INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

AM transmitter modulation circuit design question

AM transmitter modulation circuit design question

(OP)
I'm currently in the process of building an AM radio transmitter, and it's working, but not great. When modulating the signal, a section of the negative part of the wave is being cut off. Does anyone know what might cause this to happen? Here's an image pulled from the oscilloscope, with the input signal (yellow) and the modulation (blue):

RE: AM transmitter modulation circuit design question

What sort of modulator circuit are you using?

https://en.wikipedia.org/wiki/Radio_transmitter_de...

The most straight-forward concept is 'Plate Modulation'. This is where the high power audio signal is imposed on the 'Plate' (or solid state equivalent) voltage in a final power amplifier stage.

You might simply be over-modulating, and it just so happens that you're running out on the negative side before the positive. In other words, if it's 'Plate Modulation', your plate voltage is going negative.

RE: AM transmitter modulation circuit design question

The most frequent cause of THAT picture is your amplifier is operating too close to its cut-off and is not amplifying the lower part as much as the upper part. That's why it is very flat and 'quiet' down there. You need to move it's operating point up and possibly not go for as much gain or raise it's bias voltage.

It would be very useful to show us the schematic. Use the picture with the circle in it.

Keith Cress
kcress - http://www.flaminsystems.com

RE: AM transmitter modulation circuit design question

Increase the negative supply voltage.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close