need help again

[chinsoon]

Hi there,

I have some questions here to ask. Please refer to the circuit diagram that I have posted.

It is a RF circuit for anti theft wireless system. But the problem is the receiver module. Here in the circuit diagram, it shows the connection without the pin number. I have referred to the data sheet and I assumed that the antenna is connected to the FM RF In, ground to GND. But I can’t figure out the other 2 more terminal, which is connected to the transformer.

Besides that, is the transformer X2 used for impedance matching?

And for the receiver, must I use external tuning circuit so that it can be tuned to the transmitter frequency in this case? If yes, then what is the variable capacitor doing there in the transmitter circuit. At first I thought that the transmitter is the one to be tuned.

Thanks in advance for the feedbacks.

 

[Nigel Goodwin]

Hi there,

I have some questions here to ask. Please refer to the circuit diagram that I have posted.

It is a RF circuit for anti theft wireless system. But the problem is the receiver module. Here in the circuit diagram, it shows the connection without the pin number. I have referred to the data sheet and I assumed that the antenna is connected to the FM RF In, ground to GND. But I can’t figure out the other 2 more terminal, which is connected to the transformer.

They are clearly marked as 'speaker outputs' on the diagram you posted.

Besides that, is the transformer X2 used for impedance matching?

Yes, but mostly to increase the signal level, and to reference the speaker output to ground (presumably it's not so in the module?).

And for the receiver, must I use external tuning circuit so that it can be tuned to the transmitter frequency in this case? If yes, then what is the variable capacitor doing there in the transmitter circuit. At first I thought that the transmitter is the one to be tuned.

If the receiver can't be tuned, then tune the transmitter to match it.

 

[chinsoon]

They are clearly marked as 'speaker outputs' on the diagram you posted..

but i have checked the data sheet and there is no label about the speaker output. so can i assume that the 2 pins are the amplifier output in the IC?

If the receiver can't be tuned, then tune the transmitter to match it.

because i refer to some old radio, and there are tuner at the receiver(using the same CXA1019 IC), but i was thinking in this case the tuner is at the transmitter. So just to double check is that correct. by the way, if the IC cannot be tuned, then what is bandwidth of the IC? as in what is the range of frequency that the transmitter should transmit so that the IC can detect?

 

[Nigel Goodwin]

You need tuning at both transmitter and receiver, but they don't have to be adjustable. Do you have the datasheet for the receiver module?, to me it looks like it's expecting a modulated signal, and the transmitter is just an RF carrier.

 

[audioguru]

The transmitter circuit is garbage since it will change its frequency if anything moves near its antenna.

The CXA1019 radio IC was used in Sony Walkman radios many years ago.

The radio produces inter-station noise when the transmitter is not received which keeps the relay off. When the FM carrier is received then inter-station noise is much less and the relay is activated.
Hopefully there are not many FM stations around.

 

[chinsoon]

You need tuning at both transmitter and receiver, but they don't have to be adjustable. Do you have the datasheet for the receiver module?, to me it looks like it's expecting a modulated signal, and the transmitter is just an RF carrier.

for me was that the IC has some sort of bandwidth at the receiving pin. which means that when the antenna is connected, it can receive signal within a specific bandwidth. so the receiver does not need to be tuned.

the problem is that i have gone through the datasheet for the CXA1019 and i cant find any bandwidth. and since now you say that the receiver has to be tuned also, so i think what i know is wrong.

so what kind of external tuning circuit i need?

 

[audioguru]

Sony's datasheet for their old radio IC shows only 3 tuned circuits for FM. Two of them are for tuning different station frequencies.

It doesn't tune the station before the RF amplifier so it overloads easily.
It doesn't have tuned circuits in its IF amplifier so its selectivity is poor.
It makes a very cheap radio.