Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

FM Transmitter simulation in spice

Status
Not open for further replies.

micro7311

Junior Member level 2
Joined
Jun 21, 2015
Messages
22
Helped
0
Reputation
0
Reaction score
0
Trophy points
1
Activity points
199
Hi, I am looking for some input on a FM transmitter I am trying to simulate in LT Spice. I am simply just trying to modulate an input sine wave with a carrier frequency. My circuit is attached with the simulation results, as well as the oscillator simulation with the input sine wave removed. I don't have any specific design requirements as of yet, I am just trying to simulate a simple modulation circuit. I am trying to achieve ASK with this, and the output waveform does not look correct (Also the simulation stops oscillating soon after I start the simulation).

Any suggestions for improvement are welcome, thank you.
 

Attachments

  • Transmitter.png
    Transmitter.png
    16.3 KB · Views: 42
  • oscillator without Vin.png
    oscillator without Vin.png
    156.5 KB · Views: 34

Audioguru

Advanced Member level 5
Joined
Jan 19, 2008
Messages
9,238
Helped
2,136
Reputation
4,266
Reaction score
1,960
Trophy points
1,393
Location
Toronto area of Canada
Activity points
58,057
Your audio input level is WAY too high (1000 times too high). Instead of 20V p-p try it with 20mV p-p (0.02V p-p microphone level) but you will not see the deviation on a 'scope.
Also the audio input must use a series coupling capacitor so that the transistor is never turned off. The RF oscillation in your SIM stops when the input voltage goes negative which turns off the transistor.
I think your R1 value is much too low, try 47k.

Did you notice that your 'scope shows AM modulation which is normal for this very simple circuit that produces AM and FM at the same time.
 

micro7311

Junior Member level 2
Joined
Jun 21, 2015
Messages
22
Helped
0
Reputation
0
Reaction score
0
Trophy points
1
Activity points
199
Thank you for your help. Actually, my input signal is going to be a sine wave about +/- 9V with a variable frequency (not audio). I want to modulate and transmit this signal via RF. The receiving end will receive and correlate the signal to a specific frequency. I know there exists FM transmitter chips/micros that can do this, but I want to build it will discrete components. My initial thought was just a simple FM transmitter with ASK modulation. Is this something which can be achieved by the means I am proposing?

I will work on decoupling cap and the value of the discrete components.
 

micro7311

Junior Member level 2
Joined
Jun 21, 2015
Messages
22
Helped
0
Reputation
0
Reaction score
0
Trophy points
1
Activity points
199
Or any other simple modulation scheme would be welcomed. I am looking for a simple OOK circuit for this.
 

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
48,381
Helped
14,245
Reputation
28,751
Reaction score
12,932
Trophy points
1,393
Location
Bochum, Germany
Activity points
280,020
I don't understand your problem. OOK (another word for ASK) is achieved by turning the oscillator power supply (or a bias voltage) on and off. No FM involved.
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top