How can I improve the range of my AM transmitter?

[medwatt]

Hello,

I have built an AM transmitter and now I can only receive signal within just 1m radius of the transmitter. I use a colpitt oscillator to get a carrier of 740khz. For the modulating signal I'm using a 1khz tone from a signal generator although I've also used an electret microphone. I am using the mc1496p IC with the circuit provided by the data sheet as a double sideband suppressed carrier am modulator.

I have tested every stage and observed a perfect sine wave for the oscillator with a peak amplitude of 0.5v. Similarly the modulated signal from the output of the modulator at pin 6 was about the same value.

The carrier signal was successfully modulated because I could see when the 1khz tone signal superimposed on the carrier from the oscilloscope.

My problem is I can only get the transmitted signal at a distance of about 1m which does not make sense and completely defies the who idea of the design.

What I have done unsuccessfully were to use a 1m long wire and use the areil from a radio as a radiating antenna which I connected to the pin 6. I noticed no change when I moved my radio about. I could only receive anything very close to the transmitter.

I hope someone can give me an idea what to do to improve the range so something like 300m which should be actually the minimum distance for AM signals.

Here is the modulator circuit if you wish to see it: http://www.unipa.it/laser/edt/MC1496.pdf

 

[skeptic2]

You didn't say which of the circuits on that data sheet you're using. If you're using anything but figure 27, that could be your problem.

 

[vk6kro]

If you are measuring 0.5 volts out, you should be able to hear it anywhere in the room, at least, even with such a short antenna.

Have you checked that the carrier frequency really is 740 KHz? Could you be listening to a harmonic of a lower frequency?
If you have an oscilloscope, check that the distance between carrier cycles is about 1.35 µS.
Do this by measuring the time between successive positive cycle peaks with no modulation.

 

[medwatt]

I'm using Fig 24 on page 8 as my modulator which happens to also be the same as Fig 27.
I have checked the frequency I'm receiving using a radio with a digital display and the frequency was 742 khz. When I switch of the carrier I get the noise sound typical whenever there is no signal.

What you're saying: >> If you have an oscilloscope, check that the distance between carrier cycles is about 1.35 µS. << I've already done that before even putting on my radio and it was the result that helped me in tuning to the calculated frequency 1/1.35.

I was wondering if I have to boost the signal in any way before transmitting?

 

[vk6kro]

You could make the antenna longer. 20ft would be OK. If you do this, the "ground" of your circuit should be connected to something large and metallic like a kitchen sink.

The "ground" is the junction point of the negative lead of the 12 V power supply and the positive lead of the 8 volt power supply. This is not made clear in the diagram, but this point is shown with an "earth" symbol in various parts of the circuit.

The output impedance of 40 K is quite high and an amplifier could be added as in the following article:
http://ftp.ing-steen.se/text/tektext/digital-modulation/Exp100.pdf
It is on about the 3rd page.

I have used this chip and found it difficult to use because of the large number of components and the need for dual power supplies. However, it did work OK and had substantial gain as a balanced modulator for double sideband generation.

You could possbly get a boost in signal by increasing the size of the 51 ohm resistor in the input. This could be raised to 470 ohms and reduce the loading on your oscillator.
You may have to put a similar resistor in series with pin 8.

 

[jim hardy]

""I'm using Fig 24 on page 8 as my modulator which happens to also be the same as Fig 27.""

Well almost same.
look at resistor values in carrier null pins 1&4

as cautioned in the text fig 25(hence 24 too) is incapable of am .

Therefore, the modulator may be modified for AM
operation by changing two resistor values in the null circuit
as shown in Figure 27.

i don't know what resistors you have in there.

 

[skeptic2]

The IC you are using is a balanced modulator meaning it is designed to suppress the carrier. In order to receive the signal on an AM radio, you need to restore the carrier. Figure 24 is NOT the same as Figure 27. Please note the values of the two resistors next to the 50K null pot. In Figure 24 they have the values of 10K whereas in Figure 27 a value of 750. See this excerpt from page 9.

