I am putting together a pre-amp / headphone amp for my radio. The CMOY headphone amp is pretty famous, and it works great. The only modification I made was the addition of precision adjustable resistors for R4. That will allow me to adjust the gain without messing around. I used a prototype board from Amazon, ElectroCookie Snappable PCB, Strip Board with Power Rails, one of the easier boards to use. I have built this exactly as shown in the schematic and it works great, except I use one 9 volt battery. My next iteration should have better placement on the board. Inching my way toward an AM/FM Stereo SW radio. I'm cheating on the FM radio, using a module. Going to use a TDA2822M Audio Amplifier Board 2.0 Channel Stereo module for the speakers. My goal here was to build a great AM, SW. The rest is modules.
I went over my breadboard several times before I posted this. It appears that I short the coil to ground when I connect the C3 to the tank. Poking around , when I connected to the gate of Q1, it worked great, with one battery powering the whole thing, no noise or hum. I know that is also the antenna, but when I remove power, the signal drops off dramatically, power it up, it increases the signal. Also, If I remove Q2, the signal drops .
I'm using the time station at 3330Mhz from Canada, which is usually burred in noise for me. It comes in strong with this preamp. I'm sure I am missing something, but it works. Probably getting the signal back via ground from the emitter.
The only DC path to ground I can see in your regen would be for a drain to gate short in the jfet oscillator to occur.
Maybe C3 is bad? Is it possible that the whip preamp itself is oscillating? If it is, you could get all sorts of weird stuff happening.
Those are huge value coupling capacitors in that whip preamp. I would think 10nF or even 1nF or less would be fine for the frequencies your regen is operating at. The two circuits may be too tightly coupled together.
Thanks for you having a look, I appreciate it very much. Ooops, I posted the wrong schematic. Here is the one I used. When I connect C3 to the antenna connection below, using the same battery and ground, it just hums like a bad circuit. When I use two batteries, and don't connect the grounds, it works but there is distortion. Using one battery to power everything, when I connect from from the gate to the antenna, it works perfect. I did try substituting the caps, but no luck. I'll try again. BTW, the performance is unbelievable, so I hope I get it to work.
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Unknown member
Sep 29, 2022
@LarryDee First, use two batteries (the working configuration) and use your multimeter to measure the circuit voltages at strategic points. Then, connect both circuits to the same battery and confirm the circuit voltages are the same. This will confirm that you have not made any basic wiring errors.
If the circuit voltages all look OK, then: It's conceivable that connecting both circuits to the same battery is now causing the entire receiver to motorboat (oscillate at some audio frequency), preventing any reception at all. Probably half of the time I spent debugging my regenerative superhet was (and still is) related to stopping kind of unwanted AF oscillation.
The usual way to fix this is to insert RC decoupling networks at strategic locations in the power supply line. You can read a little about this here: http://www.ke3ij.com/amp.htm .
Also, your schematic does not show any AF amplifier. I assume that you are using one. How is your AF amplifier powered? Is it using the same battery as the antenna amplifier and the regenerative detector? If yes, then this again can cause problems with unwanted AF oscillation, due to the typically high current draw of an AF amp. As the AF amp draws more current, it may slightly change the voltage seen by other, earlier stages. These earlier stages may then end up trying to amplify this voltage change, causing a feedback loop and AF oscillation. This AF oscillation can then prevent reception of any signals as the whole circuit is busy amplifying its own self-generated noise and oscillating.
The issue is that both circuits - receiver and preamp - work fine when each is connected to its own battery power source, but does not work at all when one battery powers both?
These two circuits work great. Does anyone know how to run these both off of the same battery? I tried connecting the grounds and positive to the points on the schematics, but the whole project goes dead. I thought I could do a hack with ultra low current voltage regulators for each, but would like to know the proper way to integrate these circuits. The preamp works so good I'm going to make a stand alone version. The copper plate is a small copper clad board. I use a wire, but the plate does work, but it's meant for mounting outside, really cool either way. As it is, it pulls in signals I could never get before, and it isolates the antenna. I feed the output of the preamp as is directly to the antenna, but connecting the grounds caused it to fail. Thanks.
I finally got my circuit to work the way I wanted. It tunes from below 3 MHz to just over 17 MHz. With qrp-gaijin suggestions, I figured it out. I m only refencing my findings to this circuit, so what I say here may not be useful in other circuits.
