The "40 Meter Tweeter"
A Regen Receiver Designed Exclusively for 40m CW listening
by Rick Andersen, KE3IJ, May 25, 2005
How this design came about
My own prototype works just fine, so I wasn't always sure what answer to give when an inquirer emailed and asked why his clone didn't work right.
Just as I was considering whether I shouldn't just pull that article from the pages, Rob, KB3BYT, emailed me in early May and told me he'd like me to consider designing a new 40 meter receiver made just for listening to CW. Rob, like most hams, is into all kinds of fun stuff (see his web page at http://www.learnmorsecode.com) and had an enthusiasm level that made mine look like that of a snail in a salt mine, but he laid down a good challenge and I accepted:
Could I build him a simple, duplicatable, single-band receiver for CW only, that had no "exotic" parts in it, that had an internal audio amp and speaker, and that could pick up that 40m Morse Code for his Boy Scouts with nothing more than a whip antenna?
After wondering what I'd gotten myself into, I sat down and got to work. My plan was to use the "Universal Regen", found elsewhere in these pages, as my starting point. Using PNP transistors instead of NPN because I happened to have a lot of them around, and because I wanted to be able to ground the toroids in the circuit rather than tie them up to +Vcc -- Rob didn't care either way, as long as they were common, available transistors (could be done with either type, plus some shuffling of component positions in the circuit), and several days of futzing and kludgeing, I came up with the circuit below:
Clipping a 2 - 4 foot piece of wire onto the whip will improve reception considerably, even though doing this will also spoil the high Q of the LC tank somewhat. You'll also find a nice improvement in sensitivity if you run another wire from the chassis ground to a grounded point in your home (the little screw holding the electrical switch plate or outlet plate makes a good "trick" ground). You'll also greatly reduce the phenomenon of body capacitance (you reach toward the receiver to tune it and notice that the tuning is pulled off-frequency by the nearness of your hand) when the receiver is grounded.
If an external antenna is desired, I recommend you install a jack on the back panel of the receiver chassis, and either
1) run the center conductor of that antenna jack to a small value ceramic capacitor (something like 10 pF) whose other lead connects to the bottom of the whip / top of the first LC tank, or, alternatively,
2) add 2 - 3 turns (a new, separate winding) through the center of the first LC tank's toroid, making a primary whose wires are connected to the antenna jack and circuit ground.
The "L" of the LC tank is about 23 turns of #28 enamelled copper wire on an Amidon T-50-2 toroid. The secondary, 10 turns, steps down the high impedance of the resonant tank to the 600 ohms or so of the input impedance seen at the base of Q1, the RF input amplifier. This matching helps prevent the transistor from loading down the LC tank and lowering its Q.
The RF amplifier Q1 is a common emitter amp with collector feedback bias and a collector load resistor of 1.5K, at which point the amplified RF is ready to be delivered to the regenerative detector stage's LC tank... which I do with what old-timers know as a gimmick capacitor -- you simply take two pieces of insulated wire and twist them together so that they're in close physical contact but there's no actual electrical connection between them. I did this because I didn't have very many small-value ceramic caps lying around; you can substitute a ceramic cap somewhere between 3-10 pF in value and it will do the job.
The point is that we don't want to load down the Q of the regen detector's LC tank, or overload the sensitive regen with too strong an input signal, or allow any of the oscillating regen's signal to get back out to the antenna and radiate -- so we use very light coupling. So if you're one of those people who hates gimmicks, (like those "Bonus Cards" they give you in supermarkets so they can fool you into thinking you're getting a discount, when they should just have low prices in the first place!) use a cap instead;-)
Q2, another "upside-down" PNP transistor, is the heart of the receiver. Like my "Universal" regen circuit seen elsewhere in these pages, it's a Colpitts-style oscillator -- and this one really is an oscillator, in that I have it rigged so that it is ALWAYS in its oscillating mode, for "Beat Frequency Oscillator" action with CW Morse Code signals [or with SSB if you re-tune it higher in the band]. Since Rob requested that this radio be designed specifically to copy CW, why have a REGEN control at all? Just bias the transistor such that it's always oscillating, and our Regen is acting like a Direct Conversion receiver. The biasing is accomplished by the two 3.3K resistors on Q2's base.
Notice that the second toroid, also having about 23 turns, is the "L" of the LC tank for the regen detector; it is paralleled by a screwdriver-adjust trim capacitor that mounts on the copper ground plane circuit board (not on the front panel). This cap sets you inside the 40 meter band, when calibrated properly (we'll tackle that later in this article).
Q2 gets its feedback path through a 22 pF ceramic cap across its collector and emitter. Try to use an "NPO" type to prevent detuning due to changes in ambient temperature.
The RF is filtered out of the audio by a .047uF cap in the resistor network in Q2's emitter circuit; from there the audio is sent on to a 741 operational amplifier IC which acts as a preamp before the audio is power amplified by an LM386 IC and sent to a 3" speaker that sits behind holes drilled in the front panel.
Back to the Q2 detector circuit: Notice that its power lead is marked "Vz". Elsewhere in the receiver there's a 270 ohm resistor feeding a 6.8v zener diode and a 100uF electrolytic capacitor; this is a voltage regulator circuit that drops the 9v battery level down to a well-regulated and filtered 6.8v for the detector. I found this circuit to be absolutely necessary to prevent the detector oscillator from continuously drifting in frequency, to the point that calibration would be lost after a minute or so of listening.
