Back at it again with CB radio...but this time, with a twist.
If you have a 23-channel (or really any CB radio for that matter) then this mod is STUPIDLY SIMPLE to do. We're feeding a clock signal into one of the crystal sockets or the PLL clock input...depending upon the radio. The donor radio I'm using is an old Midland 18-853 AM-only CB radio. Included features on this model will include:
* Delta Tune - fine tuning with the push of the encoder, should be around 5KHz or so in steps of 100Hz
* Expanded frequency range - Allows use on the 10 meter AM & CW segments (I CAN DO IT!!! I am licensed...if you are not...don't even think about it. Leave 10m to the hams please)
* OLED display - efficient and quite bright, signaled via I2C
* Si5351 synth - stable clock output with frequencies from 8KHz to 160MHz+
Inspiration came from the CircuitSecrets YouTube channel and webpage. Here is what we'll need...
* A donor radio - crystallized ones work best, but PLL ones will work okay too.
* Arduino Nano or Pro Mini, or RP2040-based boards - At least $1 or more on AliExpress
* Si5351 board - Also $1 or less on AliExpress (you'll also need coax for the CLK output. RG174 is fine, RG316 is better)
* Rotary encoder
* OLED screen
That's it, really! Now you'll need to find some space to fit it all in your radio if you can. If not, you can make it an outboard VFO.
The Radio
The radio I want to use is a 1976 Midland International 13-853 AM-only CB radio. It has 23 channels and well...that's about it! It is crystal controlled, and I'll explain why I got crystal controlled later on...
I bought this radio off of eBay for around $25 used, and it was my very first eBay purchase! While it had some cosmetic wear on it, I think that I can take care of that. The S-meter is officially broken - I cannot get it to move but the modulation light lights up when I talk into the mic!
This is about as basic as CB radio can get. I have several Midland products, such as UHF walkie-talkies and even a weather radio - I can't complain, they're a great company and they are based not too far from me, in Kansas City, Missouri.
Why Crystal Control and not PLL???
First off - crystal oscillators are my favorite electronic component. I fell in love with crystal control radios when I first discovered my dad's Montgomery Ward GEN856 scanner (see the previous few articles).
PLL (phase-locked loop), while stable and non-compensating, just doesn't have the same "charm" as crystal control. In fact, PLLs may be limited to how far you can pull them. The Maxon SM4450 I realigned to 70cm is controlled via PLL. Basically, a PLL circuit is determined by a VCO, or voltage-controlled oscillator. Despite this, the irony lies in us installing a PLL into a crystal-controlled radio...
For crystals, it's quite simple...in CB radios, crystals are mixed to get a product of 26 or 27MHz. This is why on most CBs, 10 meters was easy to modify in. A different value of crystal would put you on the 28MHz band - and I hope you had or have a license if you move(d) it to this band. Most, if not all 23 channel radios were crystal controlled. 40 channel radios will always be PLL controlled. As of the early 80s, 23 channel CBs became illegal to sell (like produce and make, not sell second-hand such as eBay). CBs produced today must be 40 channels to comply with FCC regulations.
To modify a crystal-based CB, you only need to remove one of the crystals, namely, Channel 1. You'll then need to figure out the mixing equation. For example, on the Midland, Channel 1 has 23.290MHz as one of the crystals. This is mixed with a 14.950MHz crystal, to get 38.240MHz. Now it is subtracted from 11.275MHz (TX) or 11.730 (RX), giving us 26.965MHz (TX) or 26.510 (RX). This translates to a 455KHz offset between the two frequencies - the IF that most AM equipment operates at. Cool isn't it?
So, here's the plan: We will manipulate the 1st channel using the Si5351. The frequencies we can step up by 10KHz which will, if we still use our formula, equate to all of the CB radio channels...and then some... we'll just need to add our 23 or 24MHz signal to 14.950 and subtract based on TX and RX frequencies.
10 Meter Conversion...or add-on...
So...many hams will convert old CBs (namely SSB ones) to operate on 10 meters. Fortunately, 10 meters also has an AM section, situated around 29.000 to 29.200MHz. 29.000MHz is actually recommended as the AM calling frequency.
