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Analyze This! ... The Desert Ratt 2
and other "Regen" Receivers

One of the easiest and best performing receivers to build, the "regen" offers a unique and incomparable operating experience.
Once you hear the characteristics whistles and pops when tuning in a station, you'll think of your grandfather's radio in his den while
you lay listening to the 1939 Yanks pound the Philadelphia Athletics. And you'll also hear dynamic range and crystal clarity that is hard
to duplicate with others receivers ... even today!

Overview

Many of us have built transmitters - QRP or otherwise - but how many have built a receiver? It's actually not too difficult to do it with some of the integrated circuits around these days, Gilbert mixers like the SA512 and the like. But have you ever considered whipping up a "regen" receiver? What a ball it can be and you'll impress the neighborhood teen (and yourself) with how simple it can be, yet still give incredible listening pleasure.

This episode of CWTD will overview the basics of a regenerative receiver, and go through some classic designs that are really fun and easy to build. And per usual, we spotlight one specific design, that of the Desert Ratt by designer and homebrewer extraordinaire Paul Harden NA5N, and we'll analyze it from the ground up ... with the detail and discussion perhaps you'll even catch the bug to build your own regen!

73, George N2APB & Joe N2CX

Audio Recording ... (Listen to the MP3 podcast)

Discussion Notes:

<20:05:24> "Paul - wa0rse": I have a box of tubes that I'd like to put to use!
<20:07:28> "George - N2APB": Loooooove tubes here!
<20:09:32> "Paul - wa0rse": :) I got it...
<20:09:55> "Joe N2CX": Edwin Armstrong
<20:10:34> "Paul - wa0rse": My first transmitter was a single 6L6 crystal-controlled loaner from my Elmer, W0AUS.
<20:13:24> "Joe N2CX": It makes a distinctive sound when it "bloops" into oscillation hence their moniker of "blooper"
<20:13:29> "Paul - wa0rse": No, but I wanna!
<20:14:16> "Paul - wa0rse": My first receiver was a crystal set with a cat's whisker :(
<20:16:01> "Paul - wa0rse": Whoa! That's cool, Rich!
<20:16:10> "Dave AA3UR": My first: Knight Kit Span Master (top of page) http://www.dxing.com/rx/span.htm
<20:20:08> "Alan W2AEW": would you call it a homodyne?
<20:20:48> "Pete - WB2QLL": Corrrectly designed regens don't "bloop".
<20:28:07> "Joe N2CX": A homodyne is somewhat different - search for the homodyne wiki
<20:29:13> "Joe N2CX": Oops the wiki sez that the dc rx is a homodyne...
<20:33:00> "Joe N2CX": Small correction adding an RF amp does not make a superregen
<20:37:41> "Joe N2CX": Hum modulation is discussed as the last bullet in "Regen Negatives"
<20:43:19> "John ZL1AZS": Another resource: http://aa7ee.wordpress.com/tag/wbr-regen/
<20:45:48> "Joe N2CX": Plus the regen control is like a manual choke!
<21:04:11> "George - N2APB": Moving hand around the antenna can pull the oscillator if not properly isolated.
<21:04:44> "George - N2APB": Q2 is a common-emitter design.
<21:05:03> "George - N2APB": Stable 3V regulated voltage helps with stability.
<21:05:33> "George - N2APB": Regen controlled by varying a resistor in the emitter to control the amount of current in the circuit, and hence the degree of oscillation.
<21:06:29> "George - N2APB": The varicaps vary from 8-90pF, by providing a variable voltage 0-9V with the tuning pots.
<21:07:06> "George - N2APB": Q3 is an emitter follower used to isolate the detectors from the oscillator Q2 ... otherwise direct connection would "pulls" the oscillator unpredictably.
<21:08:21> "George - N2APB": Q4 is used as a splitter transistor to present differential signals to the op amp, thus doubling the drive level to the amp.
<21:08:56> "George - N2APB": Sometimes need to be careful about putting a speaker on the front panel because the vibrations will affect the regen stage!
<21:09:06> "Alan W2AEW": 1N4001's used as varactors - interesting
<21:11:48> "Al K8AXW": https://www.google.com/search?q=desert+ratt+regenerative+receiver&hl=en&rls=com.microsoft:en-US&prmd=imvns&tbm=isch&tbo=u&source=univ&sa=X&ei=qBtZUKarDob20gGu8YDQBQ&ved=0CCAQsAQ&biw=887&bih=416
<21:12:26> "Frank N3PUU": I find a bunch of pictures of the 3 (some on njqrp), but not a 2 yet
<21:12:58> "Ted - AB8FJ": The build on QRP-Tech was the WBR design published in QST a few years ago.
<21:13:19> "Ted - AB8FJ": I built it using the board from FAR Circuits and found the same problems with very low audio output
<21:13:46> "Ted - AB8FJ": Seems that there was speculation that a kit for the Desert Ratt was going to be offered. Did that ever happen?
<21:15:21> "Alan W2AEW": but the turn count would have to change
<21:17:44> "Frank N3PUU": Here's a built desert ratt 2: http://www.aoc.nrao.edu/~pharden/hobby/DR2_PIX1.pdf
<21:20:02> "Kevin - kb3omm": Possibly using the copper as an electrostatic shield between the primary and secondary
<21:23:43> "Pete - WB2QLL": Looks like a toroid on the 3 here:
<21:23:44> "Pete - WB2QLL": http://www.njqrp.org/qhbextra/2/2a.html
<21:26:52> "Craig -KC2LFI": Like to see a good regenerodyne circuit.
<21:32:47> "Ted - AB8FJ": Great topic. Thanks guys!
<21:32:48> "Kevin - kb3omm": 73
<21:33:18> "Terry WB4JFI": Thanks for all the work Joe and George. 73
<21:33:26> "Charles WC5MC": Really fun topic tonight
<21:33:27> "Frank N3PUU": great job guys, that was a good session.. 73!
<21:33:33> "Todd K7TFC": Yes, many, many thanks!

