Design of AM receiver 195kHz to 400kHz

[raiya_23]

I want to design a receiver that receives a morse code signal in the 195Khz to 400kHz band. Is it possible using components easily available in the market?

 

[jiripolivka]

I want to design a receiver that receives a morse code signal in the 195Khz to 400kHz band. Is it possible using components easily available in the market?

There are several options:

a. you can design a "direct-conversion" receiver, starting with a mixer fed by a variable-frequency oscillator tuned over 195-400 kHz. The mixer IF output is the audio beat between the oscillator frequency and received signal. So you can make an audio amplifier to feed a loudspeaker at output.

b. you can try an old transistor long-wave radio which would cover a part of your frequency band. You can modify its oscillator and leave the other circuits. Most such radios use 455 kHz IF, so the local oscillator can tune from {455-195} to {455-400} kHz, 55 to 260 kHz

c. There are good amateur receivers on the market, some cover 100 kHz to 30 MHz, some include the BFO, so one can listen to CW Morse code signals.

I would recommend to find radio-amateur and radio-astronomer websites; there are good and simple receiver circuits described and available.

 

[BradtheRad]

This would not be too different from making an AM radio. You could even do it as a homebrew crystal set, although the antenna may need to be longer in order to be sensitive enough to the longer wavelengths.

The band is used primarily for aviation beacons. Each one has a different identifier. It broadcasts this as a series of morse code letters every few seconds.

There's the question whether (a) you will need only to detect the simple presence of the carrier as is normally done with morse, or (b) whether you will need to demodulate the occasional signal riding on a continual carrier.

 

[chuckey]

45 years ago I used to test these transmitters at the manufacturers. For your curiousity, They were 80W carrier, 40% modulated with 400 HZ, which was keyed on and off with a Venner mechanical switch. We had to set the airfields call sign on the edge of the rotating disc by moving little segments. Because of the fact that they used a whip aerial as longwire aerials were not permitted on airfields!!, there was a really posh aerial tuning unit which stood in its own glass fibre sentry box. You could draw 2" arcs from the coils if you were brave. In spite of the extremely high Q coils (>470!!), you could only get about 20W into the aerial.
I suspect , like me, these units have been pensioned off and replaced with a semiconductor based piece of kit, Though if these can withstand lightning strikes is something else.
I would be surprised if you can receive more then one of these transmitters at any one location because of the level of backgound noise unless you have a very narrow bandwidth and a directional screened ferrite rod aerial.
Frank

 

[biff44]

There is a whole heck of a lot of noise in the AM band of 195 to 400 KHz! You are going to want to filter out that noise. I would use a mixing front end to upconvert the 195-400 KHz band either to 455 KHz, or 10.7 MHz. I would then use a standard IF receiver chip and ceramic or crystal filters to amplify the signal. You can detect the AM either with a diode detector after the IF amp, or possibly use the AGC signal available from many chips to mimic the AM envelope level.

 

[raiya_23]

There's the question whether (a) you will need only to detect the simple presence of the carrier as is normally done with morse, or (b) whether you will need to demodulate the occasional signal riding on a continual carrier.

YEP I would like to demodulate the signal

Thankyou everyone first of all I would like to start with the tuning circuit, I just want to use variable inductors and fixed capacitors for that purpose. Is there any free software available that will allow me to design a filter in the 195kHz to 400kHz band? I downloaded TINA TI but its free version does not include the filter design option

 

[chuckey]

" I just want to use variable inductors", Are you going to use a goniometer?, because that what we used in the aerial tuning units! The main coil was about 100mm diam annd inside it was a smaller coil, say 70mm diam. The smaller coil was wired in series with the main coil and could rotate on a shaft running at right angles to its axis, through the centre of it. The effect was that the smaller coil could rotate +-60 degrees so its mutual coupling could go +- to that of the main coil, so modifying the total inductance. Of course our one was rated for 20KV+ RF, I suspect you will be using a ferrite slug, but its a thought.
Frank

 

[raiya_23]

yep ferrite slug it is...or should I go for another type of filter? Are there any ICs available which might work as filters for this type of band?

 

[chuckey]

Frequency range = 195/400 KHZ, therefore inductance swing = 4 :1. With a slug I would have thought 1.2 :1 more likely?
Frank

 

[raiya_23]

Elaborate please?

Frequency range = 195/400 KHZ, therefore inductance swing = 4 :1. With a slug I would have thought 1.2 :1 more likely?
Frank

 

[chuckey]

For tuned circuits F = 1/ 2 X PI X (L X C) ^ 1/2. So to double the frequency of a tuned circuit you must have 1/4 X L.
Frank