Tuesday, 14 April 2015

Onto Jupiter

Hello!

I spent some grand time on the bench this Fall and Winter. In 1 month, I'll cease bench experiments and get outside in our garden. To keep in touch with other radio and QRP work bench enthusiasts over the spring + summer, I'll employ Twitter.

In the next 30 days, I hope to finish my Jupiter receiver and post a few of the 'unique circuits'. Reaching for something fresh, I'm avoiding cliche circuitry to learn and enjoy more.

Here's a quick glimpse at the front end filter — amplifier in test mode. I swept for S21 and performed DC measures last weekend.


 

Solid State Regenerative Receiver Community

I invite you to please join us at the Google Plus Solid State Regenerative Receiver Community.


To do my part —  I hope to post a few more of my regen ideas/experiments over the Spring and Summer. However, please post your experiments and enjoyment of those simple to moderately complex regen radio sets that hold us in thrall.

Dave, AA7EE wrote about this budding new community in a nice post: Click here.

I'll end with some random photos. Thanks for reading. 73!


Above — I also design and make solid state guitar amplifiers --- although, I've never posted any. 1 day perhaps?


Above — Egad — I still have some tube stuff hanging around but may purge it out.


Above —2 of my homebrew DC supplies


Above —1 of my first regens.  Ugly construction, of course. Всё как есть.


Above — Delightfully they go: cats and electronics. радио кошек.


Friday, 3 April 2015

Regen #5

Greetings — Привет! 

In winter 2015, I built 5 HF regens plus 2 VHF super regenerative receivers. I’ve run out of my better quality air variable capacitors, potentiometers and room — each version seems larger and uglier than the previous. With all these experiments, I’ve advanced about 2 mm up the regen receiver design learning curve — and in standard form factor, more questions arose than answers.

Briefly/frankly: I’m more a science officer Spock type than a green smoothie, quinoa and tofu devouring new ager — thus I prefer to avoid emotional messaging and hyperbole. I share my experiments to kindle interest, invoke dialog and sincerely hope we’ll all improve and enjoy what’s left of the SWL bands + analog radio design. Out of the gate — I think of this radio as OK; enjoyed making it and wish you well with your own experiments.

Photo of Regen #5 Front Panel.

 Above — Photo of Regen #5 Front Panel.

1.  Schematics

Regen #5 RF Board Schematic


Above — Regen #5 RF Board Schematic

Regen #5 AF Board Schematic

Above — Regen #5 AF Board Schematic

2.  RF Preamplifier

A common gate amplifier provides reverse isolation. IMO, batteries are best suited for moors and trail. As a dedicated AC power supply enthusiast , I won’t run a regenerative receiver without this isolation to prevent my leaked signal getting 60 cycle modulated and coming back into the receiver antenna port (the cursed common mode hum thang), or perhaps, making the local Hams irate with my QRM. 

Thanks to stuff like marijuana grow ops, our neighbor’s switch-mode lights, dimmers and other dumb dumbs; low band reception proves vexing in many bigger cities. I don’t want anything I’ve built adding to the radio listener interference burden.

I agree with the conclusions of regen wizard Charles Kitchin — the preamplifier should run at least 2-2.5 mA source (or emitter) current to help it avoid rectifying strong local stations. Lay the JFET flat side down on the copper board and solder the gate lead as close to the JFET plastic body as possible to squash UHF parasitic oscillations from that lead’s inductance. Further, the drain ferrite bead shown could just as easily be a low-value resistor. For example, 22 to 51 Ω.

The primary, 4 loose turns on a T68-6 = 181 nH (XL = 6.82 Ω at 6 MHz) generates enough of a magnetic field to couple the JFET output to the Q-multiplier (Q-M) tank. My circuit gives a wide range of input signal amplitude variation without too much overall gain, plus light coupling to the Q-M tank.

Close up of RF preamp circuitry and Q-multiplier inductor.

Above — Close up of RF preamp circuitry and Q-multiplier tank inductor. I wound the 3.34 µH inductor with 22 gauge enamel coated wire on a T68-6 toroid.