Now again, I will warn you...if you are NOT a licensed ham, DO NOT DO THIS MODIFICATION!!! I will not be responsible if you choose to do so. Leave 10 meters to us hams - if you wanna use it, get licensed. It's that simple. I will be doing this mod as I am a licensed ham. In the code, I recommend adding the 29MHz frequencies in steps of 10KHz until you reach 29.200MHz. That way, you'll have 5KHz of offset either way - technically full coverage of the AM portion! 12 meters could also be an option too, but I'd only do this on an SSB radio rather than an AM-only. There's not much AM activity on 12 meters but...who knows? You could be the first to start it!
Actually...see below for a POSSIBLE bonus mod...
More Modifications to Consider
Besides a channelized VFO, a CB radio is an excellent place to start when it comes to modding radios. I've seen several mods which I'll list down here...
* LED S-meter - Using a few passive components, an LM3914, and an audio amplifier, the old analog needled S-meters of the past can be upgraded to a smooth, cool-looking S-meter.
* Roger Beep - Another potential microcontroller project. It'd be cool if it sounded like the Quindar systems the Apollo moon missions used!
* Frequency counter - An obvious one, if you don't have one already.
* ...and so much more!
The sky is the limit when it comes to CB, and to be honest, as I've said before - as long as you're not being a menace to your neighbors or any other user, let your CB imagination run wild.
Installation
First things first, we need to remove the crystal marked 23.290MHz. Remove the crystal and solder the center wire of the RG174 coax to the crystal's input. If you ever want to reinstall the crystal, you can put in header pins to make a crystal socket! You will not want to ground this side of the coax as this will cause ground loops to occur within our radio, causing noise and interference. A decoupling capacitor on this side in series is optional but recommended to keep DC off of the coax. Next, take the other side of the coax and solder the center and the braid to the appropriate Si5351 CLK pins. Solder that to the crystal end between the coax and the crystal board.
Connect the Si5351 module to the microcontroller via I2C control (SDA, SCL, Vcc, and GND). Remember to keep wire runs short and conservative. Too long of wires can introduce noise into the system. Connect the rotary encoder to the appropriate pins, upload the code, and you're done! See how easy that was? Not too difficult on a crystal-controlled CB.
To use this, you'll need to keep the channel selector on Channel 1.
BONUS: Make it a CW Rig!!!
"Just key the transmitter in unmodulated AM and build & install a BFO so that I could receive CW."
"To transmit a CW signal, a person only needs to interrupt the PTT circuit of an AM rig."
^ (These lines change EVERYTHING!!!) ^
So it has been brought to my attention, via this website, that AM-only rigs have the capability to key CW! I've been a ham for 5 years and this is news to me but makes sense, since CW is no more than just the unmodulated carrier that AM produces.
Anyways in order to do this, we can build up an independent 455KHz oscillator (i.e., a 74HC04-based one is easy, at just 5 components!), or we can use another clock output from the Si5351 set to the IF of our AM rig (usually 455KHz) to make the CW signal audible on receive! This will also allow us to hear SSB signals, but for the sake of simplicity, we won't be making an SSB rig - that's tooooo much of a modification!!! In theory, this is a crudely BFO'ed AM radio that can receive both SSB and CW, injected directly at or around the IF transformer site. All you have to do is narrow the audio bandwidth down to your desired frequency, i.e., between 600 and 800Hz. You can use a passive audio filter to achieve this. If it were me, to avoid the possibility of retuning the IF stages, place the BFO close to the 455KHz IF stage. That way, it will inject the signal wirelessly without any on-board modification.
"Keying" the transmitter involves shorting the PTT to ground - that's it. No audio, no phase shift...nothing. Just shorting PTT to ground. You wanna make a computer-controlled keyer? Optocoupler from RTS/DTR and ground and the output pins on PTT and mic ground.
In essence, this sends an unmodulated carrier over the air, which by definition is what traditional CW is! On receive, we may have to adjust our frequency offset in order to receive CW with the included BFO. We can set our frequency limits of our BFO to be between 450KHz and 470KHz, though in theory, 455KHz is where we want to be.
CW, especially with a stock 4 watts or less, is MUCH more efficient than AM. In fact, it has an awesome SNR in comparison to other voice modes at such low levels. Stay tuned as I embark on this exciting and wonderful adventure!!!
Oh, and the Si5351 clocks I have work GREAT and are STABLE!!!
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