News
Spring 2024
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Another P3ST Transceiver Kit Batch is
Open for Sales
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The first batch of the modular P3ST 20-Meter Transceiver kit was larger than fifty units. This batch will close when the number reaches twenty. As soon as it does, parts for the entire batch will be ordered and shipping will commence thirty days after that (Why? See "What is Mostly DIY RF, Anyway" below). It shouldn't take long for the batch to fill up--there are fourteen buyers on Tindie who signed up to be notified when orders would be opened. If you're too late to snag one of the kits before the batch closes, email me and I may be able to reopen sales.
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This batch will include improvements to the motherboard, as well as some fixes of errors discovered by builders of the first batch. It will also include some beefing up of documentation and build instructions.
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!! Warning: New Product Offerings Ahead !!
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Bidirectional PG-TIA IF Amplifier
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This version of the PG-TIA amp follows the original Hayward and Kopski design by being bidirectional. By powering the left to right amplifier, the signal goes that way. If powered right to left the direction is reversed. This is the way the IF amps in the classic BITX architecture work. Bidirectional amps make it easy to use the same filters, mixers, and modulators for both transmit and receive.
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A termination-insensitive amplifier (TIA) such as this one always presents a 50Ω impedance at both its input and output ports, providing a stable and predictable impedance to whatever it’s hook up to. This simplifies the design of the mixers, modulators, product detectors, and filters these TIA amplifiers serve in a complete radio system.
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Like the other PG-TIA amp MDRF offers (the red one), the gain of the bidirectional amps is set by selecting the value of the collector, the feedback, and degeneration resistors. New to the bidirectional version, though, these gain-setting resistors are not soldered in plated-through holes.
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Instead, the PCB pads for them are large squares on the component side only, and you solder the resistors in place as you would with “Manhattan” or “Muppet” construction. This makes them easy to install and just as easy to replace if you want to “program” for a different gain setting. By the way, there’s no reason the left-to-right amp has to have the same gain as the right-to-left amp. There are occasions you might want different gain profiles.
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GPAA-1 General-Purpose Audio Amp
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After several months of off-and-on development and testing (and with some help from Steve AA7U), the well-known general-purpose 3-or-4 stage audio amp by Richard Andersen, KE3IJ (SK), is now ready for prime time. You can use it with all four stages or you can bypass the first stage using jumper blocks.
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The amplifier has a 2N3904/2N3906 push-pull output stage, and it runs on 12VDC (with a built-in 9V regulator). A diode provides reverse-polarity protection. If you’d like to skip the usual (boring) LM386 and put out some real power (2+ Watts) using good ol’ discrete components, give KE3IJ’s amp a try. For a schematic, click here.
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Dual-Gate J310 JFET
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One could call this arrangement of J310 transistors a JFET tetrode. They are similar to dual-gate MOSFETs, but with the characteristics of a junction FET. Once widely used in their TO-92 through-hole versions, the J310 is now available only as a surface-mount device. Pete N6QW pioneered the use of the 310 in a dual-gate arrangement. These little half-inch breakout-boards might make experiments and home brewing with them a bit easier.
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Here's an application that uses J310s in dual-gate configuration as a product detector for a direct-conversion receiver.
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The breakout boards come with the J310s pre-installed. You can add wires or leads as you desire. They would work particularly well for “Manhattan”-style construction.
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XTO-1 Crystal Test Oscillator
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The flood of inexpensive “xtals” produced for computers and other digital gadgets makes homebrewed SSB filters quite practical. In addition to a little advanced knowledge–and a good calculator–a few pieces of (also homebrewed) test gear are needed.
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Some of the now-ubiquitous nanoVNA instruments can be used, especially for measuring and visualizing the pass-band of a completed filter. For the measuring the characteristics of individual xtals, though, the 101-point scan resolution of a hand-held nanoVNA makes it’s use problematic.
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So enter, then, the XTO-1. It’ based on the classic xtal test oscillator devised by G3UUR to find the resonant frequency and other important parameters of a batch of crystals. It is used in conjunction with a good frequency counter (with a resolution to 1Hz). With the help of computer software (several pieces of which are free), you’d then use this data to design a filter of a given 3dB bandwidth, shape factor, pass-band ripple, and other important characteristics. For more on the G3UUR method, watch M0NTV’s video, “Crystal Filters for the Fearful.”
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The XTO-1 is also useful to determine the series resonance of individual crystals to sort them into narrow batches (with a frequency spread of less than 50Hz) for use in filter designs that do not rely on the motional parameters calculated using the G3UUR method. The XTO-1 is available as a kit of PCB and components (including SMA connector) only. Assembly time (including making a cup of coffee and heating up the soldering iron) is less than one hour.
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PWR-1 Power Protection and Filter
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Most of us have been there. For whatever reason, and in fact contrary to all reason, you’ve reversed the polarity of your power connection and now one, some, or maybe all of the active devices in your homebrew rig–transistors mostly (including those built into ICs)–are toast.
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All of us have been somewhere else, too: the “undiscovered country” of noise conducted through our power supplies. Because it’s very uncommon to shield the power cables we connect to our rigs, a lot of noise is also picked up in them, fed into our power inputs, and then bounces around inside our sensitive circuitry. What’s more, we often generate noise inside our gear, commonly from digital devices such as Arduinos, displays, rotary encoders, and the like. In short, we’re happier when we can either eliminate or reduce noise sources–certainly those over which we have some control.
