This is the current state of my receiver. Unlike earlier designs this one uses NO VARIABLE CAPACITORS! This helps because (a) it reduces cost, (b) makes it easier to build for anyone (it's hard to hunt down identical variable capacitors), and (c) allows it to be totally voltage controlled so microchip or PC control of frequency becomes trivial. Tuning over the entire 40m band is achieved with 3 LEDs reverse biased acting as varactors (wow!). The knobs are potentiometers. The whole circuit runs on 5v.
In the spirit of furthering my knowledge of AC circuity, I'm trying to build a 100% homebrew transceiver. Yeah, QRSS and ultra-weak signal, ultra-narrowband communications is still fun, but it's not the same thrill as actually engaging in real time communication with somebody! My goal is a transmitter / receiver in a box. The basic features I desire are (1) multiple bands (at least 40m, 30m, 20m), (2) FULL-band coverage, (3) direct conversion receiver, (4) 10W transmitter, (5) digital frequency display, (6) common standard components (nothing mechanical, no air variable capacitors, everything must be easily obtainable on sites like Mouser and DigiKey), (7) SMT capability, (8) inexpensive ($20 is my goal, but that's a tough goal!). My designs are changing daily, so I'm not going to waste time posting schematics every time I write on this blog, but here are some photos and videos of the product in its current state.
(I just found that last video - it was one of my favorite songs as a teenager, performed live!)
UPDATE: I got a cool dual 16-bit counter IC made by TI, a SN74LV8154N - very cheap, and can be configured as a 32-bit counter. It seemed like a better option than multiple 8-bit counters, and this chip is about $0.60 so if I can make it work I'll be happy! I breadboarded it up (see circuit diagram) and it seemed to work. I started wiring it on the perf board, but haven't written software for it yet...
UPDATE - I just found this video on youtube I never posted on my blog, so this seems like an appropriate location for it:
This blog has become too much about electronics. Yeah, electronics projects are good but there's more to life than electronics! Right now I'm in dental school and... [pause] oh, yeah I don't want to reflect on that. Let's talk electronics! I keep planning to eventually write what dental school is like from an honest perspective for its archival significance, but the thought of spending time writing about dental school makes my gut churn. There are so many other things to do than to reflect on "that place". My home is my escape, this website is my escape, and when I'm ready to talk, I'll talk. This is a note to myself to, at least once before I graduate, write what it's like to be in dental school. Back to tinkering. I leave you with a selection from Dvorak's Piano Quintet Number 2 (opus 81). It's not a wonderful recording, but I appreciate the performance.
EDIT: link updated in 2020 because original video is down. The referenced portion of the song is this video starting at 27:30.
Today is a very special day, as it's the day I first made a contact with a radio transmitter I built completely on my own! The plans were copied from no where (although the concepts were obviously learned elsewhere), so it's somewhat of a unique design (likely because it's not very good!). I'll be the first to admit there is MUCH room for improvement, but my goal was to design and build a multi-band transmitter which would produce RF (not necessarily efficiently) at multiple bands by dropping in crystals of different frequencies.
My first QSO was with Bob, KC8MFF in West Virginia at 5pm today on 7MHz. He heard me calling CQ and replied! He gave me a 559 which made my happy. I was sending about 8 watts at the time into a Mosley Pro 67 Yagi at 180FT and receiving from a 40m dipole at 150FT at the W4DFU Gator Amateur Radio Club station in Gainesville, FL. Although he's was about 650 miles away, I hope to make a more significant contact as the band opens up later tonight. It's such an exciting feeling! The aluminum plate gets very hot (even with the fan) and there's a slight smell of smoke whenever I transmit, but it adds to the fun I guess! Here's some information about the build, though I'm confident it's less than optimal.
I'll preface this by stating that my goal was to produce an experimental platform which I could use to investigate construction techniques of small moderate-power transmitters. This is by no means a finished product! Much work (and some math) must be done to calculate the best number of turns on each coil for each band, including the RF choke on the power (resulting in class C amplifier behavior), the RF transformer, and the inductor/capacitor values of the low pass filter - all of which were determined empirically (watching output on an oscilloscope while adding/removing turns on a toroid). At 10W, it's not QRP, but it's easy to tone down to QRP (5W levels).
One of my desires was to create a transmitter which could be built at minimal cost (total value of this is probably about $10). The microcontroller (ATTiny2313) was what I had on hand ($2), the buffer chip acts as a small amplifier ($0.50), and the power amplifiers are IRF510 MOSFETs ($1). The rest of the components are junkbox, and their values aren't really significant! The power supply is a 19V 3.6A power supply from an old laptop - small, convenient, awesome! Hopefully with some tweaking I'll have a nice transmitter which I'm proud to share and have replicated...
The overall schematic represents a crystal clocking a microcontroller at the transmit frequency, where the CKOUT fuse has been set, producing 5PPV square waves. These trigger an inverting buffer which (a) amplifies the current of the signal and (b) provides an easy source of inverted signal. The two (inverse) signals then fire a pair of IRF510s in tandem, each acting as a Class C amplifier producing about 60PPV waves (not quite as square-ish). The output is low-pass filtered with a Pi filter (3 pole Chebyschev), then sent to an antenna. Nothing special has been done to match the output to the antenna, so SWR with a 50ohm load is currently a bit high, but I imagine a variable capacitor on the output LPF would give me something to adjust to improve this. I should probably go back to square 1 and re-do the math from start to finish and follow my impedance values more closely.