I could not find or get to work properly, any RF preamp circuit or a grounded gate circuit with this circuit. They just messed it up, gain was terrible. As configured, it sounds great, so I stopped trying to add the preamp. I use a Fielect Toroid Core Ferrite Choke Iron Powder Inductor Ferrite Ring 19.8x33x11.1mm from Amazon. The wire is Fielect 0.55mm Inner Dia Magnet Wire Enameled Copper Wire Winding Coil from Amazon. I found that the thicker the wire (lower gauge) the better the gain. But as you'll see below, you can only go so far. Through experimentation, I found that the winding really affects performance. You can wind it to any frequency range you want, with a 365 pf capacitor, but there seems to be a point where it works, but not well. You need a good amount of turns to get a high quality signal. That presents a problem for frequency coverage. With 40 turns = around 8uH C5 at 470pf, and a 365pf tuning capacitor, I could cover 3 MHz to almost 10MHz. If you use this calculator.
https://goodcalculators.com/resonant-frequency-calculator/
You'll see that it should go much higher. But in practice, it doesn't. Qrp-gaijin mentioned I should try a 4700pf for both C5 and C6, and changing the values led to success. Qrp-gaijin suggestion had me focus on the correct components, or I never would have gotten this to work in the higher frequency ranges. Since raising the values didn't work, I thought, maybe they should be lower. When I lowered C5 to 100pf, and changed the coil to 16 turns, the circuit tuned around 8 MHz to 17 MHz with nice fidelity and good gain. C6 is left at 470pf. The amount of turns on the core is very critical, less turns gets higher in frequency but no gain, more turns and it doesn't cover the band I want, and with high turns and a low value C5, it oscillates real bad. As it is, the selectivity is really great, but the tuning is tight. A gear down would work well. You should take notice of the 5K pot VR1 instead of 2 K, it just works better. I will have to use a switch to move the coil and cap in and out, but this little thing works as good as my Tecsun PL310 and has great fidelity, using a 2 foot wire. For bigger or powered antennas I will couple to the ferrite core with a few turns to keep it from transmitting. Next step is to get it off the breadboard onto an all copper board, Manhattan style (I think that name sounds cool) Hope you find this interesting. If anyone would like please let me know if you want a post with the parts list and suppliers.
I looked around and actually found a circuit that is made for 9 volts, draws 8 ma, and uses available parts. Doesn't use phantom power, that makes it all easy for me. But this output is low impedance also. Would I just use the FET portion of the circuit and not the 2n2222. Get my signal at C2 ?
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Unknown member
Sep 23, 2022
Replying to
That should work fine. The input to the Q1 JFET is high impedance, which should still allow you to get a good signal from the whip antenna. You can then try connecting the output of the amp (C3) to a few-turns link winding on your regen's coil.
Maybe you can also just try taking the output from Q1 at C2, but I guess the second transistor Q2 improves the performance so you might want to first try the circuit as designed.
I built the preamp as is and it works great. Allows me to pull in 3333 Mhz time no problem. The 2n2222 although old, is quiet. It's a quiet preamp. But here is where I come up short. I took the output from C3 as in the schematic, and plugged that in on the tank coil directly, it works but it's hummy. When using turns on the coil to connect the preamp , the performance drops dramatically. I am using 2 batteries, one for the radio, one for the preamp. Can't connect the ground, it stops working, I'll need to try it all on one battery. More work to do, but in the end, it should really perform well. Just need to get it coupled correctly. Tried a 10k resistor across the output, no change. Need to fiddle with it.
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Unknown member
Jul 19, 2022
@LarryDee Regarding your circuit -- you might get better performance by connecting the J1 drain to a link winding on L2, and eliminating the coupling capacitor C1. Just a guess, but I think that using a link winding would allow for more signal transfer than a choke/capacitor.
Great to see that you decided to use LTspice to draw your schematic. Good choice! You're in for a world of fun if you decide to simulate the circuits later. You can test a lot of circuit behavior without needing to fire up the soldering iron. If you post the LTspice file, I'll be happy to have a look and see what, if anything, needs to be done to make it work in the LTspice simulator. Maybe a separate thread would be a good idea for this topic about using LTspice.