3.  Q-Multiplier

I sought a low distortion, high Q, negative resistance oscillator as the heart of this receiver and came up with this fun Colpitt’s variant. You can spend years learning, building and testing a ton of oscillator topologies and I plan to work towards this over time.

(FFT) The worst case distortion of the Colpitt’s Q-multiplier at 6 MHz : 2nd harmonic =  -46 dBc.

Above — (FFT) The worst case distortion of the Colpitt’s Q-multiplier at 6 MHz : 2nd harmonic =  -46 dBc. With lower amplitude, it may drop as low as –55 dBc across the 4.9- 8.33 MHz tuning span.

The regen control changes the oscillator amplitude.  1 annoyance with a Colpitts — as you adjust the amplitude or “regen” control potentiometer, the oscillator frequency changes since bias changes affects the transistor input capacitance (mainly through collector to base inter-element capacitance at the pn-junction).

A diagram showing the various internal and parasitic capacitances that affect a transistor circuit
Above — A diagram showing the various internal and parasitic capacitances that affect a transistor circuit. 

To reduce the tank tuning effects from bias change, I employed 3 strategies that worked:
  • High fT transistors (low input C)
  • Cascode oscillator topology (reduces Miller effect)
  • High tank C to L ratio [(weak effect) --- also minimizes stray C plus may boost resonator Q and decrease phase noise]
Initially, some friends reacted negatively to my transistor choice — the 2SC3355, a low–noise 6.5 GHz BJT. Yes, that’s a bit crazy fT-wise, but I bought 20 for $1.00 from a dying electronic store’s bargain bin and quite frankly — they're amazing.  I ran ferrite beads and VHF-UHF bypass to prevent parasitic oscillations and sniffed out none with my spectrum analyzer and DSO.

All my other BJTs above the fT of the PN5179 and BF199 are SMT parts. I wanted the RF board to only house leaded parts soldered in 100% Classic Ugly Construction plus — no low-Q, unknown temperature coefficient cut or glued pads anywhere on the RF board. Actually, leadless SMT parts probably offer the better BJT choice with respect to wiring parasitics.
It also might be better to run MPSH10 (PNP), BF199, PN5179 or other transistors with an fT between ~1- 2.5 GHz compared to my mega fT choice to reduce the chance of spurious oscillations while still fronting a low input C — I’ll leave transistor choice up to you.

The cascode configuration boosts the main Colpitts BJT’s output resistance to present a higher QL to the resonator. Better quality oscillators often run higher QL to isolate the tank from transistor variations and/or to reduce phase noise and possibly some temperature (frequency) drift.  



RF board built with 100% Classic Ugly Construction.

Above — RF board built with 100% Classic Ugly Construction.

RF section prior to wiring up the pots plus switch

Above — RF section prior to wiring up the pots plus switch. More bolts were added later.

4.  Detector

 

While factoring each particular JFET’s characteristics, the physics behind this detector at various signal levels and Q-multiplier amplitude settings looms complex. I’ll just give a hypothesis for some points.

The detector is directly connected to the Q-M tank. To decouple it, I center tapped the main inductor. 


A review of regenerative receiver operation 

AM

For maximum AM sensitivity on a signal, advance the Q-M amplitude pot until you hear some high pitched AF noise or oscillation. Then slowly lower the Q-M amplitude just enough to eliminate this audio buzz. Instead, for weak signal listening, we may choose to autodyne the signal by further increasing the Q-M amplitude while adjusting the fine tuning capacitor to zero beat.


SSB/CW

For SSB, advance the Q-M amplitude pot until you hear some hiss and then go look for the familiar duck quack of a SSB signal. Fine tune around the signal and tweak the Q-M pot until you hear your desired audio quality. CW is straight forward — just find a nice beat frequency. If you run the RF gain too high, signals may get phase or frequency modulated.