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This is where the PWR-1 comes in. Its P-Channel MOSFET instantly (58ns) shuts off power if polarity has been reversed, and its low-pass and common-mode filtering keep noise down to a tolerable level. Some circuits–such as a direct-conversion receivers–are susceptible to common-mode AC hum. The PWR-1 suppresses that to inaudible levels.
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The PWR-1 has one other optional function: it can sense the total current draw through it, sending an analog signal that can be read by the ADC input of an MCU (Arduino, Xiao, etc.). This is done with a Hall-effect sensor, so full voltage and current are available.
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So the PWR-1 is available in three versions: one with just the fuse and reverse-polarity protection, one that also includes noise filtering, and one for the whole . . . enchilada. Each is available only as a kit.
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Now in Development ⇨
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MAR-6SM+ MMIC Amplifier
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The Mini-Circuits MAR-6SM+ MMIC is small but powerful. It has a bandwidth from DC to 2GHz, a gain of 22dB at HF frequencies, a very-low noise figure of 2.3dB, and–wait for it–a termination-insensitive I/O of 50Ω. It runs on 3.5VDC at 16mA. It’s pretty small, but it will come pre-installed on a half-inch square PCB similar to our BFR106 and dual-gate MOSFETs.
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UDVBM-2 Universal Digital VFO/BFO
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An all surface-mount Universal Digital VFO/BFO Module a quarter the size of the through-hole versions currently available. A Raspberry Pi RP2040 microcontroller and Si5351A clock generator will share the same small PCB and, like its bigger cousins, the UDVBM-2 will feature a filtered power supply and all the digital-bus access (I2C, SPI, 1-Wire) and peripheral connections. The RP2040 can be programmed in either Arduino C/C++ or MicroPython. It operates at a clock speed of 133MHz and has 2Mbytes of flash program memory. The UDVBM-2 will be programmable through its USB-C connector.
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CWAZ Low-Pass Filter
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A low-pass filter (LPF) attenuates the “spurious” harmonic output of a transmitter to an acceptable level. The filters MDRF will offer are based on the “CWAZ” (Chebyshev with a zero) design by Jim Tonne, WB6BLD, and popularized by Ed Wetherhold, W3NQN.
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The virtues of this design is that it uses standard-value capacitors, and that it provides extra attenuation of the second harmonic without increasing the filter’s SWR within the band it was designed for. MDRF will offer the filters for the 160, 80, 60, 40, 30, 20, 17, 15, 12, and 10 meter bands. Initially, they’ll be available for output powers up to 10 Watts, and either as a kit of PCB, toroids, capacitors, and enameled wire, or as a fully-assembled and tested unit.
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dB or not dB? That is the Question
The DSA-1 will be the Answer
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Usually, we Hams want all the signal strength we can get, but there are times when plenty is too much. Some of our circuits are sensitive and can be overdriven into distortion or even damage. Fixed attenuators are handy things to have around, but a step attenuator is even more useful as you can set it for the exact number of dB down you want. Such devices, controlled by switches, have been around a long time, but they’re often big and clunky (and you end up paying more for the clunk).
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But digitally-controlled step attenuators have been developed for the cell-phone industry, and we can make use of them, too. MDRF's DSA-1 will use two Mini-Circuit’s DAT-31A-PP+ chip for step attenuation from 1dB (the insertion loss) to 62dB. It takes care of maintaining impedance, it’s good from DC to 4GHz, and it’s controlled by a five-bit parallel port–yet one more task for our beloved (ahem) microcontrollers.
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What is Mostly DIY RF, Anyway?
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That really should be a "who" and not a "what" question. Mostly DIY RF is just a trade name for me, Todd Carney, K7TFC. I'm a garden-variety amateur-radio enthusiast, and I don't mind admitting I'm also an amateur at manufacturing and business as well. Naturally, I try to do my best at all three, but nevertheless as a true amateur (from the Latin amare: to love; doing something for the love of it).
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As a business, MDRF is the smallest possible: it's just me, K7TFC. There's no other workers or employees, and there's certainly no other investors or sources of capital. The good side of this is that I don't have to answer to anyone else, nor do I need to meet their expectations of profit or capital gains. I can offer products to fellow amateurs that no properly-capitalized company would ever bother with, and I can do so at prices that are lower than what a high-overhead firm could get away with. In fact, my business overhead is the same one I live under, and though I do have some dedicated space for MDRF work, my kitchen table has been pressed into service more than once.
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The downside of being small and self-funded is that I can't produce products quickly at high volumes. In general, I cannot maintain a stock of pre-assembled and tested items (such as crystal QER filters or TIA-AGC amps), so on the website and in product listings I indicate these can take up to fourteen days to ship. My ability to maintain large stocks of parts and components is limited as well, more so for specialized items than for "popcorn" parts. I may need to wait a few days for parts to arrive before getting an order out the door and on its way to the buyer. Given the worldwide disruptions of the component "supply chain" in recent years (a situation still in recovery), delays may last for more than a few days. As I have done in the past, though, I let buyers know what's going on.
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I case you were wondering, I take the mostly in Mostly DIY RF seriously. Selling a QER crystal filter to a homebrewer for his otherwise-scratch-built rig, or a surface-mount dual-gate MOSFET breakout to a builder with aging eyes and shaky hands, or an IF amp with AGC to anyone who doesn't feel up to putting one together (because he chooses his battles carefully), is pretty-much the niche I want to roost in. You know, sometimes there's just too many things to do to get a project working, in addition to all the other things in life. I can't help with any of those, but I am happy to offer some little thing or two that will get your project doable, done, and on the air.
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