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Unknown member
Jul 19, 2022
Replying to
LTspice files are just plain text files, so if nothing else works you can just open the file in a text editor, and copy/paste the text into a post or comment here, like this:
Example LTspice file af-poweramp.asc
Version 4SHEET11552680WIRE112-9616-96WIRE176-96112-96WIRE16-8016-96WIRE176-80176-96WIRE1120112-96WIRE17616176-16WIRE-4848-6448WIRE1648160WIRE48481648WIRE-6480-6448WIRE11216011296WIRE128160112160WIRE208160192160WIRE288160208160WIRE-64176-64160WIRE112208112160WIRE208208208160WIRE112304112288WIRE208304208288FLAG176160FLAG-641760FLAG1123040FLAG2083040FLAG288160 AF_out
IOPIN288160 Out
SYMBOL Misc\\cell 176-80R0WINDOW12300 Left 0WINDOW3900 Left 0SYMATTR InstName V1SYMATTR Value 1.2SYMBOL npn 480R0WINDOW03834 VRight 2WINDOW37191 VLeft 2SYMATTR InstName Q3SYMATTR Value 2N3904
SYMBOL res 0-96R0SYMATTR InstName R9SYMATTR Value 1k
SYMBOL res 96192R0SYMATTR InstName R2SYMATTR Value 50SYMBOL cap 1632R90WINDOW0032 VBottom 2WINDOW33232 VTop 2SYMATTR InstName C1SYMATTR Value 4.7µ
SYMBOL voltage -6464M0WINDOW12300 Left 0WINDOW3900 Left 0SYMATTR InstName V2SYMATTR Value SINE(0{vin}1k)SYMBOL cap 192144R90WINDOW0032 VBottom 2WINDOW33232 VTop 2SYMATTR InstName C3SYMATTR Value 4.7µ
SYMBOL res 192192R0SYMATTR InstName R5SYMATTR Value 32TEXT72-184 Left 2!.tran 050m 0.010u
TEXT72-152 Left 2!.step param Vin 300m 600m 50m
Here is the common gate circuit I found on line. Pretty Simple.
https://worldradiohistory.com/UK/Bernards-And-Babani/Bernards/222-Solid-State-Short-Wave-Receivers-For-Beginners.pdf
I have been working on this for a while, trying to tweak the best out of these transistors that I could. Here is a schematic of what I came up with. It tunes from 3 to about 12 MHz, but it sounds great. The sensitivity is OK, not bad, but not crazy good, using just a 2 foot wire. Please let me know if you see something jarring. R1, R3 , and R% were replaced with precision adjustable potentiometers to tweak performance. What that allowed me to do was set the values here precisely. J1 is a grounded gate circuit I found in an OLD radio book online. It just so happens to fit this circuit nicely. I feed the output to a cheap LM386 amp you buy on Amazon and run headsets. I'll post a parts list soon. I only used LTspice to draw this up, I don't think it will actually run a simulation, but you are welcome to the file if you want.
I don't have an SDR receiver - yet - but when I was building my simple SSB transceiver and playing with mixers, I got one of the new 8 bit digital oscilloscopes that did FFT spectrum analysis so I could "see" what was going on, and ultimately concluded that in many instances the displayed results were wildly misleading.
So I invested in the 14 bit depth Red Pitaya which provides much more reliable information, but is still probably not infallible compared to a real analog spectrum analyzer.
If these RTL-SDR devices are 8 bit ( or worse ) the results should be treated with skepticism and cross checked, if possible.
qrp-gaijin provided this circuit and I built it. It worked perfect first time. I also bought a small shortwave radio from Tecsun, PL-310-et . Using these, I was able to figure out what kind of whacky harmonics my RTL-SDR was displaying. By tuning this oscillator circuit to a spot displayed on the SDR, I could verify the signal with the Tecsum radio, thereby ignoring the harmonic display. After I found the correct frequency display, I was able to do the same thing with the 2 transistor shortwave I am building here. I found that the radio circuit will only work between 3 MHz and 12 MHz or so . Using a 0 to 320 PF capacitor and an iron ferrite toroid core wound to 3uh. Using the 3uh coil, it only oscillates in a very narrow range of the capacitor. With a 6uh coil, it tunes approx. 4Mhz to around 10 ish. I don't know why the radio circuit below will only oscillate no higher than 12 Mhz, but it does sound good.
Direct sampling mode is a hardware modification that allows the tuner chip in RTL-SDR dongles to be bypassed, allowing reception of signals between 0 – 14 MHz. However, after performing this mod there is no filtering and images from higher frequencies such as broadcast FM can be problematic.