Of course, for AM, or SSB/CW, a delicate interplay exists between the various controls so that you'll often dial in the best sounding audio by expert knob tweaking — the so-called, “art of regen” stuff that conjures mystique and nostalgia. This takes practice — regen receivers sure engage you!

Regen #5 Particulars

Further muddying the waters, I ran a front panel switch that the places an 8K2 resistor in parallel with the 22K detector source resistor to make a "higher current" setting. In my particular case, that’s ~5974 Ω which sets the JFET source current to 441µA. I’ve probably mislabeled this switch on the receiver’s front panel: it might be better to just say lower current and higher current mode than SSB/AM.

When switched to the lower current setting, the measured source current = 138 µA.

 

AM


The gate-to-channel diode provides detection and the JFET is working as a square-law detector. Thus demodulation distortion will be a function of signal level and bias on the JFET since the square-law operation is only good over a certain range.  The JFET is biased near to cutoff.

Normally, I run Regen#5 with the switch in high current mode for it offers maximum sensitivity. In certain cases such as when tuning strong AM signals, switching to the low current detector mode offers less AF distortion. Often, the switches' effects on recovered audio distortion sounds subtle.

I determined the 22K source resistor experimentally during listening tests.  The goal = to find a source resistor that gives the best audio fidelity.

 

CW/ SSB

 

Ideally, we want our detector to operate as a mixer — in this case, we’re running a direct conversion receiver.  In SSB/CW reception, you run greater Q-multiplier signal amplitude than while detecting AM signals and we’re probably get some square law detection within the Q multiplier. 

1 theory is that rectification results in DC that may be enough to actually drive the FET toward pinch off which kills the gain. Switching in higher JFET source current will help keep the RF detected from the Q multiplier from pinching the JFET off. 

This winter, I built some different, very low distortion AM detectors. In 1 design, I ran a low current pair of BJTs with heavy feedback. While remarkable for AM, they really sucked for CW/SSB detection. So the direct-coupled hybrid cascode detector shown is a compromise circuit that works OK for both but better for SSB than AM in terms of AF distortion via listening tests.

This detector begs for further experiments. For example, what happens when it is DC coupled to the Q-M tank and a high ohm gate resistor is added? Should the 8K2 shunt resistor be replaced with a pot, or perhaps a switch to allow 3 or more different JFET source currents?

5. Audio Frequency Board


AF circuit breadboard

Above — AF circuit breadboard

Regen#5 features an op-amp plus discrete component AF board. I love analog design and the exquisite control that choosing discrete parts offers. My goal =  low distortion + low noise from AF input to loudspeaker.

Preamplifier

 

Nothing special here. A 0.1 µF input bypass rolls off noise and prevents local AM signals from entering, getting rectified and amplified by the AF chain. I opted to not include a multiple pole low-pass filter chain for dissecting CW/SSB pile ups, or a tone control circuit, but in a keeper-grade radio, I would add 1 or both.

I don’t run band-pass AF filtering in my receivers, but that's another option. With all the free, online software, designing good AF filters has never been easier.
You may boost the gain of the op-amps by increasing the feedback resistor value. Too much gain on strong signals may cause a spasm of feedback and create celestial noises.


PA


I’ve discussed this circuit before on this blog post.
Except now, I’ve solved a distortion problem caused by closing the loop in the op-amp connected to the power followers. In high gain feedback amplifiers, it does not take much time delay or phase lag to cause oscillation at high frequencies near the upper end of the bandwidth.  The small 22 pF feedback capacitor lowers the closed loop bandwidth so that there is insufficient gain at high frequencies for oscillation to occur.


FFT of the audio amplifier board.

Above — FFT of the audio amplifier board. Swinging 10 Vpp with the strongest harmonic at - 57 dBc between a 12.2 to 0 VDC single-supply rail provided 1 of the happiest accomplishments in my fledgling amateur radio designer career. I hope to better this 1 day, but that will prove difficult.