AM Modulator
The circuit shown in Figure 26 may be used as an
amplitude modulator with a minor modification.
All that is required to shift from suppressed carrier to AM
operation is to adjust the carrier null potentiometer for the
proper amount of carrier insertion in the output signal.
However, the suppressed carrier null circuitry as shown in
Figure 26 does not have sufficient adjustment range.
Therefore, the modulator may be modified for AM
operation by changing two resistor values in the null circuit
as shown in Figure 27.

 

[medwatt]

If you are measuring 0.5 volts out, you should be able to hear it anywhere in the room, at least, even with such a short antenna.

Have you checked that the carrier frequency really is 740 KHz? Could you be listening to a harmonic of a lower frequency?
If you have an oscilloscope, check that the distance between carrier cycles is about 1.35 µS.
Do this by measuring the time between successive positive cycle peaks with no modulation.

I did today what you suggested by replacing the 51 ohm resistances. Well, I noticed an improvement in range even with no antenna. But there was a little noise. I tested it with a 1khz tone signal of 0.2v pk-pk.
I have noticed other things. I mentioned I was using a Colpitt oscillator and I've attached a schematic of the one I'm using. When tested individual (isolated from the modulator) I could get a perfect sine wave with appreciable pk-pk voltage. When I connected it to pin 8 of the modulator, the sine wave was heavily distorted. I thought maybe I should not connect them directly and I built an emitter follower (common collector) and from there I took the output. I did not notice much difference. I wonder if that has anything to do with the range.

Again I am using a dual power supply with a single common line and two voltage lines; one for positive and one for negative. I have connected the common from the dual supply to the same line where the ground of the modulator circuit is connected. Is that ok?

Something else which I didn't understand is when I have taken the wire from the oscillator and connect it to pin 8, things were as described above. However immediately I take the lead from the ground of the oscillator and connect it to the ground of the modulator, everything seem to diminish. I n the modulating diagram I provided there was just the indication of Carrier Signal In and no indication of whether there should be a common ground for the oscillator and modulator circuits and I just took it for granted that I might just ground them both and carrier completely diminished.

Thanks for reading

 

[vk6kro]

You will be able to hear your oscillator directly with the receiver, so you should aim at getting a clean sinewave and clean modulation at the output of the modulator. You have an oscilloscope, so this should be possible. Ignore what you can hear in the receiver.

That amplifier you showed would have trouble driving a 470 ohm load and you may have to put a 2200 ohm resistor in series with the output going to the modulator. This will reduce the drive to the modulator, but it may help in keeping it undistorted.

If the oscillator is running from the same positive power supply as the modulator, then it should be OK (and essential) to join their grounds together between the oscillator and the modulator. They will both work even if you don't do this, but you could get noise pickup if you don't.

You have the dual power supply connected correctly. The common lead should connect to the "ground" of the modulator.

Your emitter follower may need to have its bias adjusted. An easy way to do this is to put a 4.7K resistor in series with a 100 K pot with a knob on it. You substitute this for the bias resistor in the circuit and optimise the bias to give the best waveform out.

 

[medwatt]

>> That amplifier you showed would have trouble driving a 470 ohm load and you may have to put a 2200 ohm resistor in series with the output going to the modulator. This will reduce the drive to the modulator, but it may help in keeping it undistorted.

I forgot to mention that I have tried that and there was a drop in output FM signal because I could notice an increase in noise from the radio.

I have also be searching elsewhere and I don't seem to be the only one having problems with range. Check this out: http://www.ramseyelectronics.com/forum/showthread.php?t=2995

 

[vk6kro]

First you need to have something to transmit that is powerful and undistorted. You really should be getting about 6 volts p-p.

So, something that reduces distortion is worth doing even if it temporarily reduces range. There are plenty of ways of increasing signal level but these are no good if the signal is already distorted.

I notice that you are now not using an antenna at all. This is not a good idea. You need at least 10 ft (3 M) of wire as an antenna to have any hope of receiving this signal.

When used normally, this chip would be followed by several stages of amplification and mixing before it is connected to an antenna.

What happens if you rotate the "carrier null" pot? It should null out the carrier in the centre position to produce double sideband output. This would sound distorted on an AM receiver, so you would have to rotate the pot off centre until conventional AM is produced.