Note that the circuit so far described, up to the trimmer cap, is in itself a complete regenerative detector/tuner..... but it tunes way too "fast" and wide in frequency range. What we do now is put a varactor diode in parallel with the band-setting trim cap; if we vary the reverse DC bias on that varactor, its capacitance changes by several picoFarads and we now have a "fine tuning" control that we re-name the "Main Tuning" control, and we're doing it with a cheap, available 10K potentiometer instead of a hard-to-find mechanical tuning cap!
In my circuit we don't look for a fancy varactor (they make 'em, but they're a little bit "exotic" and Rob wanted simplicity and easy availability of components); instead we use a common 1N4001 rectifier diode, which works just as well and gives more tuning range than we care to have at 7 MHz.
Notice the 15K ohm resistor in the bottom leg of the Tuning pot: With that value, we limit the tuning of this receiver to about 30 KHz. That may not sound like much, but if you calibrate your receiver so that the bottom of the dial is around 7020 KHz, you'll be able to tune up to 7050, covering the QRP frequencies of 7030 and 7040 with ease. (You will have to retune the trimmer cap for the Novice band.) If you decide you want a wider tuning range, lower that 15K to something else (say, 10K or 4.7k) and you'll get more range but "faster" tuning (the stations will sweep by faster and it will take a steadier hand to tune them in).
The audio circuit, as mentioned earlier, consists of a 741 op amp in a non-inverting, single-supply configuration, with a voltage gain of about 213. A 330 pF ceramic cap across the 1 Megohm feedback resistor provides some low-pass filtering to mellow out the otherwise piercing CW notes at high frequencies.
The output of the 741 preamp is AC-coupled to another pot, which acts as a VOLUME control; it too is bypassed with a small cap to soften the frequency response of the audio amp. The Volume pot is combined with the ON/OFF switch for the receiver. The pot wiper feeds audio into the LM386 IC which power amplifies it to about 100 mW to drive an 8 ohm, 3" speaker.
Now before you jeer at me for all my nasty remarks in my other articles about how I hate the LM386, you ought to know that the two of us (the chip and me) have kissed and made up, and are good buddies now. I knew I was asking for trouble when I configured it for maximum gain (x200 I think) by adding a 10 uF electrolytic cap between pins 1 and 8 of the IC; this meant I would be tormenting the chip with the temptation to launch into its famous squealing/motorboating mode.... but I added two bandaids that made everything better: Bypassed/decoupled the power pin with a 10 ohm resistor and 470uF electrolytic cap, and hung that blasted 10 ohm resistor / .1uF capacitor pair off the output pin to ground. (Believe me when I tell you that this hasn't always worked in other projects I've built. But it does now. Riiight.)
The radio is built in the usual KE3IJ manner: Solder two pieces of 5x7" copper-clad pc board (from Radio Shack) into an "L" so that there's a bottom and a front panel; build the circuit over the bottom piece, wiring the ungrounded circuit points up on top of 10 megohm resistors used as "standoff insulators", and soldering grounded component leads directly to the copper floor.
[Eventually I will take some digital camera pics of my monstrosities and post them within these articles; meanwhile I invite you to use your powers of visualization and imagine how much better yours could look than mine.]
NOTE! Once you've heard some wobbly-sounding signals during your clone's maiden voyage, verifying that it works, you will want to drip some candle wax onto your toroid coils, to cement them in place; otherwise your CW will sound like Ethel Merman singing "Everything's Coming Up Roses" on Broadway with opera-singer vibrato. I put a little sliver of wood under mine (they will be detuned if you lay them right down on the copper ground plane) and then dripped the wax over them. No more wobbly CW.
Calibration / Alignment
My solution was to build a very simple auxilliary circuit: A crystal calibrator. The circuit below is lifted straight out of the "Pixie"-style transceiver (see my RixPix page)-- it's a simple oscillator consisting of a crystal [frequency of your choice], pullup resistor, two ceramic caps, and an emitter resistor, all supporting a 2N3904 or 2N2222 transistor. Oh yeah, a 9v battery and clip, and a .1uF bypass cap also:
Here's how you use it: Put a 7040 KHz crystal into the calibrator (or any frequency within the 40m CW band), connect a battery and put it somewhere near the 40 Meter Tweeter receiver. You'll have to determine how near to the receiver (you wanna hear it, but not overload it which might pull the regen's frequency into lock with the crystal oscillator -- don't want that!). Take a thin plastic tuning wand (or a small metal screwdriver; non-metallic if you can) and carefully tune the receiver's trimmer cap screw up and down its range until you hear a quick "whoop!"... then try to fine-tune until you zero-beat the receiver to that whoop-turned-carrier-squeal. The tuning pot on the receiver would be, in this case, set approximately half way between min and max; this would assure you of covering the QRP frequencies within the 40 meter band. Adjust the above procedure to your particular situation and tuning needs. The point is, the crystal oscillator serves as a handy, rock-solid carrier transmitter for a reference frequency to which to calibrate your 40 Meter Tweeter.
Well that's about all for now. Since the prototype for this radio was only finished a few days before this article was first written (May 25, 2005), I expect there will be some feedback leading to a revision here and there. Let me know of your success or failure with this little receiver, should you decide to build it.