AF and RF circuit boards mounted and wired in Regen #5


AF and RF circuit boards mounted and wired in Regen #5

Above — AF and RF circuit boards mounted and wired in Regen #5


5. Thoughts

I’ve seen countless regen schematics spawned by the good and the great. Wow! — how can such a relatively simple concept garner so much attention and appear in so many forms?  I hardly feel I’ve contributed to the regen knowledge base, but gained valuable knowledge of what I want or hope to achieve in the future.

Regen#5 behaves like any regenerative radio should. The Q-M amplitude control feels precise — almost to the point of being too touchy. But does the regen amplitude control work any better or worse than other designs? I don’t know. The frequency shift associated with changing the Q-multiplier bias is minimal when compared to my other builds.

I applied standard LC VFO temperature stability techniques and felt surprised when I didn't have to add negative or positive tempco caps to stabilize it. This receiver stays on frequency for hours even when tuning CW and SSB signals.

Regen#5 suffers from microphonics as a SSB/CW receiver — and reminds me of an unbalanced or single-balanced DC receiver without the broadcast band nor in-band AM detection. The scratchiness of my dust-laden, ancient, air variable caps gets multiplied greatly by the sensitive RF chain. I’ll get some Caig Lab’s contact cleaner and have a go on those caps.

Still, too, the AM and SSB detector poses compromise — maybe it’s better to run 2 separate  optimized detectors with a switch to pass the signal through the best detector for the desired demodulation mode? I’ve got lots of experiments ahead, and more regen circuit ideas to share but plan to stop all regen work until next fall.


6. Out Takes and Sound Bytes

 
My prototype PA board

Above — My prototype PA board

FFT of the prototype PA board

Above — FFT of the prototype PA board.


Hartley VFO ideas from Regen #3

Above — Hartley VFO ideas from Regen #3

FFT of the signal from the above Hartley


Above — FFT of the signal from the above Hartley.


Sound Bytes

 

I heavily compressed some sound bytes to show the receiver in real world conditions. You'll hear me tweaking the Q-multiplier control on these recordings:

Mostly SSB during a contest March 28 with lots of QRN   Click

Noisy conditions and weak signal CW listening  Click

SSB during noisy conditions. I probably ran the RF gain too high  Click


Thanks for reading.

Saturday, 14 March 2015

Radon Measures - Daughters of decay

Greetings!

Occasionally, I abandon scope — and get on my apple box to rant and wave.
So click here if you want something about circuitry.

A dear friend suffers in the terminal stages of lung cancer. She, a non-smoker, can't figure out how this happened. We'll never know, however, might it have been radon gas exposure in the former old basement suit she lived in for decades?

We bought a radon gas meter that runs a photodiode, a microcontroller and some DC circuitry to perform alpha spectrometry and display the data means.


Above — Day 1 of radon gas measures in my basement lab.

Above — Data at 2 weeks.

I won't write much about radon gas since good links abound Link1  Link2. Canadians measure radon concentration in becquerels per cubic meter (Bq/m³), while in the USA, they express it in picocuries per liter (pCi/L). I hope my  3 -12 month radon measures stays low and look forward to viewing the changes over seasons and after rainstorms. Hopefully I won't have to make home renovations like my friend Greg did. He lives in our general area.

I get 3 IEEE publications. IEEE Spectrum ran a DIY Radon Detector article last year and here's a link although it's sans the code and schematic.

Remember all the noble gasses? The chemistry fascinates me and its worth reading about. For much of us, alpha radiation exposure primarily comes from inhaling radon and its decay products — it's now the leading cause of lung cancer in non-smokers.

Don't assume that because Bob and Sally's house down your street tested OK, that your home is safe. This is another "To Measure Is To Know" test case. It's horses for courses.

Actually, some of us guys get a bit casual about cancer prevention: How many of you still smoke, go out in the sun all day with no UV blockade, ignore our doctor's advice to get a colonoscopy, or to receive the dreaded prostate sweep? Feeling uncomfortable? This all pales in comparison to the suffering of our friend with lung cancer,

Back to my lab with a radon detector gathering data. 
Best!