Note that this feature makes use of direct sampling and so aliasing will occur. The RTL-SDR ADC samples at 28.8 MHz, thus you may see mirrors of strong signals from 0 - 14.4 MHz while tuning to 14.4 - 28.8 MHz and the other way around as well. To remove these images you need to use a low pass filter for 0 - 14.4 MHz, and a high pass filter for 14.4 - 28.8 MHz, or simply filter your band of interest. (Note that that 28.8 MHz is downsampled on chip resulting in the 3.2 MHz bandwidth)
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Unknown member
Jul 02, 2022
So here's a quick video showing how confusing it can be to try to interpret the output of the RTL-SDR waterfall display. I am running the RTL-SDR not in direct sampling mode, instead running it in it's "normal" mode of operation. I disconnected my homebrew upconverter for this test. For the antenna, I connected a small 4-turn loop of wire to the RTL-SDR antenna connector through a short length of coaxial cable.
For the signal source, I'm using the two-transistor oscillator (the dip meter) circuit that I linked earlier (https://www.b-kainka.de/bastel53.htm). I'm using the dip meter as a VFO to generate a signal to be picked up by the RTL-SDR. Just as a guess, with the coil I used, it probably tunes from about 6 to about 24 MHz.
The RTL-SDR is tuned to 30 MHz. In the below video, I start with the VFO at around 6 MHz and tune it always upwards in frequency up to about 24 MHz. (Note: There is no audio with this video.)
So, what do we see on the SDR software screen? A bunch of signals -- some strong, some weak, moving both upwards and downwards on the waterfall display! So we're seeing various harmonics of the VFO frequency, plus those harmonics are somehow getting mixed with the LO in the RTL-SDR to create spurious signals that move in the "wrong" direction -- downwards on the frequency display, even though the generated signal from the VFO is only moving upwards in frequency.
And things get even more confusing if you add another mixer stage, as I did by adding a broadband HF upconverter in front of the RTL-SDR. That again provides ample opportunity to create unexpected mixing products.
So it can be a real challenge to correctly interpret what you are seeing.
The same caveat applies to using your portable shortwave receiver for testing -- it will in all likelihood be a superheterodyne receiver with one or more mixer stages, so again you will pick up your regen's radiated signal at multiple locations on the dial.
I still think that if you are observing that your regen's radiated signal is tuning in the "wrong" direction on the RTL-SDR display (tuning the regen up in frequency results in the signal on the RTL-SDR display going down in frequency), you're probably looking at some spurious mixing product and not the fundamental oscillation frequency of your regen. Tune the RTL-SDR lower and lower in frequency and keep looking for the regen's radiated signal at lower frequencies. I'm guessing you'll find it.
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Unknown member
Jul 02, 2022
@w5jag wrote:
I don't know how it is showing up at 24 something MHz unless your regen oscillator is overloading the RTL-SDR
The RTL-SDR has an internal gain control for the LNA, and turning it up too high can lead to spurious signal generation. You can see this in my video here at 02:18.
Here's a best-guess at the internals of one model of the RTL-SDR.
However, I think Larry is using the RTL-SDR's direct sampling mode, which may be bypassing all the mixer circuitry (and the LNA) and sending an RF signal directly into the chip. But still, my guess would be that even in direct sampling mode, spurious signals can still be generated.
Using a local VFO to generate a controlled test signal could help in understanding if and how the RTL-SDR is overloading.
Yes, I am using direct sampling mode. I do actually listen to WTWW at night sometimes, it's music format is like WABC New York 1965-75. I'm in Jersey, so I get anything from Nashville, and Boston, like it's in my backyard. I'm going to pick up a Skyworks SI4734 based TECSUN PL-310ET radio to compare all this. And when I build the oscillator for testing, I can compare it to that radio too.
WWCR is at 4.84 MHz in the evenings with a whopping big signal out of Nashville, and is likely beaming in Larry's direction.
It's a whopping big signal here off the back of the antenna.
I don't know how it is showing up at 24 something MHz unless your regen oscillator is overloading the RTL-SDR. You might want to back the antenna away from your regen.
WTWW is on 5085 KHz in the evening with a big signal out of the Nashville area and they are beaming in your direction. Might want to give them a listen.
73
Win W5JAG
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Unknown member
Jul 01, 2022
@LarryDee Your RTL-SDR software display shows it is tuned to 23.962318 MHz.
You said you are receiving a Nashville shortwave station around 4 MHz. I'm guessing you're talking about WWCR at 4.840 MHz.
If we multiply 4.840 MHz by 5, we get 24.200 MHz. That's pretty close to where you are tuned, at 23.962318 MHz (your RTL-SDR's oscillator might be slightly off frequency).