Future work will be invested into adding an iambic keyer property to the microcontroller, as well as a button to send CQ at various speeds. It may be interesting to clock this from a Si570 digital synthesizer, allowing me to transmit on any frequency and no longer be crystal-bound. Additionally, using the same oscillator source to power a direct conversion receiver would yield obvious benefit, allowing transmit/receive from a home-brew device at minimal cost. Currently, I'm locked into using a commercial rig as a receiver. We'll see how it goes...
Anyhow, that's that. I wanted to document this because I know I'll look back in the future and laugh at how poorly designed this project is. I'm just amazed it works, and for now this represents a gigantic step step in my learning and growth as an engineer. As poorly designed as it may be, it's something I'm very, very proud of!
Great inspiration has come from Wes Hayward's Experimental Methods in RF Design text. I've been checking it out from the library every few weeks (Interlibrary Loan, from Vanderbilt University to the University of Florida) but I finally got my own copy for Christmas. It's such a great resource! The IRF510 push-pull idea came from figure 2.101.
PS: The image below is of a MOSFET I exploded in the development process. Too much current... oops!
I decided to sit down and build something last night, and I'm surprised by how functional it is! Nothing about it is extraordinarily complex, and it's extremely flexible, accommodating almost any crystal you want to drop in. Although I doubt I'll use this exact design for a permanent transmitter, it was fun to build and I'll post photos hoping to inspire others to tinker with RF circuitry as well! The final device worked on 7.000MHz and had 3 components: power supply, oscillator/amplifier (making 20mW), and amplifier (making 1.5W).
First, I needed an oscillator. I had an easy source of one because I had a pile of ATTiny25 microcontrollers. Often I run a microcontroller at my transmit frequency with a crystal (applied to XTAL1 and XTAL2 pins) and collect the convenient 5V square wave on the CKOUT pin (after the appropriate fuse setting is applied). However, although the ATTiny25 has both XTAL and CKOUT pins, they overlap! This means that CKOUT cannot be obtained when using a crystal. This complicates things slightly...
I ended-up getting a nice sine wave from the XTAL1 pin, although it was less than 1PPV. I tried having this signal directly switch an N-channel MOSFET as an amplifier, but it didn't work that well (a transformer might help increase PPV, but that complicates things). I instead used a 74HC240 (8 inverting buffers on one chip) to help boost the signal. However, 1PPV wasn't enough to get the buffer oscillating. I therefore added a 2 resisters and a capacitor to the first inverting output, such that a persistent low would slowly raise the voltage of a wire, and I attached that wire to the input of the buffer chip. This way, although 1ppv wasn't enough to start oscillations, a few milliseconds of time allowed the inverting output (high when the input is low) to raise voltage of the input until it was enough to fire the buffers. Once it starts, it starts! I'm trilled, because a voltage divider or a potentiometer would have been a pain, and required specific parts.
The result is about 20mW of power with no tuned circuit! This means it will work on pretty much any crystal you can pop in the micro-controller. This may be suitable for a QRSS transmitter, and since we're not pushing any of the components very little heat is produced, should it should be thermostable and easy to regulate. Modulation is achieved by a reverse-biased LED varactor diode varying crystal capacitance to ground, discussed elsewhere on my site so I won't go there again.
Power supply is one I built a while back and had available. 5V for the microcontroller, and 12V for the amplifier. Simple!
Amplifying the signal was pretty easy as well. The 5V signal output of the buffer goes from 0V to 5V, which was enough to trigger an IRF510 N-channel MOSFET with a convenient packaging that I screwed into a huge heatsink. I push the MOSFET a lot, and a lot of heat is produced, but as long as I keep it separate from the oscillator the heat shouldn't affect frequency too much. Although on my workbench I use exposed wires connecting components, this is prone to getting RFI so obviously use shielded cable of some sort, or use extremely short leads. The MOSFET is arranged as a class C amplifier, with a RFC inductor at the drain.
In retrospect I'm doubting that 5V is enough to fully activate the IRF510. I should probably use some method to bring voltage just below firing threshold, so the 5V can more fully open the gate. I'll try that later! The output is filtered with a PI lowpass filter. I use two 1nF capacitors and a coil which I wind until the output on the scope looks acceptable. I know there are more exacting ways. Anyhow, I had fun, so I thought I'd post. I'm just tinkering at this point!
It's putting out about a watt and a half into 50 ohms. How cool? Adding a code key is trivial, as the 74hc240 has "gate enable" pins for easy on/off control - even from a microcontroller! Food for thought... 73!
UPDATE - I decided to slap a 10.140MHz (QRSS window) crystal in there and see what happened. I saw my signal locally (AJ4VD/W4DFU grabber), but not elsewhere, so I left it up for about a day. [Vince Adams, N9VN]() spotted it in IL (about 1,000 miles away) and made a post on a mailing list asking who it was. Awesome! Note that for QRSS I used a lower-current power supply, so I don't actually know what power output was, but I'd estimate it to be about 500mW.
(It's the "V-shape" at the bottom)