Saturday, 7 March 2015

Etherkit Si5351A Breakout Board Update

Jason, NT7S recently shipped the Etherkit Si5351A BOB's and I got mine this week.






This BOB made a huge splash and forms a bedrock circuit to anchor future test and measurement products from Jason.

Stay tuned for updates from Etherkit or the NT7S Twitter feed.




Saturday, 21 February 2015

Regenerative Receiver #4

Hi gang!

This Winter, after a ~4 year hiatus from regenerative receivers, I built 4 while studying the plethora of published designs in print and on the web. How do you describe the sounds that regens deliver? Perhaps the adjective "screechy", or adjective-verb phrase "alive with vibe" might do ---  and when tuning across a SWL band they emit those classic wee-woo sounds that signal we're close to receiving some yet unknown, far off, station.

Many regen authors boast "high performance" or, like a Markov chain, their boosted outcome appears dependent on the state of whatever modulation mode your tuning -- or which knob your tweaking at the moment. Few measures are ever shown in the regen literature. Some designers embrace minimalism, while others unleash parts with fits of fury. Compromise, subjectivity, mysticism, sci-fi, nostalgia ---- it's all here. Joy.

As as mere regen receiver student, I'll offer my experiments with my regen #4 in hopes they might prove useful at some level.

My inspiration for this particular receiver comes from the work of simply amazing builder Makota, 7N3WVM. While a minimalist, his designs pitch common sense innovation and deliver maximum spark for us experimenters.

Makota's website:  http://www.qsl.net/7n3wvm/

The base receiver for my design may be seen at  http://www.qsl.net/7n3wvm/regen.html

It features a Colpitt's oscillator AC coupled to the tank for the Q multiplier plus a separate, low current FET detector. Many have copied/adapted his core regen design, including Dan, N1BYT in the WBR. Dan added his balanced LC circuit with ultra super-light coupling.  Dave, AA7EE also applies a SPRAT fueled Makota design in his Sproutie receiver.

Regen #4 schematic Part 1

Regen #4 schematic Part 1

 
Above — The entire regen #4 schematic.

I'll discuss the stages from left to right.

RF Preamplifier

We hear it over and over --- regens leak RF from the tank to the 'tenna. The often used, common base, or common gate RF amplifier serves as my favorite way to boost antenna-to-tank isolation, plus allows the use of crappy antennas when antenna space gets constrained.

The 500 Ω pot also reduces leakage and I normally just open it "a crack" since my antenna is a full sized 1/4 wave vertical on the 40M Ham band and the preamp offers gain.

No question — if you run the RF gain pot too wide open -- on a strong signal, you may cream the otherwise well behaved regen circuitry and make some bad noises.

The worst case antenna port leakage of regen #4: ~-41 dBm.

Above — The worst case antenna port leakage of regen #4: ~-41 dBm.  In this experiment, I turned both the RF gain and the regen pots fully clockwise! The rig was writhing in spasm with such a setting --- normally both are set close to fully counter clockwise.

Typical AM reception antenna port RF leakage.

Above — Typical AM reception antenna port RF leakage.  I listened to some AM stations like Radio Habana and others farther up the 49M band. With typical AM RF gain and regen control settings, the leakage out of the antenna connector was -73 dBm.
So the leakage will fall somewhere between the worst case and typical for AM reception -- I can live with that.

Feel free to drop the preamp current by increasing the 3K3 resistor. When I hear the word preamp, I think "run some current" -- even, still, it's just 2.24 mA.

Regen #4 breadboard

Above — Regen #4 breadboard

Regen #4 breadboard front view.

Above — Regen #4 breadboard front view. I once read you gotta run a chicken-head knob to really wrestle the audio pot into submission on a regen.

Tank and Q Multiplier

I ran a single, grungy, air-variable capacitor for tuning. My receiver tuned 6.69 - 9.2 MHz
but lacked the fine tuning needed for proper SSB and CW work. I suggest that builders apply standard VFO techniques like a few C0G/NP0 fixed capacitors for temperature stability and some way to provide fine tuning such as a small delta C capacitor in parallel with the other tank parts, or perhaps, a gear reduction tuning knob.

