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- Radio News and ReviewsI have been using (Slugging through and learning) KiCad This project looks awesome. It lets you import a KiCad schematic-netlist and sets up a breadboard for you ready to go https://hackaday.com/2024/05/13/sandwizz-promises-to-reinvent-the-breadboard/ https://www.microaware.com/sandwizz/
- Solid State RadiosI found this interesting circuit from a German experimenter: https://www.youtube.com/watch?v=Eqwfb-cSlRA Kurzwellen Audion mit Rückkopplung Schaltplan - eflose #909,https://www.youtube.com/watch?v=Eqwfb-cSlRA The schematic is included in the video. He uses a direct-coupled three-transistor AF amp, similar to what I have been using in my 1.2-volt regenerative superhet. However, he has some important changes compared to my circuit: 1. Detection: He directly uses the first transistor in the 3-transistor chain as an envelope detector, by feeding modulated RF (not AF) into the first transistor, and adding a 10 nF filter/bypass capacitor onto the output of the first transistor. In my circuit, I used a separate detector transistor ahead of the AF amp. 2. Low-Z headphones: His last transistor in the 3-transistor chain directly drives low-impedance headphones. I had tried this before, by trying to modify my existing 3-transistor chain that had high-impedance output, but was unsuccessful in finding the right values for the biasing resistors to get good gain with a low-impedance output. He seems to have found a good biasing solution. In my circuit, I used a separate power amp after the 3-transistor chain to drive low-Z headphones. I'll have to run some LTspice simulations to see if his 3-transistor circuit (including detection and driving low-Z headphones) has comparable gain to my 5-transistor circuit (using an extra detector and extra power amp). 3. Headphone grounding: He connects the headphones through a 220 uF capacitor across the final 100-ohm load resistor, between the last transistor's collector and Vcc (not ground). I imagine that grounding the headphones at Vcc instead of ground might help avoid some motorboating issues that might otherwise occur due to small internal battery resistance. 4. Grounding regeneration pot wiper: For the regenerative stage, he uses the standard cross-coupled oscillator, with a potentiometer wired as a voltage divider controlling the emitter voltage to control regeneration. He also grounds the wiper of the regeneration control potentiometer, by using a 10 nF bypass capacitor, which he says is important. On the other hand, I don't ground the wiper of the regeneration control potentiometer (and neither does Alan Yates at http://www.vk2zay.net/article/128), and it seems to work fine, but I suppose it's conceivable that omitting the bypass capacitor on the wiper might allow some small amount of RF into the Vcc line, which could cause various instabilities. I solved my instabilities by introducing a separate "regulated" Vcc_2 line providing 0.7 volts, via a 0.7-volt voltage divider consisting of a 51 ohm resistor and a 100-nF-bypassed silicon diode. Perhaps by following his design -- grounding the pot wiper, and grounding the headphones at Vcc instead of ground -- I might be able to do away with the need for the Vcc_2 regulated supply line in my receiver. 5. Tapping down on the coil: He connects both the detector transistor and the regenerative amplifier (the cross-coupled pair) to a low-Z tap on the coil. (Burkhard Kainka, another German experimenter, also does the same thing: https://www.b-kainka.de/bastel3.htm .) In contrast, I use not a low-Z tap, but a low-Z secondary winding (which is not directly part of the main LC resonant circuit), and connect only the detector to the low-Z winding, but connect the regenerative amplifier across the whole coil. He shows that with no regeneration applied, he can still receive signals from a short whip antenna connected to the unregenerated tank, so his tapping-down scheme probably loads the tank less than my scheme and achieves a better impedance match to the Q-multiplier. At the middle of the YouTube video, he also disconnects the whip antenna and shows reception with only the solenoidal tank coil, which he wound on a toilet paper roll.Like
- Solid State RadiosA Google Image search for Direct Conversion receivers using the NE602 (NE612 SA612) double balanced mixer will reveal numerous, perhaps over 100 similar schematics designed for the 80 to 10 meter amateur bands. Kits used to be sold by Vectronics and Ramsey. I was never happy with their performance and for this reason I built a simple superhet using an LM386 audio amplifier as a regenerative IF detector-amplifier. Using 10 turns on a ferrite rod as a preselector enables the receiver with the values shown to use both high side and low side injection to tune from 3.4 MHz to 10.2 MHz in 2 bands. No external antenna is necessary for casual listening and selectivity and performance are comparable to my best commercial portable shortwave receivers using their built in whip antennas. I use it most evenings to listen to SSB on 80 and 40 meters up and down the East coast of the U.S. The IF transformer is a red medium wave local oscillator transformer which in my case is tuned to 1.7 MHz. A variable local oscillator frequency of 5.1 to 8.5 MHz permits 2 band coverage using both low side and high side injection by tuning the front end preselector variable capacitor. Image rejection is good and by properly arranging the IF and LO frequency values there is little chance of overlap. Unlike a pure regenerative receiver, a superhet using a regenerative IF permits tuning across the bands without having to vary the regeneration control. The LM386 audio amplifier is set up as a high gain regenerative RF envelope detector which when oscillating allows the reception of CW and SSB signals. The first photo shown below is that of a stock QRP Kits EASY Direct Conversion receiver which has a varactor fine tuning control and after modification uses the original audio volume control for regeneration. The second photo is a modified version of the stock receiver in which I have added a ferrite bar inductor, a PVC tuning capacitor, a red MW local oscillator transformer, a trimmer preselector variable capacitor and an audio jack and modified the wiring to match the schematic of the new receiver. The stock varactor fine tuning may be used in conjunction with the main PVC tuning capacitor, but for SSB the new regeneration control varies the frequency slightly and also may be used for finer tuning. https://www.qrpkits.com/ezseries.html#ezreceiver Link to EASY receiver construction and schematic: https://www.qrpkits.com/files/EZRX20170527.pdf STOCK QRP RECEIVER MODIFIED QRP RECEIVER The first schematic is the stock Direct Conversion QRP receiver. The second schematic is the superhet version with regenerative IF.Like