To change frequency use a LC tank circuit and diode varactor capacitor with a 10 turn vernier potentiometer with built in turns counter to tune by changing the reverse bias dc voltage of the varactor diode. Much smaller than a variable air capacitor or a variable permeability tuning coil. Works well for me ! 😇
I like it. This is what amateur radio is all about, not the 'station in a box' style of radio. There is a reason the government authorizes hams to design, build, repair, and operate their own gear on any ham band. The government gives us these billions of dollars woth of bandwidth in the hopes that we will learn RF. That knowledge is the difference between hams and CBers, or should be. Putting the VFO in a box is mostly for thermal stability, just dead air space to slow down thermal frequency drift during long QSO's. That and it will neutralize frequency fluctuations due to capacitance change from hands getting too close to the circuit, which in your build is not a problem. Back in the day, before crystal referenced synthesizers, a perfectly stable VFO was the Holy Grail of radio perfection. I agree, build the oscillator first. It DOES something. Kids want something that does something. THEN add the pieces that do MORE.
Bill, I definitely think PTOs are the future in analog tuning. The homebrew variable inductor coils are not reliant on commercial manufacturing for a precise component like an air variable capacitor. The future for homebrew gear is still bright. On another topic, I couldn’t help noticing some frequency shift when you moved your hand away from the tuning knob. It’s not objectionable and I could live with it in my LO circuits but it is there. Kudos for your presentation! de Rick, WD5L
This is a very helpful video. Thanks for sharing grounding the screw to prevent hand capacitance. I have a suggestion to stabilize your tuning screw. Use a thicker piece of wood than your 1/4 " thick plywood. OR no matter which thickness you use just make the wood go all the way across the face of the receiver PLUS fasten that wood to stabilize each end so the wood won't move won't move when the screw is tuned. A 3/4 " thick piece of wood might be an improvement by giving the screw more surface area in which to work. If you can tolerate it, a piece of 2X4 would stabilize the screw even more. Oil or light grease on the threads of the screw might be needed to make tuning smoother, especially with a thicker piece of wood. Your idea of using finer threads on the screw will definitely give a slower tuning. You already know the 1/4 inch designation is the screw diameter. The 20 tells how many threads per inch of the screw length. The 28 tpi (threads per inch) can be homemade IF you're willing to buy 1/4 inch rod and a thread cutting machinist die (not prohibitively expensive IF you make a lot of tuning screws) which you'll use to make threads on the rod. Getting the thread cutting die will negate having to wait for special order screws with the 28 tpi threads. I'm not sure if a thread cutting die with more than 28 tpi are avaiable, but if so, you could make the tuning even more precise. Thanks again for sharing all the information.
Another way to make an indicator for this setup would be an old school solution, have a string that winds or unwinds as the brass rod turns. Have the string movement move a needle indicator for frequency indication.
Thanks but our goal here is to build a SIMPLE analog receiver. I'm sure the mechanism you describe would work, but it wouldn't be appropriate for this project.
Peter-VK3YE covered a one transistor osc/mixer/det 40M Direct conversion reflex receiver concept as well as a Homebrew software defined radio in two of his RU-vid videos. Since almost all students have access to a Windows PC with a soundcard of some sort, a simple SDR arrangement like the TinySDR for 80m band (search) would serve as a catalyst to their learning of building real radio receivers and the technology used. 73 de WB9BBC-Bill
A possible way of getting a frequency scale would be to have a needle indicator with a pivot at it's base and have the movement of the brass rod push or pull the needle. The needle movement would be representative of the frequency.
Hey! I should use this idea, or variations thereof, for my son's ham radio club at his school!? This might be a bit over their heads--or beside their heads, as the case may be. But dead bug/Manhattan construction, the MMM as their first RF-emitting device. The whole thing is a mighty barbaric YAWP that I could contribute to right here close to home. Always appreciate what you do, Bill. VY 73, W0XO Jonathan
Jonathan-san: We plan on providing an initial JOO MMM-like experience by having the students build the PTO first. They will get the PTOs oscillating on the first or second day of the build. JOO! JOVO! Then they will go on to build the rest of the receiver. 73 Bill
Here is the second suggestion I think I found why it won't post. variable inductor with two coils. the outer is stationary and the inner rotates. you can find photos by searching google, I can't post the link but they are out there and very easy to build.
Unrelated to my other post here, I have been encouraged by your PTO and glue stick experiments and have 3-D printed out a variety of standard size PETG filament toroid cores. (T37, T50, T100, sizes...etc.). Being essentially air core toroids. I would be interested in their use in oscillator (for stability) and mixer designs. Thanks for your inspiration! 73 de Bill-WB9BBC
Hi Bill! Great video as always. Thanks for the name check too! I'll probably do a video of my Glue-Stick PTO experiments at some point. One thing I have noticed is that Farhan's PTO circuit that I'm using (the same one as you I think - from the Daylight Again) runs on 5V (via a 7805 regulator) but this means it puts out very little power: I measured -10 dBm (98 uW) which is OK into an active mixer like a NE602 but is certainly far short of the +7 dBm that a diode ring DBM would want to see. Are you running yours at a higher voltage? I might experiment with a couple of J310s configured as a Dual Gate MOSFET. Hand capacitance is a big issue too but as I am screwing iron dust toroids into the coil then I don't have the option of grounding as you did. The thing itself is pretty stable mind - I was amazed at how little drift there is. Anyway, thanks again for inspiring me to work on something new. I love the DCR project for the school. Keep up the great work. 73 Nick M0NTV
Nick: Thanks. I got some good info on how to deal with the Glue Stick backlash issue -- I will post on it soon. I think you should take another look at your build of the Daylight Again circuit. I am running mine off an 8 volt Zener and am getting enough out to turn on the diodes of the ring mixer. With the output connected to the input of the diode ring I measure about .640 volts on the input to the toroid. No need for a dual get MOSFET -- you should be able to use the Daylight Again VFO with a diode ring. I am running mine on 40 meters. Please let me know how this works out. 73 Bill
I am fairly new to Solder Smoke but very much appreciate the broadcasts. This is an excellent project for young people. If nothing else it will teach them that you can do a lot with low-tech when hi tech is not available or become unreliable. A thought for your tuning; why not go back to the old valve (tube) broadcast radio trick of string and pointer. Wrap one end round the knob spindle, a pin or a pulley at a point on the left and then take the string across to an opposite point on the right and tension the string with an elastic band or spring to keep the tension on the string... seems in keeping with the project. 👍😀
You need another nut or 2 to take out the looseness of the knob. If there is backlash you can tighten the nuts into one another to take that out. Machining tight fitting threads calls for tighter tolerances and costs more so standard hardware is always loose fitting.