Are you sure that your regen is not oscillating at 4.840 MHz? Could you check the RTL-SDR around 4.840 MHz and see if the regen's oscillation signal is detectable there?
Because the RTL-SDR lacks (good) front-end filtering, it is conceivable that powerful shortwave stations could appear at multiple places on the dial.
Wow, that's a great answer. I need to look into it. I have a cheap shortwave radio that did match up, but the frequencies are limited. Yes, radio stations appear all over the place on the SDR. That might be what is happening. Gonna take some more fiddling.
I didn't built the oscillator yet. But I did tune to a radio station. I tuned the SDR to a high frequency radio station, then tuned the radio to that station and they matched. I did this to verify it is in fact tuning to a high frequency station. The station isn't listed anywhere, I couldn't figure out where it was coming from. It is a Nashville station normally at 4 something Mhz. I'm not a fan, just use it for markers. Please excuse my announcing, didn't even try to polish it.
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Unknown member
Jul 01, 2022
@LarryDee My first idea is that the variable capacitor might be bad, which can cause intermittent contacts, shorted plates, and weird capacitance values. But you said you already checked that.
My next suggestion would be to build a simple variable-frequency signal source (a VFO), which will allow you to generate a test signal that you can then receive in the regen. By systematically tuning the regen to receive the VFO signal, then slightly adjusting the VFO tuning and again adjusting the regen tuning, you can observe the behavior of your regen's tuning step-by-step and get a clearer idea of if there are any jumps or reversals in your regen's tuning.
Here's a simple VFO circuit for this purpose. It was originally intended as a dip meter, but you can omit the diode and everything to the right of the diode. Just connect any coil and a variable capacitor, and it will oscillate and generate a test signal that you can then receive on your regen. You can check the frequency of the test signal with your RTL-SDR. As W5JAG said, you need to be careful about interpreting your observations on the RTL-SDR because you may be observing higher-order harmonics.
I don't know anything about regenerative receivers.
But one thing that jumps right out is that your oscillator is orders of magnitude stronger and sharper in the "low" band, than it is in the "high" band, assuming your SDR has uniform sensitivity over that frequency range.
Are you possibly seeing the second or third harmonic of the oscillator? Seems like the tuning might be backward for that, but might be worth investigating. That's the first thing that comes to my mind.
Thanks, I need to look into that. I think the sharpness was due to me not pushing the regen control higher when it got into the high bands because I was holding the camera. It does get pretty high oscillation there too. It actually tunes and demodulates the higher stations no problem. I also checked the variable capacitor, thinking it was maybe hitting weird values, but it checked out fine. And I swapped it with a metal air capacitor and same thing. And once it starts tuning (Oscillating) the high band, I have to fiddle with the regen control to get it back to the lower band. Kinda weird
I am putting together a pre-amp / headphone amp for my radio. The CMOY headphone amp is pretty famous, and it works great. The only modification I made was the addition of precision adjustable resistors for R4. That will allow me to adjust the gain without messing around. I used a prototype board from Amazon, ElectroCookie Snappable PCB, Strip Board with Power Rails, one of the easier boards to use. I have built this exactly as shown in the schematic and it works great, except I use one 9 volt battery. My next iteration should have better placement on the board. Inching my way toward an AM/FM Stereo SW radio. I'm cheating on the FM radio, using a module. Going to use a TDA2822M Audio Amplifier Board 2.0 Channel Stereo module for the speakers. My goal here was to build a great AM, SW. The rest is modules.
I went over my breadboard several times before I posted this. It appears that I short the coil to ground when I connect the C3 to the tank. Poking around , when I connected to the gate of Q1, it worked great, with one battery powering the whole thing, no noise or hum. I know that is also the antenna, but when I remove power, the signal drops off dramatically, power it up, it increases the signal. Also, If I remove Q2, the signal drops . I'm using the time station at 3330Mhz from Canada, which is usually burred in noise for me. It comes in strong with this preamp. I'm sure I am missing something, but it works. Probably getting the signal back via ground from the emitter.
@LarryDee First, use two batteries (the working configuration) and use your multimeter to measure the circuit voltages at strategic points. Then, connect both circuits to the same battery and confirm the circuit voltages are the same. This will confirm that you have not made any basic wiring errors.
If the circuit voltages all look OK, then: It's conceivable that connecting both circuits to the same battery is now causing the entire receiver to motorboat (oscillate at some audio frequency), preventing any reception at all. Probably half of the time I spent debugging my regenerative superhet was (and still is) related to stopping kind of unwanted AF oscillation.