I can't stand temperature drift in my receivers, but varactor (or rectifier diode) tuning might work.

The 8:29 turns inductor ratio is ripe for experiments. As you reduce the smaller winding from 8 turns too a lower N, more of the RF amp's signal will shunt to ground via the collector's 0.1 µF bypass capacitor. I also built a version with no RF amp.

I built the Colpitts oscillator first, added the LC tank and then watched the signal amplitude and frequency in my 'scope + frequency counter as I turned the tuning capacitor. The old make a stage ---  then test that stage thang.
Then I built the RF preamp,detector and AF preamp.

A 470K resistor isolates the Colpitt's parts from the front panel 10K DC bias (Colpitt's amplitude) control. The 22K resistor sets the maximum Colpitt's amplitude and the 3K3 R, the minimum. You may have to experiment with the 3K3 resistor to find the resistor value that allows the Colpitts to just barely turn off but not go to 0 VDC.

If you omit this resistor, you'll hear what it does -- as you slowly turn the 10K pot clockwise from fully counter clockwise, a "bump" sounds as the BJT turns on. You apply the nearest standard value resistor that just eliminates that bump; for me it was 3K3 Ω.

I omitted the 10 µF bypass capacitor 'tween the wiper and 470K resistor in my design from Makota's original design since it causes a small response delay when changing the 10K pot.

Detector and AF Preamplifier

I extended Makota's detector by morphing it to a hybrid cascode.The hycas detector hikes reverse isolation plus lifts the output AF voltage and impedance.

1 experiment worth considering: Remove the 6K8 source resistor and solder a really short lead to a 10-25K pot wiper and 1 outside terminal. While listening to some signals, adjust the pot to find the sweet spot yielding the best sensitivity + AF gain. Remove the pot, measure its resistance and replace it with a nearest standard value resistor. After that, you might try the same procedure with the 2K7 collector resistor while listening to some strong signals. That's how I came up with my values as shown in the schematic.

In regen #4, most of the AF voltage gain occurs in the detector plus the following feedback pair.
The shunt feedback pair is a low-level only, cheesy, popcorn preamplifier I often throw in my experimental receivers. While this preamp looks crazy and won't win awards for temperature stability, its harmonic distortion is reasonably low:

An FFT of the regen #4 AF preamplifier @ ~1 KHz.

Above — An FFT of the regen #4 AF preamplifier @ ~1 KHz. The 2nd harmonic lies 52 dB down while all others are > 62 dB down. For low-level applications only. 2.52 mA total current.

You've noticed the 180 pF cap from output to input? Without a 100 - 220 pF cap in parallel with the 100K feedback cap to lower the preamplifier bandwidth, you'll hear audio oscillation.

DC Voltage

I ran my standard, homebrew, well filtered 12.2 volt DC power supply for my regen and heard no hum. Many prefer a battery supply and in that case, its a good idea to add a voltage regulator to keep a steady DC current in the Q multiplier and detector stages as the battery fades. Ensure you decouple and bypass well in any case.

An example ripple filter used to scrub off noise from the DC supply


Above — An example ripple filter used to scrub off noise from the DC supply. I might place this after a voltage regulator going to the Q multiplier and detector. If you use such a filter, a decoupling resistor plus bypass capacitor is still required in every stage of the receiver as shown in the regen #4 schematic.

AF Power Amplifier

In most regens, you'll see a 10 µF or so capacitor between pins 1 and 8 of an LM386 — this cranks up the gain to 200 and with it; noise and distortion.

I'm going to make 2 bold statements: [1] The LM386 in the low gain mode with some minor tweaks makes a very nice audio power amp. [2] It's unlikely you could build an AF power amp with the same clean average power capacity having the same or less distortion.

