Shack in the Box: A 2m/70cm FM transceiver

This is a project that I started last year and, after a few weeks, lost interest in it until recently.

During my usual web snooping for interesting parts, I came across these VHF and UHF modules being sold by Nice RF in China.  About the size of three side-by-side postage stamps, each is a complete (except for the low-pass filter and audio power amp) transceiver for the VHF (136-174 MHz) and UHF (400-470 MHz) bands.  The modules are good for about a Watt output, and are programmed serially via a series of AT commands - perfect for interfacing to an Arduino!

The modules are the ultimate black-boxes and contain all of the RF, switching, demodulating, and CTCSS/DCS circuitry, so building this rig has been much more of an effort in programming than it has in building hardware, which is partly the reason that I lost interest in it last May; I had gotten stuck in how to program a few functions and temporarily burned-out.  But, after taking a break from it, I've been making good progress over the past few days.

One of my peeves about many 2m/70cm rigs is that they're not at all intuitive to operate.  My ancient Kenwood TR-7950, on the other hand, is a breeze; even someone unfamiliar with the radio could easily figure out how everything works on it, so I used that as my "model" for the user interface.

Basically, one can either enter the desired frequency via the keypad (VFO mode) or recall it from one of 16 memory channels, selected using the pot to the right of the display.  The display shows the frequency, repeater offset, tone frequency and mode, and also indicates whether the selected memory is in, or locked-out of, the scan list.

The keypad itself is a 16 switch matrix that I purchased from Ebay.  To conserve I/O, I've interfaced it to the Arduino via a resistor divider network, allowing it to be read using only one analog I/O pin.  Likewise, the memory selection is done using a pot and another analog I/O pin.

Serial communication between the Arduino and VHF/UHF modules is done using the "Soft Serial" libraries, which allows using other I/O pins for serial comms and keeps the UART free for programming/debugging.  

Not shown on the VERY INCOMPLETE schematic below is the LCD module, which talks to the Arduino over the I2C buss. 

Cosmetically, I've long been intrigued by the Paraset tranceivers that were provided to the resistance during WWII, so the vague resemblance isn't coincidental.

Obviously, this is still very much a work in progress, so I'll be posting a few follow-ups as things progress.  I just got this to the point to where I've made a few QSOs with it, so I wanted to share it in its raw, al fresco state.

Preliminary (very!) and incomplete schematic - for inspirational purposes only, do not duplicate!!!

 One may wonder why anyone would go through the trouble of building something like this when you can order something like a Baofeng for $30.  That's a valid question, and my answer is simple: Because I can!

Giving A Voice to the $20 Bill - In CW.

It's been said many times that there's nothing like the sound of a direct conversion receiver, and a couple of weeks using the $20 Bill has certainly affirmed that for me, the direct-to-audio experience with a competent DC rig is indeed something special.

But, I didn't get into this hobby just to listen, so it didn't take long (maybe two days) before I started noodling and doodling a transmitter board to install inside the rig.

Being true to the theme of the original rig, I used all common and inexpensive transistors: 2N3904, 2N3906 and a pair of BD-139s in push-pull to deliver roughly 5 Watts when using a 13.8V supply.  Nothing fancy and nothing being pushed very hard makes for a predictable and stable QRP transmitter.

"Splicing" it in to the rig was simple: The VFO from the receiver is "hard wired" to the input of the transmitter, meaning that it is always connected to both circuits rather than switched between RX and TX.  At the other end, I connected a relay at the junction of the mixer and bandpass filter to switch the filter/antenna circuit between the two modes.  Similarly, a relay is inserted between the audio driver and power amps, allowing switching between receiver audio and transmitter sidetone.

A few hundred Hz of VFO shift is needed when transmitting, else people will reply at your zero-beat frequency.  To accomplish this, one set of relay contacts switches a gimmick capacitor in parallel with the main tuning capacitor.  The gimmick is simply a pair of insulated wires twisted together, the number of twists was determined experimentally.

To use the rig as a transceiver, you simply tune stations to the lower side of zero beat, flip the T/R switch and pump the key.  It doesn't get much easier.

Since building the transmitter board, I've made several QSOs and, other than my signal being weak, the reports are that the tone is pure and free of annoying clicks and/or chirp - everything I look for in a CW transmitter.

Simple rigs like this are a blast to operate.  The appliance guys don't know what they're missing!


 

Cheap and Easy Audio Filter For DC Receivers

I've been having a lot of fun with the $20 Bill rig, it works great, but, as with anything, there's always room for improvement.

When I designed the thing, I was going for simplicity, so the audio bandpass filtering was very rudimentary, basically consisting of the coupling capacitors to limit the lows and a couple of shunt capacitors to roll-off the highs.  I like a wide receiver, but after a few days, even I thought it was a bit too wide. 

My first few efforts used discreet components, but these either proved marginally effective or used more parts than the rest of the rig, so, alas, I broke down and resorted to the magic black boxes.  But, rather than use a modern DSP subsystem, I went somewhat old-school and used an IC that's been around pretty much forever: The LM324.  The '324 is a quad op-amp that's very common and dirt cheap; yeah, there are better parts, but the ol' '724 is good enough for this application.

 There are excellent active filter tutorials elsewhere on the web, so I'll just describe the circuit at a very high level:

Each stage is comprised of a high-pass filter (3.3K in series with .01uF) on it's input, and a high-cut filter (68K paralleled by .01uF) in it's feedback loop.  By cascading eight stages, I was able to get very decent stop-band attenuation; four stages probably would have been perfectly adequate, but more is always better, right? 

Here's what LTSpice says the passband should look like:

 

Built in the physical realm, it's a bit tighter than that, likely due to the loose tolerances of the parts I used, which is to be expected when the resistors and capacitors cost less than a penny each when bought in bulk.  But, it's good enough!

With any filter, the real test is in listening to it: What's it sound like?  Actually, very nice - it reminds me a lot of the 3KC filter in my SX-101A, very pleasant when listening to SSB signals and more than adequate for casual CW.  My only gripe is that it's a bit noisy, but that may be due to where I have it tacked into the $20 Bill - I inserted it after the volume control, and a better place for it may be at the input to the power amplifier stage.  Maybe I'll try that tomorrow, or maybe I'll do something else.

I'll close with this teaser:  There's another hardware defined transceiver on my drawing board (screen?) and it'll have a very unique architecture - something I haven't seen anyone do before, and there may be a very good reason for that (like, it doesn't work!)