The CB Forum > How CB radio works
Understanding how CB radio works
The Radio Shop:
Thanks Klondike Mike.
In the next few post I want to start with a new subject. Getting to know you way around the radio. I will take a pic of the inside of a Cobra 29 and break it down into 4 quadrants.
PLL section
Transmit mixer
Receive and output
Audio section.
There are actually 5 sections. The fifth section which is the middle of the radio contains IF filters, the AM demodulator and the squelch circuits. But we will leave this section for a later time.
Once you know your way around the radio this makes repairs a lot easier.
The Radio Shop:
Ok, back to Getting to know you way around the radio
Please excuse the crude photo I am attaching.
As you can see I have divided the radio into 4 main groups. Remember, some sections do over lap with other sections. But that is fine for now. Mostly all radios today are built under this design. You will see a few that is a little different. But when you get the understanding of where things are in the Cobra 29 you will easily be able to identify other boards. In my first post is a schematic with certain area\'s labeled. Take the time to open that schematic and find the labeled sections on the schematic. That will give you a better understanding as to where the parts are in relation to the schematic.
PLL
In this area you will find the PLL chip and it\'s supporting components, 10.240 reference oscillator, The VCO and supporting components. You will also find the 5 volt regulator in this area.
Output and TX mixing
In this area you will find the begining of the transmit stage, the transmit mixer, predriver (buffer), driver , and the final
Audio and modulator
In this area you will find the audio stages, the AMC stages and modulation limiting circuits, voltage regulators for TX & RX functions..
The Radio Shop:
What are all these components ??
I know when you are first starting out you look in a radio and say wow. These things are packed full of stuff. Let me see if I can help you out a bit.
TR 1- RF AMP TR 13- Squelch Clamp
TR 2- Buffer AMP TR 14- Final Amp
TR 3- Squaring AMP TR 15- Driver AMP
TR 4- Pulse AMP TR 16- PreDriver
TR 5- Pulse clamp TR 17- Mic audio AMP
TR 6- RF Gain Clamp TR 18- AMC Clamp
TR 7- RF AMP TR 19- ANT Ind switch
TR 8- 2nd IF AMP 1 TR 20- REC/2nd IF osc
TR 9- 2nd IF AMP 2 TR 21- TX supply switch
TR 10- 2nd IF AMP 3 TR 22- Dimmer switch
TR 11- Squelch inverter TR 23- Common 8 volt regulator
TR 12- Squelch AMP TR 24- ANT Ind inverter
FET 1- First RX mixer FET 2- Second RX mixer
IC 1- PLL chip
IC 2- VCO/Mixer
IC 3- TX Mixer
IC 4- Audio output / modulator
D1 & 2- Protection Diodes / Front end
D4- AM Detector/ AGC
FL1- First fliter
FL2- Second filter
The Radio Shop:
Tonight I am looking at showing you all how to convert a cobra 29 into a stable signal generator.
I will put what I can together and try to get this layed out. The generator will need to be 50 ohms, and produce a clean 100uv signal. This will be unmodulated. You will also have the option to drop the 100uv at 10 increment levels. this will be done with a simple divide by 10 network of resistors.
What we need.
A clean stable 100 microvolt signal. ( 100 uv is enough to set a S-meter to S9)
A way to drop the signal for testing the sensitivity and alignment of the receiver. (peak the stages)
The idea of the signal generator is that when you have a radio on nearby, and you turn the generator on with out being connected to the radio you want to test, you should not hear anything coming from the generator. This is going to take a bit of work and a lot of shielding to stop this from happening. The one I built years ago from the RCA co-pilot was mounted inside a steel chassis. The tuned circuits you build will also have to have there on fabricated boxes. These can be made of very thin tin sheets. As long as you can bed it and solder to it. No aluminum. You can also fabricate the boxes from double sided circuit board. This is easy to work with and has good sheilding.
So how do we start?
Looking at the cobra 29 schematic find the TX mixer. This is IC 3. Pin 9 of the mixer chip is your rf out. This goes to L20, the first tuned circuit in the output stage. From L20 we go to R63 then to C65. This is as far as we need in the circuit. Everything from the antenna side of C65 towards the antenna is useless. This is the section we will pick our signal off at and convert it to a usable signal.
At this point in the radio we are going to have around a 1000 ohms impeadance @ about 1 milivolt. As you can see we have to go a long way to get this to 50 ohms @ 100 microvolts (uv). The idea is to get this signal through a cable and to the radio under test and not through residual radiation from a poorly built leaky generator. So what we will do is take our 1 milivolt 1000 ohm signal and force it to 50 ohms @ 500 uv via a 1/2 divider network. To do this we need 3 47 ohm 1/4 watt resistors, 2 100 ohm 1/4 watt resitors.
If anyone see\'s error in what I am saying please jump in and let me know..
More later
The Radio Shop:
Ok,
Here is out network to complete the output stage of the generator. As you can see the divide by 10 stages can daisy chain from each other. Each section must be in it\'s own sealed box. All componet leads must be as short as possible. You can use the small teflon coated coaxial cable to connect is stage. Do this by drilling a small hole to pass the coax through then fold the braid back and solder to the enclosure.
Remember, ever component in this stage radiates RF.
I suggest finding and enclosure that will house the radio and other component. Also to house future upgrades that can be added. I would even reinstall the original covers on the radio. Everything must be grounded and sealed. So get you some aluminum tape.
More later
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