An LM386N-4 schematic with the needed bypass and feedback tweaks to make it sing sweetly


Above — An LM386N-4 schematic with the needed bypass and feedback tweaks to make it sing sweetly like Montserrat Caballé.  I showed the functional schematic earlier. Most LM386 users stick a capacitor between pins 1 and 8 which bypasses the 1.36K emitter resistor and this application (OK in some situations) resulted in the LM386 getting labels such as "a distortion machine", or a "compromise part". I continue to applaud the team that designed this part.

I've applied the LM386 like this for years, but have never shown any measures. Here we go:
 
An FFT of an LM368N-4 driven with ~ 1KHz sine wave to output 3.06 Vpp into an 8 Ω resistor load.

Above — An FFT of an LM368N-4 driven with ~ 1 KHz sine wave to output 3.06 Vpp into an 8 Ω resistor load. The second harmonic is ~ 64 dB down! At this drive, the power = Vpeak ^2 / 2R -- so (1.53 * 1.53) / 16 = 146 mW.

This is roughly the clean power you'd get from a 2N4401/2N4403 complimentary pair;  however, to get distortion this low would take some serious engineering, parts and measurement for the average Ham experimenter.

I cranked up the drive to output 5.27 Vpp

Above — I cranked up the drive to output 5.27 Vpp: Average power now = (2.635 * 2.635) / 16 = 434 mW. Look, the distortion remains low.

The time domain output with my LM386 driven to the point where I can just detect signs of distortion: 5.71 Vpp or 520 mW.

Above — The time domain output with my LM386 driven to the point where I can just detect signs of distortion: 5.71 Vpp or 520 mW.

 Adding back the FFT shows the harmonics are still down ~ 52 dB  from the fundamental. Still reasonable at over 1/2 W.

Above — Adding back the FFT shows the harmonics are still down ~ 52 dB  from the fundamental. Still reasonable distortion at over 1/2 W power.

Pushing the drive to get obvious clipping of the sine wave results in a 2nd harmonic of -40 dBc.

Above — Pushing the drive to get obvious clipping of the sine wave results in a 2nd harmonic of -40 dBc. That's my threshold of allowable distortion tolerance in a popcorn radio AF PA. P = 599 mW. At normal room volume, you might get this on some stronger signal peaks.

The 4 controls on a regenerative receiver keep your hands busy. Some combination of those 4 tweaks will give you the best sounding audio and hopefully keep the signal level where minimal clipping of peaks occurs.

The LM386 seems OK in my book. If these experiments don't show the benefits of measurement, then I'm afraid nothing will. 

In regen #4, my strategy included building up most of the the AF voltage gain with the hycas detector + a feedback amp using relatively low noise 2N4401 BJTs.  The LM386 adds some gain, but minimal and the result is a popcorn, relatively low noise + distortion AF chain that sounds OK.

No question, some builders can build a better AF mousetrap and I encourage them. However, for a low-fi regen receiver, I posit the LM386 in low gain mode with some tweaks will save a lot of parts plus space and give reasonable AF into a speaker. As ever, you'll decide what works best.

Out Takes

A double-tuned tank featuring varactors as the variable C.


Above —A double-tuned tank featuring varactors as the variable C. At some point, I'll couple it to a Q multiplier and take a look. I built a ton of regen-related circuits in Jan - Feb 2015. Great fun.

A sweep of the above double tuned circuit tuned at 6. 37 MHz.

 Above — A sweep of the above double tuned circuit tuned at 6. 37 MHz. The higher than expected insertion loss arose from the mismatch at the output end ( 29t : 10t ), plus the low Qul of the varactors.

A GPLA generated transfer function

Above — A GPLA generated transfer function. I designed the 49M band double-tuned circuit with the LADPAC software that ships with EMRFD.

Addendum:

A ton of people view this blog by clicking on links from Twitter. I joined up -- see My Links

Thank you

Todd, -VE7BPO-

QRP-POSDATA for April 9, 2015

Please click here for an interesting thread post by  'Bear' NH7SR who built his own version of Regen #4

Also consider looking at the slightly tweaked version of the detector in Regen #5. Click here.

Thanks, Todd