SESSION NOTES .... Analyze This! ... The Desert Ratt 2, and Other "Regen" Receivers

"A regenerative radio receiver is unsurpassed in comparable simplicity, weak signal reception, inherent noise-limiting and agc action and, freedom from overloading and spurious responses. The regenerative radio receiver or, even super-regenerative radio receiver or, "regen" if you prefer, are basically oscillating detector receivers. They are simple detectors which may be used for cw or ssb when adjusted for oscillation or a-m phone when set just below point of oscillation. In contrast direct conversion receivers use a separate heterodyne oscillator to produce a signal. " ... Ian Purdie, VK2TIP, as an intro to the Charles Kitchin N1TEV regen receiver project.

The Twinplex design ... a simple 1-tube regen with superb performance for the simplicity offered.
(See Greg Latta's beautiful AA8V Twinplex web page ... http://faculty.frostburg.edu/phys/latta/ee/twinplex/twinplex.html

Background

Regens were used in commercial and ham receivers form the 1920s to the early '30s ... easy, cheap, great performers
After the Great Depression, easier to buy superhets ... Regens were relegated to poor-performing "introductory sets for beginners"
The by the '60s and '70s, direct-conversion receivers became vogue and remain so to this day

Basics

An oscillating detector heterodynes (mixes with) an incoming RF signal at roughly the same frequency
It outputs the baseband audio, and also serves as a very high-gain amplifier and Q-multiplier
So ... a regenerative circuit oscillates, heterodynes, multiplies Q and amplifies all at the same time!
The use of positive feedback provide audio output of several hundred millivolts
Hi-Q means the regen stage is very selective to start with ... so BPF needed before it
Regens can detect nearly all types of modulated signals ... AM, CW, SSB, Digital and FM
Few parts = easier to build, cost less, consume less power
High-quality audio is possible from a regen ... low distortion, wide BW, low noise characteristics surpass the quality of most superhets

More Regen Positives

Selectivity is fully under operator control ... even allowing reception of FM!
Variable selectivity permits operator to optimize the slope of the receiver's amplitude vs. frequency characteristic. (Versus the fixed, high selectivity of most superhets prevents them from effectively demodulating FM signals by slope detection.)
Regen radios operate differently depending on whether they are adjusted above or below the oscillation threshold.
For AM reception, regen adjustment is made just to the oscillation threshold, yielding the best sensitivity and selectivity. Frequent adjustments are necessary.
For SSB and CW reception, the detector is set to oscillate and the regen should be tuned away from the carrier in order to produce a beat note. (An oscillating detector is far more sensitive than any other.)

Regen Negatives

RF leakage is possible out the antenna ... The set may need to have some sort of isolation to prevent radiating the oscillator signal!
"Blocking" = when the oscillating detector locks onto a strong incoming RF signal ... A variable input attenuator can minimize this condition
Severe blocking may prevent a regen from being used next to a high power transmitter (e.g., as a keying monitor) ... a separate sidetone oscillator on the transmitter can solve this one.
Hum modulation at higher HF frequencies (>14 MHz). When used in oscillating mode (e.g., for CW and SSB reception), the RF output from the oscillator gets back into the antenna to mix with the incoming RF signal ... Again, good front end isolation can help.

Regens are similar to Direct-Conversion Receivers

Both mix an oscillator at the receive frequency with the incoming RF signal to yield the audio output signal
The D-C design gets its selectivity from the front end filtering/amp stages preceding the mixer, and thus is limited to a given band(s) by those front end components.
And the audio gain from a D-C set comes from amplification after the mixer
The regen design has the mixing, selectivity and amplification all built into the same stage, offering better possible performance ... the gain and selectivity are multiplied 1000-fold during the mixing process! (Although the gain and selectivity can drop dramatically in the presence of high RF input signals.)
But the D-C radio is easier to operate than the regen.
Good analogy ... An automatic transmission car (the D-C receiver) is very easy to operate, while the manual transmission car (the regen receiver) needs good design and has several tricky controls to skillfully operate, but is capable of far better performance.

Three Classic Books on Building and Understanding Regen Radios

And the best of them all ... by Bruce Vaughn, NR5Q:
(available from Electric Radio ... http://www.ermag.com/ )