CMC707AMRADIO

It's November 6. How fast time flew by.

     Let's discuss a bit about mixing frequencies. Two frequencies can be mixed in a MIXER  stage. This is evident in the ordinary BC AM radio. The first tranny is the mixer stage.  The antenna tunes to the desired station with a frequency F1. A local frequency F2 is generated and these two frequencies are mixed. The result is the sum and difference of F1 and F2, and the two individual frequencies. Usually, the difference is chosen and is amplified. In the AM band, this difference is chosen to be 455khz. So, the local osc is always 455Khz below the incoming signal. It could well be above too and the 455khz will appear. This is called the intermediate frequency. Thus, a powerful RF amp is fixed tuned to 455khz. All stations are converted to this frequency. The local oscillator can be a single tranny and is usually done in the SW band. For the BC band, the mixer can also be tasked to handle the local oscillator, thus the stage is called the MIXER-OSCILLATOR stage.

     BUILD AN AM BC BAND RADIO AND LEARN ALL THESE PRINCIPLES.

     The tuning capacitor in an AM radio has tqo sections. One is for the antenna and the other one for the osc. They are ganged together. As you tune to a different station, you are also tuning the local osc so that the difference will always be 45khz.

Hi Friends. It is November. Over the long weekend, I completed my FM Receiver using a single FET transistor. MPF102, @N5457, etc...even the 2N5486. The Audio amp is the ordinary LM386 but with a transistor driver. It's load enough to drive a speaker. I even picked up the Control Tower with a change in capacitor. I built the tuning capacitor version and the slug tuned version. The later is compact and will be ideal for our FM Walkie Talkie!

     We can actually make any receiver(except for the very high frequencies) with just one FET transistor. Then we can use a common hi gain audio amp. We shall be moving towards building receivers using the intermediate frequencies, 455KHZ and the 10.7Mhz.

Hi, it's a new week, Oct 29. November is near.

     Anyone who would like to build this simple Walkie Talkie can contact me for advice.


Antenna: The longer the antenna the better but it may not fit your enclosure.  Also a pi network has to be included to match your antenna. It is a simple 2 capacitors before and after an inductor.

FREQUENCY:

     Let's talk about frequency.  The higher the frequency, the shorter the wavelength. Remember that. That is why making a WT in the AM broadcast band about 1 megahertz is not practical. 27 Megs happens to be just the allowed band by the U.S. FCC. Then they allowed at 49Megs. Newer WTs for the US market are on 49Mhz. They allowed for shorter antenna. Now, if we can go 100 Mhz, which is in the FM band, then antenna could be much smaller. A good example is the FM wireless microphones. Can we build a walkie talkie in the FM band?Well, just outside the commercial band  so where it is  unoccupied.  It is easy to make the wireless mic. Circuits are all over the WEB and Alexan Commercial carries a local kit. Now, if we can make a simple DIY FM receiver, that will be fun. An FM walkie talkie. It has clarity advantage but not so much on range. Perhaps we can increase the TX power so we can go 100 to 200 feet! Not bad if it just for camping and biking trips. Or Boy's Scout camps. Or sleeping in tents in the HS campus. Future projects!

                                                TRANSMITTER CIRCUIT ENLARGED


     Here is an enlarged circuit diagram of the transmitter. The Crystal is Base to ground, making this a Miller oscillator. The Pierce oscillator hjas the Crystal Base to Collector.

                                                          RECEIVER CIRCUIT




Great, we are able to move the schematic of the Receiver circuit for a Walkie Talkie Project. Take note, the power to the collector has been greatly reduced by the resistors in series, the 220 ohms and the 4.7k. The 33ufd is a stabilizer. In addition, the crystal was reboved and the 403 cap was directly connected to ground. The audio output is marked.

BK

Hi friends, it's Tuesday, 10/23.

     Well, for today I will continue on the transistor. Look at the transistor as a faucet. Water will flow but controllable by the handle. So with the tranny. The Base is the handle, the Emitter the input and the Collector the outflow. We like the faucet to be opened at a certain point. So with the tranny. We call it biasing. A bit of pus to turn it on. We can uise a single resistor from the plus line to the base , such as 180k. But it is more stable to put a voltage divider. In fact if you are designing, use a variable resistor across the power lines. The wiper moves across choosing the right bias to the Base. Or fixed one and make the other one variable.
     

    In the diagram shown, the biases are the 33k to plus and the 15k to minus. Here it is shown on the TX mode. Note the Crystal to ground via a 403 to Base.