The usual way to fix this is to insert RC decoupling networks at strategic locations in the power supply line. You can read a little about this here: http://www.ke3ij.com/amp.htm .
Also, your schematic does not show any AF amplifier. I assume that you are using one. How is your AF amplifier powered? Is it using the same battery as the antenna amplifier and the regenerative detector? If yes, then this again can cause problems with unwanted AF oscillation, due to the typically high current draw of an AF amp. As the AF amp draws more current, it may slightly change the voltage seen by other, earlier stages. These earlier stages may then end up trying to amplify this voltage change, causing a feedback loop and AF oscillation. This AF oscillation can then prevent reception of any signals as the whole circuit is busy amplifying its own self-generated noise and oscillating.
The issue is that both circuits - receiver and preamp - work fine when each is connected to its own battery power source, but does not work at all when one battery powers both?
What type of battery (ies) are being used?
That's a head scratcher.
73,
Win W5JAG
These two circuits work great. Does anyone know how to run these both off of the same battery? I tried connecting the grounds and positive to the points on the schematics, but the whole project goes dead. I thought I could do a hack with ultra low current voltage regulators for each, but would like to know the proper way to integrate these circuits. The preamp works so good I'm going to make a stand alone version. The copper plate is a small copper clad board. I use a wire, but the plate does work, but it's meant for mounting outside, really cool either way. As it is, it pulls in signals I could never get before, and it isolates the antenna. I feed the output of the preamp as is directly to the antenna, but connecting the grounds caused it to fail. Thanks.
I finally got my circuit to work the way I wanted. It tunes from below 3 MHz to just over 17 MHz. With qrp-gaijin suggestions, I figured it out. I m only refencing my findings to this circuit, so what I say here may not be useful in other circuits.
I could not find or get to work properly, any RF preamp circuit or a grounded gate circuit with this circuit. They just messed it up, gain was terrible. As configured, it sounds great, so I stopped trying to add the preamp. I use a Fielect Toroid Core Ferrite Choke Iron Powder Inductor Ferrite Ring 19.8x33x11.1mm from Amazon. The wire is Fielect 0.55mm Inner Dia Magnet Wire Enameled Copper Wire Winding Coil from Amazon. I found that the thicker the wire (lower gauge) the better the gain. But as you'll see below, you can only go so far. Through experimentation, I found that the winding really affects performance. You can wind it to any frequency range you want, with a 365 pf capacitor, but there seems to be a point where it works, but not well. You need a good amount of turns to get a high quality signal. That presents a problem for frequency coverage. With 40 turns = around 8uH C5 at 470pf, and a 365pf tuning capacitor, I could cover 3 MHz to almost 10MHz. If you use this calculator. https://goodcalculators.com/resonant-frequency-calculator/
You'll see that it should go much higher. But in practice, it doesn't. Qrp-gaijin mentioned I should try a 4700pf for both C5 and C6, and changing the values led to success. Qrp-gaijin suggestion had me focus on the correct components, or I never would have gotten this to work in the higher frequency ranges. Since raising the values didn't work, I thought, maybe they should be lower. When I lowered C5 to 100pf, and changed the coil to 16 turns, the circuit tuned around 8 MHz to 17 MHz with nice fidelity and good gain. C6 is left at 470pf. The amount of turns on the core is very critical, less turns gets higher in frequency but no gain, more turns and it doesn't cover the band I want, and with high turns and a low value C5, it oscillates real bad. As it is, the selectivity is really great, but the tuning is tight. A gear down would work well. You should take notice of the 5K pot VR1 instead of 2 K, it just works better. I will have to use a switch to move the coil and cap in and out, but this little thing works as good as my Tecsun PL310 and has great fidelity, using a 2 foot wire. For bigger or powered antennas I will couple to the ferrite core with a few turns to keep it from transmitting. Next step is to get it off the breadboard onto an all copper board, Manhattan style (I think that name sounds cool) Hope you find this interesting. If anyone would like please let me know if you want a post with the parts list and suppliers.
@LarryDee Regarding your circuit -- you might get better performance by connecting the J1 drain to a link winding on L2, and eliminating the coupling capacitor C1. Just a guess, but I think that using a link winding would allow for more signal transfer than a choke/capacitor.
