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Can anyone help me about this circuit - RF Cafe Forums

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Post subject: Can anyone help me about this circuit Posted: Thu Sep 13, 2007 3:26 am


Joined: Thu Sep 13, 2007 2:38 am
Posts: 3
Hi all,

I have a problem with this circuit.

This is a circuit of an ASK Transmitter module. Circuit works between 4VDC to 12VDC. The module have only 4 pins. GND-DATA-VCC-ANT. I want to understand this circuit deeply and than make my own circuit. But my circuit must have more output power and it would be very nice to have that power at 5VDC. How can I modify this circuit.

Thanks for your help.


Adress of the circuit.


Post subject: Posted: Thu Sep 13, 2007 3:20 pm

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello dgnclm,

To me this circuits looks like a Colpitts Oscillator. The capacitors C5,C6 and L3 form a 180 degrees network and set the oscillation frequency and the transistor forms the extra 180deg to close the loop.

Capacitors C7, C4 are used for matching to the antenna port.
Inductors L1, L2 are RF chockes.

The data input activates the oscillator circuit by causing the NPN conductor to conduct and this makes the ASK operation, i.e. when there is data then there will be an output carrier at the antenna port.

If you want to increase the output power you need to connect an amplifier at the antenna port, this can be a MMIC amplifier or a discrete amplifier.

It will help if you can provide the following information:

Which gain and output power do you require?

Hope this helps!


Post subject: Posted: Tue Sep 18, 2007 10:11 am


Joined: Thu Sep 13, 2007 2:38 am
Posts: 3

Thanks for your reply.

I have been searching net based on your reply. Yes, it must be colpitts oscillator.

There is no doubt that C4 and C7 are for matching and L2 is RF choke. But I am not sure with the L1.

Also C5 is generally is optional in SAW RESONATOR based designs. Please check ... mitter.pdf
One port saw resonator is a 3 terminal device there is input output and chasis. Generally input and ground is short circuited. I believe that C5 is for EMC related cap.

With the output power, This circuit has about 10dbm to 12dbm output power. I need something like 25dbm-40dbm. I hope that reducing R2 or R1 can solve the problem but I am not sure.

I have upload a better looking circuit ... otostream/


Post subject: Posted: Tue Sep 18, 2007 2:28 pm

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany

I am not sure if reducing the value of both resistors will help you to gain the required power.

The range of output power which you stated is hugh: 25dBm=0.3W, +40dBm=10W!! This has a hugh impact on the active device that you will choose as an amplifier. You should narrow your range and state exactly which output power you need.

Regarding SAW resonator, this works basically as a high-Q filter and intends to deliver the energy from the tank to the transistor. Therefore, both pins: IN ans OUT should be used, and the IN pin to my opinion can't be shorted to GND.


Post subject: Posted: Wed Sep 19, 2007 2:08 am


Joined: Thu Sep 13, 2007 2:38 am
Posts: 3

Sorry!! the values are that I wrote is in mW range. That means now device has 10-12mW, I need 25-40mW. Basically I would be enough for me to extend range %60-%70.

With SAW resonator I see maybe 10 different saw device manufacturers datasheet all of them shorts 2 terminals I do not get it. Check

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