- #1
prs725
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Local oscillators that is used in RF transmitters for AM modulation.Any help is appreciated. Thanks !
Can anyone please tell me more about local oscillator?
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Discussion Overview
The discussion revolves around the role and characteristics of local oscillators in RF transmitters, particularly in the context of AM modulation. Participants explore various types of oscillators, their applications, and the differences between their use in transmitters versus receivers.
Discussion Character
- Technical explanation
- Debate/contested
Main Points Raised
- Some participants assert that local oscillators are primarily associated with receivers rather than transmitters, emphasizing their role in mixing with incoming signals to produce intermediate frequencies (IF).
- Others mention that local oscillators can also be used in transmitters, although this is less common, and typically involve oscillators operating at the frequency of interest or multiplying from a lower frequency.
- There is a discussion about the types of oscillators used for AM modulation, with some participants noting that relaxation oscillators do not produce continuous sine waves, while Hartley and Colpitts oscillators are mentioned as typical sinewave oscillators.
- One participant provides a detailed example of mixing frequencies to produce various IFs, illustrating the complexity of the process in VHF and UHF frequencies.
- Another participant clarifies that in AM transmitters, audio modulation is applied to the last amplifier stage, contrasting this with FM transmitters where the oscillator is modulated with the audio signal.
- Several participants reference Wikipedia as a source for further information on oscillators, indicating a reliance on external resources for clarification.
Areas of Agreement / Disagreement
Participants express differing views on the role of local oscillators in transmitters versus receivers, with no clear consensus reached on their primary application or the types of oscillators best suited for AM modulation.
Contextual Notes
Some statements rely on specific definitions of local oscillators and their applications, which may not be universally agreed upon. The discussion includes corrections and refinements of earlier claims, particularly regarding frequency mixing and the types of oscillators used.
- #2
carlgrace
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prs725 said:
Look up oscillator on Wikipedia. 99% of the oscillators used for AM modulation would be the "relaxation oscillator" type.
- #3
Antiphon
- 1,685
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A local oscillator is usually mixed (through a multiply circuit) with the baseband signal. This creates an intermediate frequency (IF) that is easier to work with.
- #4
prs725
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Thank you !carlgrace said:
- #5
prs725
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Thank you !Antiphon said:
- #6
vk6kro
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carlgrace said:
Relaxation oscillators do not produce continuous sinewaves.
Typical sinewave oscillators would be Hartley and Colpitts oscillators. These use tuned circuits with varying methods of connection to a transistor or FET.
Here is a Hartley oscillator. Ignore all the capacitors at left. These are just to get fine tuning of the oscillator.
Only one (say "Ct" ) is needed.
You can read the whole article that this circuit came from here:
http://www.hobbyprojects.com/H/hartley_oscillators.html
There is a link to this site:
http://mysite.du.edu/~etuttle/electron/elect16.htm
which gives a summary of the different kinds of sinewave oscillators.
- #7
davenn
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prs725 said:
The term LOCAL OSCILLATOR is more correctly used in a receiver not a transmitter
its the freq of the LO that is mixed with the incoming signal from the antenna to produce an IF signal which depending onthe received frequency there may be more than one LO and several IF (Intermediate Frequencies) for example
a received freq of 550 MHz mixing with the first LO of 505MHz gives the first IF of 45 MHz,
it then may be mixed again with a second LO of 494.3MHz to give a second IF of 10.7MHz
and finally mixed again with another LO of 10.245 MHz to give a 3rd IF of 455kHz from which the audio is demodulated.
thats a common arrangement for VHF and UHF and up received frequencies
lower freqs such as AM broadcast bands 550 - 1620 kHz more commonly just used a single IF down to 455 kHz ( there are a number of variations around 455 kHz used + - a few kHz)
Antiphon said:
altho that does happen for the transmit side its not overly common. Its more common to use an oscillator either operating at the freq of interest or multiplying up from a lower freq
say a crystal based Colpitts or Hartly osc as VK6KRO indicated.
Also to the OP --- in an AM transmitter, the audio modulation is normally applied to the last amplifier stage of the transmitter. No Local Oscillators involved in that process. This is quite opposite to a FM transmitter, where the oscillator is modulated with the audio signal.
cheers
Dave
Last edited:
- #8
davenn
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davenn said:
to correct a typo pointed out to me
" a received freq of 550 MHz mixing with the first LO of 505MHz gives the first IF of 45 MHz,
it then may be mixed again with a second LO of 34.4MHz to give a second IF of 10.7MHz
and finally mixed again with another LO of 10.245 MHz to give a 3rd IF of 455kHz from which the audio is demodulated.
was just having a "senior moment"
Dave