Great to see that you decided to use LTspice to draw your schematic. Good choice! You're in for a world of fun if you decide to simulate the circuits later. You can test a lot of circuit behavior without needing to fire up the soldering iron. If you post the LTspice file, I'll be happy to have a look and see what, if anything, needs to be done to make it work in the LTspice simulator. Maybe a separate thread would be a good idea for this topic about using LTspice.
Here is the common gate circuit I found on line. Pretty Simple. https://worldradiohistory.com/UK/Bernards-And-Babani/Bernards/222-Solid-State-Short-Wave-Receivers-For-Beginners.pdf
I have been working on this for a while, trying to tweak the best out of these transistors that I could. Here is a schematic of what I came up with. It tunes from 3 to about 12 MHz, but it sounds great. The sensitivity is OK, not bad, but not crazy good, using just a 2 foot wire. Please let me know if you see something jarring. R1, R3 , and R% were replaced with precision adjustable potentiometers to tweak performance. What that allowed me to do was set the values here precisely. J1 is a grounded gate circuit I found in an OLD radio book online. It just so happens to fit this circuit nicely. I feed the output to a cheap LM386 amp you buy on Amazon and run headsets. I'll post a parts list soon. I only used LTspice to draw this up, I don't think it will actually run a simulation, but you are welcome to the file if you want.
You all might have found this, but it's rare to find a reduction drive air capacitor.
https://www.mikeselectronicparts.com/product/384pf-air-variable-capacitor-with-81-planetary-reduction-drive/
I don't have an SDR receiver - yet - but when I was building my simple SSB transceiver and playing with mixers, I got one of the new 8 bit digital oscilloscopes that did FFT spectrum analysis so I could "see" what was going on, and ultimately concluded that in many instances the displayed results were wildly misleading.
So I invested in the 14 bit depth Red Pitaya which provides much more reliable information, but is still probably not infallible compared to a real analog spectrum analyzer.
If these RTL-SDR devices are 8 bit ( or worse ) the results should be treated with skepticism and cross checked, if possible.
73,
Win W5JAG
qrp-gaijin provided this circuit and I built it. It worked perfect first time. I also bought a small shortwave radio from Tecsun, PL-310-et . Using these, I was able to figure out what kind of whacky harmonics my RTL-SDR was displaying. By tuning this oscillator circuit to a spot displayed on the SDR, I could verify the signal with the Tecsum radio, thereby ignoring the harmonic display. After I found the correct frequency display, I was able to do the same thing with the 2 transistor shortwave I am building here. I found that the radio circuit will only work between 3 MHz and 12 MHz or so . Using a 0 to 320 PF capacitor and an iron ferrite toroid core wound to 3uh. Using the 3uh coil, it only oscillates in a very narrow range of the capacitor. With a 6uh coil, it tunes approx. 4Mhz to around 10 ish. I don't know why the radio circuit below will only oscillate no higher than 12 Mhz, but it does sound good.
qrp-gaijin suggested oscillator
Radio Circuit I am working with
Check out the following warnings about RTL-SDR's direct sampling mode:
https://www.rtl-sdr.com/low-pass-filter-for-rtl-sdr-direct-sampling-mode/
https://www.rtl-sdr.com/rtl-sdr-blog-v-3-dongles-user-guide/ a
So here's a quick video showing how confusing it can be to try to interpret the output of the RTL-SDR waterfall display. I am running the RTL-SDR not in direct sampling mode, instead running it in it's "normal" mode of operation. I disconnected my homebrew upconverter for this test. For the antenna, I connected a small 4-turn loop of wire to the RTL-SDR antenna connector through a short length of coaxial cable.
For the signal source, I'm using the two-transistor oscillator (the dip meter) circuit that I linked earlier (https://www.b-kainka.de/bastel53.htm). I'm using the dip meter as a VFO to generate a signal to be picked up by the RTL-SDR. Just as a guess, with the coil I used, it probably tunes from about 6 to about 24 MHz.
The RTL-SDR is tuned to 30 MHz. In the below video, I start with the VFO at around 6 MHz and tune it always upwards in frequency up to about 24 MHz. (Note: There is no audio with this video.)
So, what do we see on the SDR software screen? A bunch of signals -- some strong, some weak, moving both upwards and downwards on the waterfall display! So we're seeing various harmonics of the VFO frequency, plus those harmonics are somehow getting mixed with the LO in the RTL-SDR to create spurious signals that move in the "wrong" direction -- downwards on the frequency display, even though the generated signal from the VFO is only moving upwards in frequency.