DC Receiver is the way to go not Regen receiver. The next step is A SDR receiver using the sound card in laptop or computer software like SD Radio (for Win) or Quisk (for Linux) VK3YE cover this in one of his video Keep up the work Bill N3JGH.
I have only built from scratch, in a point to point manner, a regenerative receiver 13 years ago. It actually worked well and I wish I still had the receiver. Being retired and an amateur I'd like a radio related project this winter and the direct conversion receiver has caught my interest. I am not formally educated in electronics, rather mathemstics, thus a clear schematic is needed for me. This then leads to my question, do you know of any link to a schematic for a receiver similar to yours that I can use as my start point?
The key to success here is to see the receiver as being composed of four separate stages. Build each stage on separate boards. Test each one. Then put them together into a fill receiver. (We tell everyone who asks to do this, but very few of them follow our advice, so they usually do not succeed. ) Full schematics and build instructions are here: hackaday.io/project/190327-high-schoolers-build-a-radio-receiver Please let us know how this goes. 73 Bill N2CQR
@@soldersmoke Thank you. Most appreciated. My project likely will start in November. I have much to do before winter begins and while I should be out staining a deck I am having a hard time leaving your video pdcast #247 from three months ago.
Really interesting explanation of the brass screw. I wonder if aluminium would be a good choice since aluminium threaded screws are also common with different threads. I've worked with strong magnetic fields for years but thanks to your explanation I've just realised the eddy currents in the non ferrous conductor are effectively a transformer secondary.
Thanks Drew. And thanks for reminding me of the term "Eddy Currents" -- I haven't seen that or thought about it in decades. But as you said, that makes the screw seem like a shorted secondary. BTW, just to test this all out I put a real secondary coil on top of my PTO coil, then shorted it out while the primary was connected to an LC meter. Indeed, it is like taking turns off the primary. Inductance decreases, frequency of the oscillator increases.
@@soldersmoke it's just occurred to me that, instead of a shorted secondary or moving the conductor, you could use a transistor to control how much currently flows in a fixed secondary. Then you could maybe achieve the same tuning effect using a widely available pot. (And eliminate the need for a variable cap without using varicap diodes)
@@soldersmoke Use a long-tailed pair to control the current in the secondary - then you can make a genuine VCVFO. Details in Hawker's "Amateur Radio Techniques".
I bet you wouldn't get AM distortion if you used a PLL chip to generate the VCO, because you could dial it in to exactly the frequency of the AM carrier.
No, I don't think that is correct. You can do very well with a VFO using a PTO or a VXO (Variable Crystal Oscillator). We built and used AM receivers for many decades without PLL chips!
I discuss: Shielding of the VFO -- necessary or not? Shielding the VFO circuit will reduce or eliminate microphonics from the receiver. It will also aid in the temperature stability of the VFO, reducing frequency drift. So I will respectfully disagree with your conclusion.
Scott: i know that this is the conventional wisdom on this subject, but let me ask you this: Suppose you build the receiver with an unshielded VFO and then find that you have no microphonics and no frequency drift. Should you then shield the VFO to prevent microphonics and frequency drift? Also, while we frequently hear that VFOs must be carefully shielded, the VFOs in most commercial gear are NOT in separate metal boxes. Even VFOs in Direct Conversion rigs are often not in separate boxes. Look at W7EL's Optimized transceiver. The VFO was not shielded, yet this rig is (rightly) hailed as one of the best DC transceivers out there. Have you built a DC receiver that had microphonic and drift problems that were actually resolved by shielding? Thanks and 73 Bill
@@soldersmoke Yes, microphonics and short term drift due to temperature changes, not log term drift due to component values changing over a period of time. "Should you then shield the VFO to prevent microphonics and frequency drift?" NO, if it ain't broke don't fix it.
Bill, I am loving this series of yours. I am also losing my comment and I don't know what it is that is setting it off. so I will divide it into two pieces. if you search Clickspring on youtube the last video shows a very old solution. it's a spiral indicator.
Bill, I definitely think PTOs are the future in analog tuning. The homebrew variable inductor coils are not reliant on commercial manufacturing for a precise component like an air variable capacitor. The future for homebrew gear is still bright. On another topic, I couldn’t help noticing some frequency shift when you moved your hand away from the tuning knob. It’s not objectionable and I could live with it in my LO circuits but it is there. Kudos for your presentation! de Rick, WD5L
I think I got rid of the vast majority of the hand capacitance when I grounded the screw. But you are right, some freq shift occurs when I move my hand away from the control. I checked and this happens even when my hand is just moved away from the wooden front panel. I will add some grounded metallic backing to the "front panel." I think that will take care of the problem. 73 Bill
@@soldersmokeBill, it’s fine. It’s noticeable but certainly not a deal breaker. I still have a few air variable capacitors but PTO is definitely the way to go.
