schematic AM-Receiver for Aircraft communication

AM-Receiver for Aircraft communication

AM-Receiver for Aircraft communication (118.250MHz)

This side will explain my construction of a AM-receiver at 118.250MHz (This frequency about the FM Radio receiver  frequency). Normally FM Radio using 88.0MHz up to 108MHz.

The receiver is tunable by a DC controling-voltage.

All contribution to this page are most welcome!

Background
The aircraft communication Some country in the world is still Amplitud Modulated (AM). The local airport (Axamo) use the frequency 118.250 MHz.

The reveiver I will explain is a tunable AM-receiver for this frequency. The receiver is instead manually tunable with some 100kHz around the 118MHz.

The output from the receiver is a low level output (100-200mV) so you must connect it to some kind of amplifier. I will not explain how to build an audio-amplifier.

Block diagram

The hart of the receiver is the Voltage Controlled Oscillator (VCO). The Frequency from the VCO is controlled by the tuning voltage (0-9V). The Antenna is connected to a preamplifier tuned to 118MHz.The RF and LO is then mixed in the SA602 circuit and the product is a 455kHz IF-signal.

To demodulate the AM signal, I have used a circuit called TCA440. This circuit is a complete AM-receiver, but it's range is upp to 50MHz so that is the reason I have an external LO and Mixer. II will only use the IF-part of this circuit. The output from the circuit is the audio-signal.

The complete shematic: 

More details The VCO The VCO is based on a Hartley oscillator. The frequency is determined by L1 and capacitor C1. The Vtuning voltage will change the capacitance in the varactor BB132 wich will change the oscillation-frequency. The value of capacitor C2 will determine how much the frequency can be changed by the tuning voltage. The larger value the more the frequency will change. Since the receiving-frequency is a narrow band I would prefere to make the tuning range as small as possible, else you will have difficult to find and set the receiving frequency. The RFC is RF-choke. Use a ferrite block or torroid. This component is not critical. L1 is a bit more difficult to make. This is an air-coil with 5 turns and the diameter is 7.2 mm (use 0.5 mm wire). The tap-point is about 2-3 turns from the ground. The best way to get the oscillator work is to attach a oscilloscop to the output (pin 6). Tune C1 so the output frequency is 118MHz. If the amplitud of the oscillator is weak around 118 MHz, you must move the tap-point a bit. The best way is to make 3-4 coils with the tap-point at different places and test each coil before you decide wich one you will use. The amplitud from my VCO is about 140mV. Remeber: When you are building oscillators you must keep the wires short and shiled the oscillator! then it will work nice.  The RF-Preamplifier The preamplifier is constructed around the dual-gate MOS-FET BF990A. Maybe you don't need this preamplifier, in that case you can exclude it. Look in the datasheet how to connect a tuned circuit for the input of the SA602. If you cant find BF990A, you can use another dual-gate MOSFET. There are lots of them you can use. BF998 is popular. Gate2 is connected to VCC for highest amplification. Gate 1 is connected to a tuned circuit consisting of L2 and C3. L2 and C3 makes a narrow band filter wich should be set to 118MHz. L3 and C4 is the output filter from the MOS-FET. This filter should also be tuned to 118MHz. Adjust C3 and C4 till you find the best audio-signal. L2 and L3 is (110nH) 4 turns air-coils with diameter of 7.2 mm. The antenna should be 1.27m or half of it 63 cm. Remeber: When you are building preamplifiers you must keep the wires short and shiled the amplifier! then it will work nice.  The mixer and the AM-demodulator The mixer is based on the circuit SA602, see the datasheets for more information. The TCA is a AM-receiving circuit, see the datasheets for more information. Any AM-demodulator circuit can be used. IF you don't find the TCA 440 you can buy a cheap radio with AM-receiver and locate the yellow IF-can inside. This can is for the AM IF-filter. Disconnect the input to the IF-can and attach it to the ceramic filter from the output of the SA602 circuit and you have an AM-demodulator with speaker-amplifier. If you still have difficults to identify the part inside the radio, try to find the datasheet for the main circuit on internet. I have been buying sheap radios and often found CXA1191 inside. On internet I found schematic and at the bottom-left corner you will find the IF-coil. The pin15 is the output from the internal mixer and pin17 is the input for the IF-amplifier and demodulator. Simply disconnect the wire from pin15 and use the wire as an input for your own IF-amplifier. I guess you got the point!  Building and tuning I will not attach any circuit layout or PCB, because the full construction is easy to build. You will just have to do it yourself. 

The first thing you should examine is the LO. Make sure it works and adjust it around the 118MHz. Use a frequency-counter or oscilloscope. 

Adjust the Vtune voltage from 0 VDC to +9VDC and make sure the LO frequency changes.

Now comes the tricky part, I hope you are a calm person, if not skip this project. Try to find the frequency you are searching for.
The Air-port here use the 118.250MHz so the LO should run at 118.250MHz + 455kHz = 118.705MHz.
The tricky part is that the communication is not so common at this Air-port, but if you live close to a large Air-port you will find communication all the time and won't have any problem to tune in the receiver. Tune C3 and C4 until you get the pest performance.
Remeber that the LO is very sensitive for disturbance and interferense. The frequency can change very easy. One misstake I did was to tune the LO with a frequency-counter and when I disconnected the counter the LO frequency had changed, so the best way is to tune by your ears.

One easy way to make your receiver work properly is to build a low power transmitter from a hartley oscillator like the one in this construction. Tune the oscilllator to 118.250MHz (use a frequency-counter). This is exactly the same frequency you want to receive. Attatch Vtune to some kind of signalgenerator. Ex. 1kHz oscillator or to the audio-output from a radio. Attach a short piece of wire to the output from the hartley oscillator as an antenna. The transmitting signal will be FM but it will still work good for your AM reciver.

When you got the transmitter working, put it some meters away from the receiver and tune your receiver to the best receiving condition.

At 127.200MHz I found the automatic-weather reporter wich is transmitting constantly.
I used this to make sure my receiver worked.



Future project
After spending lots of houres to tuning and listening I have decided to Improve the receiver with a PLL frequency synthesizer.
A PLL frequency synthesizer is a frequency multiplier. It multiply a reference frequence till the desired frequency.
Example: Let's say the reference frequency is 5000Hz and you want to receive at 118.705MHz, the multiplier should be set to 23741.
5000 * 23741 = 118.705MHz. How it all works will be explained later.
The synthesizer will set the VCO to desired frequency. The synthesizer will be controlled from a computer or a microcomputer like PIC16F84.

The project will be finished in Jun 2006.