And things get even more confusing if you add another mixer stage, as I did by adding a broadband HF upconverter in front of the RTL-SDR. That again provides ample opportunity to create unexpected mixing products.
So it can be a real challenge to correctly interpret what you are seeing.
The same caveat applies to using your portable shortwave receiver for testing -- it will in all likelihood be a superheterodyne receiver with one or more mixer stages, so again you will pick up your regen's radiated signal at multiple locations on the dial.
I still think that if you are observing that your regen's radiated signal is tuning in the "wrong" direction on the RTL-SDR display (tuning the regen up in frequency results in the signal on the RTL-SDR display going down in frequency), you're probably looking at some spurious mixing product and not the fundamental oscillation frequency of your regen. Tune the RTL-SDR lower and lower in frequency and keep looking for the regen's radiated signal at lower frequencies. I'm guessing you'll find it.
@w5jag wrote:
The RTL-SDR has an internal gain control for the LNA, and turning it up too high can lead to spurious signal generation. You can see this in my video here at 02:18.
Here's a best-guess at the internals of one model of the RTL-SDR.
http://aaronscher.com/wireless_com_SDR/rtl_sdr_info.html
However, I think Larry is using the RTL-SDR's direct sampling mode, which may be bypassing all the mixer circuitry (and the LNA) and sending an RF signal directly into the chip. But still, my guess would be that even in direct sampling mode, spurious signals can still be generated.
Using a local VFO to generate a controlled test signal could help in understanding if and how the RTL-SDR is overloading.
WWCR is at 4.84 MHz in the evenings with a whopping big signal out of Nashville, and is likely beaming in Larry's direction.
It's a whopping big signal here off the back of the antenna.
I don't know how it is showing up at 24 something MHz unless your regen oscillator is overloading the RTL-SDR. You might want to back the antenna away from your regen.
WTWW is on 5085 KHz in the evening with a big signal out of the Nashville area and they are beaming in your direction. Might want to give them a listen.
73
Win W5JAG
@LarryDee Your RTL-SDR software display shows it is tuned to 23.962318 MHz. You said you are receiving a Nashville shortwave station around 4 MHz. I'm guessing you're talking about WWCR at 4.840 MHz. If we multiply 4.840 MHz by 5, we get 24.200 MHz. That's pretty close to where you are tuned, at 23.962318 MHz (your RTL-SDR's oscillator might be slightly off frequency). Are you sure that your regen is not oscillating at 4.840 MHz? Could you check the RTL-SDR around 4.840 MHz and see if the regen's oscillation signal is detectable there?
Because the RTL-SDR lacks (good) front-end filtering, it is conceivable that powerful shortwave stations could appear at multiple places on the dial.
I didn't built the oscillator yet. But I did tune to a radio station. I tuned the SDR to a high frequency radio station, then tuned the radio to that station and they matched. I did this to verify it is in fact tuning to a high frequency station. The station isn't listed anywhere, I couldn't figure out where it was coming from. It is a Nashville station normally at 4 something Mhz. I'm not a fan, just use it for markers. Please excuse my announcing, didn't even try to polish it.
@LarryDee My first idea is that the variable capacitor might be bad, which can cause intermittent contacts, shorted plates, and weird capacitance values. But you said you already checked that.
My next suggestion would be to build a simple variable-frequency signal source (a VFO), which will allow you to generate a test signal that you can then receive in the regen. By systematically tuning the regen to receive the VFO signal, then slightly adjusting the VFO tuning and again adjusting the regen tuning, you can observe the behavior of your regen's tuning step-by-step and get a clearer idea of if there are any jumps or reversals in your regen's tuning.
Here's a simple VFO circuit for this purpose. It was originally intended as a dip meter, but you can omit the diode and everything to the right of the diode. Just connect any coil and a variable capacitor, and it will oscillate and generate a test signal that you can then receive on your regen. You can check the frequency of the test signal with your RTL-SDR. As W5JAG said, you need to be careful about interpreting your observations on the RTL-SDR because you may be observing higher-order harmonics.
http://www.b-kainka.de/bastel53.htm
I don't know anything about regenerative receivers.
But one thing that jumps right out is that your oscillator is orders of magnitude stronger and sharper in the "low" band, than it is in the "high" band, assuming your SDR has uniform sensitivity over that frequency range.
Are you possibly seeing the second or third harmonic of the oscillator? Seems like the tuning might be backward for that, but might be worth investigating. That's the first thing that comes to my mind.
73,
Win W5JAG