How To Make AM-Receiver for Aircraft communication

Background
The aircraft communication in Sweden is still Amplitud Modulated (AM). The local airport (Axamo) use the frequency 118.250 MHz. The reveiver I will explain is a AM-receiver. The receiving frequency is controlled by a Frequency synthesizer.The advantage by using a Frequency Synthesizer is that the the synthesizer will keep the VCO frequency VERY exact and VERY stable! There will not be any complicated tuning. The desired VCO frequency is set by a digital number in the synthesizer circuit. The synthesizer circuit uses a crystal controlled reference frequency to achieve the control of the VCO.
To control the Synthesizer I have add a micro-controller (PIC16F84). Maybe you know how to handle this circuit? IF not, don't give up! You will have to program the synthesizer-circuit in other way. The programming of the synthesizer is explained in the datasheets.
You can find homebrewed programmer for PIC16F84 all over internet.


Block diagram





The RF-signal is amplified in a FET preamplifier. The RF-signal is then mixed with the VCO signal. IF the RF is 118.25MHz and the VCO is 128.95MHz the product will be : 128.95MHz - 118.25MHz = 10.7 MHz. Most AM-demodulator doesn't work with 10.7 MHz as IF, they use 455KHz, so I have add a FM-receiver/mixer circuit wich will down-convert the 10.7MHz to 455kHz.
The MC3371 circuit has also a RSSI-signal (Receiving Strength Indicator). The RSSI will later control the MUTE function of the speaker-amplifier, so if there is no RF-signal the speaker-amplifier should be turned off, and if there is a RF-signal the speaker should be turned-on. The FM-receiver MC3371 mixes the input with a crystal-controlled frequency (10.245MHz). The product will be 10.7MHz -10.245MHz = 455kHz. Since the audio signal is AM-modulated I will not use the FM-demodulator in this circuit. In the blockdiagram you can find two 455kHz ceramic filters. One is for the MC3371 circuit to achieve the RSSI signal, the other filter is for the AM-demodulator wich will demodulate the 455kHz signal to an audio-signal.
The synthesizer adjust the "tuning voltage" of the VCO by probing the VCO-output and compare its frequency with a reference frequency. The desired VCO frequency is programmed into the synthesizer from the micro-controller unit.
I will not explain more how a synthesizer works because you can read about it in the datasheets and there are hundreds of Internetpages just about synthesiszers.
What lable has the circuit wich handle the AM-demodulation?
To be honnest, I don't know! I have found that the easiest and best way is to take your sisters old FM-AM-radio or buy a cheap beach-radio with AM and crack it open! Try to identify the IF input of the Radio-circuit. One advantage to use a radio is that you will automatically have a audio-amplifier.


There are some different ways to find the IF input :

  • Try to find a 455kHz ceramic filter (most often yellow littel plastic box with 3 input). If you find it you will see 3 pin. The middle one is conneted to ground or Vcc. One of the other is the input. To find out wich one, you can dismount the filter from the PCB and then touch one of the input with your finger. When you hear lots of noise in the speaker you have found the IF-input.
  • IF you don't find the ceramic filter you can try to find a IF-CAN, with yellow top. Dismount the CAN and make the fingertest as above. 
  • The easiest way is to identify the RADIO-circuit in the radio. Try to find datasheets on internet and then identify wich pin is the IF-input.

In this project I had a radio with a circuit called CXA 1191. I found a shematic of this circuit on Internet. I removed the IF-CAN and saved the ceramic filter. The 455KHz IF is now at the input of the ceramic filter. I saved the rest of the radio because I wanted the volum control and the speaker-amplifier. Klick on the pic to see what how I did my changes.

As you now understand it was a minor surgery, just a removal of a IF-CAN and attach a wire to the ceramic-filter as input.


More details (Click to see complete schematic:)

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. 10pF will give 1MHz/Volt. The RFC is RF-chokes. 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 3 turns and the diameter is 7.2 mm (use 0.5 mm wire). The tap-point is about 2 turns from the ground. The best way to get the oscillator work is to attach a oscilloscop to the output (pin 6 SA602). Make sure you got a nice sine-wave. Apply a variabel voltage 0 to 5V to Vtuning and make sure that the frequency changes. The VCO should be working at 128MHz when the Vtuning is 2V. Apply 2V to Vtuning and adjust C1 until the frequency is 128MHz. This is all the tuning you need to do, the synthesizer will make the fine-tuning for you! 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.
A simple way to tune this preamplifier is to build another VCO and connect a audio-source to the Vtune. Put a wire at the output of the VCO. You have just build a very simple transmitter. Tune the transmitter to 118MHz and put it at a distance from your receiver. Now you can fine tune your receiver for the best performance.
Remeber: When you are building preamplifiers you must keep the wires short and shiled the amplifier! then it will work nice. 
The mixer
The mixer is based on the circuit SA602, see the datasheets for more information.
Synthesizer LMX2337



This is a simple explanation of the synthesizer!
The synthesizer I have used is LMX2337. This circuit is a DUAL frequency synthesizer. The block-diagram above shows one of the two units inside the circuit. As you can see, the reference oscillator is connected to a 15 bit programmable divider (R-register). The output of this divider is connected to the Phase detector.
Fin is (from the VCO) is connected to a prescaler and then to a programmable divider (N-register). The output of this divider is also connected to the Phase detector. Both signal to the Phase detector are connected to the FoLD wich is an output for test purpose.
There are 3 control signal to set the different registers. Clock , Data and LE.
How can all this control the VCO?
Let's say you want to receive at 118.25MHz. The VCO should then run at 118.25 + 10.7 = 128.950MHz.
I have a crystall oscillator at 12.8MHz and I want the reference frequency to be 50kHz.
12.8MHz / 50kHz = 256 : the R-register should be programmed with the value 256.
When the two inputs to the phase detector are in phase ( the same frequency) the output of the detector will be 3-state. If there is any frequency differense, the output will go high or low. Since the output is connected to the Vtune of the VCO the frequency of the VCO will change untill the two inputs of the phase detector have the same frequency.
We know that one input is 50kHz always, and the Fin should be 128.950MHz.
128.950 MHz / 50kHz = 2579 : the N-register should be programmed with the value 2579.
NOW, If the Fin is 128.95MHz, both inputs to the phase detector will be 50kHz and the synthesiser will keep it locked there.
See the datasheets for more detailed information about this circuit!

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 128MHz. Use a frequency-counter or oscilloscope. 
  • Adjust the Vtune with a variabel voltage from 0 VDC to +5VDC and make sure the LO frequency changes.
  • There is another oscillator wich has to work and it is the 10.245MHz oscillator in the MC3371 circuit. Use a oscilloscope to verify that you have a signal. The signal might be low and the probe might interfere, so you should build a FET-preamplifier to your probe. I assume your AM-IF part is working so you can go futher with the tests.
  • To test the syntesizer I attached a oscilloscope to the FoLD output. I programmed this output to monitor the refrence divider wich should run at 50kHz. Then I programmed the output to monitor the programmable divider. I disconnected the Vtune to the VCO from the synthesiser and connected a variable voltage to the Vtune.when I changed the Voltage (0 to 5 V) the VCO frequency changed (116-141MHz) and the output frequency of the synthersizer changed from 45-55 kHz. Now I know the reference frequency is ok and the programmable frequency is ok. The synthesizer can now find and lock to the frequency 128.950MHz. At 127.200MHz I found the automatic-weather reporter wich is transmitting constantly. 
  • I used this to make sure my receiver worked. 

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