fm transmitter connect antenna ?

[petehall347]

trying to figure out where to connect an external antenna to this fm transmitter . i doubled up on pics in case 1 of each was better than the others .

[redwire]

Re: fm transmitter connect antenna ?

They typically use the headphone-in cable for the antenna.
I think the FM transmitter PA is on the far edge of the board, it seems to connect to the headphone AGND pad. If you follow the SOT-23 transistor at the far edge.
I couldn't find datasheets on the 31202 IC.

[petehall347]

Re: fm transmitter connect antenna ?

They typically use the headphone-in cable for the antenna.
I think the FM transmitter PA is on the far edge of the board, it seems to connect to the headphone AGND pad. If you follow the SOT-23 transistor at the far edge.
I couldn't find datasheets on the 31202 IC.

http://www.chipdocs.com/datasheets/d...a/TB31202.html

[petehall347]

Re: fm transmitter connect antenna ?

i found some that have an extra wire for the antenna a blue wire . and some that have the antenna etched on the board .this one has neither of those so it threw me as to where to connect and if i need to remove an attenuator or not . .

[R_J]

Re: fm transmitter connect antenna ?

The output of the 31202 should be pin9, follow that pin to the next transistor

[petehall347]

Re: fm transmitter connect antenna ?

The output of the 31202 should be pin9, follow that pin to the next transistor

i just checked and right now pin 9 doesn't seem to connect to anything . i went through everything on the same side of the board . . i dont see any transistors on the other side .
making me wonder if the antenna is maybe under the ic .

[R_J]

Re: fm transmitter connect antenna ?

That should be the output of the pll ic, it is very likely coupled to the next circuit with a small cap, so it might be hard to trace, it wont be directly connected to the next circuit. That ic output is not a power output so it won't be connected to the antenna but to the input of the output section.

[petehall347]

Re: fm transmitter connect antenna ?

i tried to absolutely everything from that pin 9 thats opposite diagonally from the dot . no beep on anything . i know its a bit strange . i randomly tried from other pins in case its coated with something but no problem tracing those pins with my meter . i will check again . i dont really want to be removing the ic to look at the traces .

[petehall347]

Re: fm transmitter connect antenna ?

just checked again a few times and not finding where it goes to . i can trace the other pins no problem ... i know it has to connect to somewhere or it wont work .

[R_J]

Re: fm transmitter connect antenna ?

I did'nt see you already linked the datasheet.

[petehall347]

Re: fm transmitter connect antenna ?

looks same as the one i already posted ... i still cant find continuity from pin 9 to anything .

[petehall347]

Re: fm transmitter connect antenna ?

2 better pics that can be zoomed in

[R_J]

Re: fm transmitter connect antenna ?

here are a couple similar schematics.

[petehall347]

Re: fm transmitter connect antenna ?

is that using pin 3 charge pump 1 for out to amp circuit ?

[R_J]

Re: fm transmitter connect antenna ?

Every diagram I look at is different, this is just the rf drive, the modulation will enter somewhere else.

[petehall347]

Re: fm transmitter connect antenna ?

i don't see any coils on this thing unless they are hiding in smd components .. i would have expected to see open coils and 1 to antenna . still cant see whats used as an antenna .it must have one must it not ?

[R_J]

Re: fm transmitter connect antenna ?

This is a simple low power device, it likely only has one small transistor for the final output. They could use a small cap to couple the r.f. to the dc input wire, audio input wire, a piece of tinfoil in the case.

What was connected to the two elongated holes in the baord?

[petehall347]

Re: fm transmitter connect antenna ?

the batteries connect there .

[redwire]

Re: fm transmitter connect antenna ?

The SMT inductors are dark gray or green, caps are brown or white. Pin 9 must have a via or trace under the IC which then heads over to the RF power amp.
I see the RF power amp as the two transistors at the edge. Note circuit AGND pad connects to the pcb ground pour through the inductor, not directly.

So what I would do is use the AGND pad for the antenna connection, and connect incoming signal GND to the main ground pour.

[petehall347]

Re: fm transmitter connect antenna ?

The SMT inductors are dark gray or green, caps are brown or white. Pin 9 must have a via or trace under the IC which then heads over to the RF power amp.
I see the RF power amp as the two transistors at the edge. Note circuit AGND pad connects to the pcb ground pour through the inductor, not directly.

So what I would do is use the AGND pad for the antenna connection, and connect incoming signal GND to the main ground pour.

pin 9 isnt connected to anything . i have checked a few times now .