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It seems that a LPF is contained in PLL and not in frequency-synthesizer. Maybe I had made a wrong judgement.

wu yi
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  • What do you mean by a frequency-synththesizer? I read that as direct digital synthesis. – RoyC May 06 '19 at 14:13
  • Pretty much any *useful* frequency synthesizer with an *analog* output will almost irrespective of architecture need to have a filter of some sort to clean up the output, pretty commonly a low pass filter or else a band pass one. – Chris Stratton May 07 '19 at 20:34
  • I have got what I need from @Bimpelrekkie. Now I have realized the question I asked is not rigorous enough. From your answers, I find the beauty of preciseness. Afterwards, I will be careful with what I may ask. – wu yi May 08 '19 at 05:22

4 Answers4

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According to Wikipedia a frequency synthesizer

is an electronic circuit that generates a range of frequencies from a single reference frequency.

A block schematic could look like:

enter image description here

And I see a Lowpass Filter in there so your statement

LPF is contained in PLL and not in frequency-synthesizer.

is untrue.

Also a Phase Locked Loop (PLL) has a block schematic which is a copy of part of the frequency synthesizer:

enter image description here

So basically a frequency synthesizer could be a PLL but with some blocks (mainly frequency dividers / counters) added to increase the range of supported frequencies.

A frequency synthesizer can also be made using a DDS which has no PLL.

Bimpelrekkie
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  • What you have said is helpful to me! You are the first one to answer my first question since I knew this website. Thank you! @Bimpelrekkie – wu yi May 06 '19 at 08:46
  • "So basically a frequency synthesizer is a PLL" is not generally true. There are frequency synthesizers that are not PLLs. See my answer. – Curd May 06 '19 at 10:06
  • @Curd fair point, updated my answer. – Bimpelrekkie May 06 '19 at 10:35
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A PLL is a specific type of building block, that will control a variable oscillator (usually a VCO, though it doesn't have to be voltage-controlled - YIG-based PLLs use current instead) such that it's phase is locked to that of a reference (not frequency! It just so happens that if you lock to a phase, you automatically lock to the frequency too).

Of course, they can put multipliers and dividers in between to have the oscillator frequency and reference frequency be different, but they are always 'locked' to one another.

A frequency synthesizer is the name we give to a block that synthesizes frequencies. This can be done internally through a PLL, but this could also be an FLL (frequency-locked loop), DDS (direct digital synthesis, basically a DAC that just outputs a signal at the frequency we want), DLLs (delay-locked loops), etc...

In modern electronics the entire thing is not always as simple as it may seem, and many high-performance synthesizers may use hybrids of different types.

So this question is kinda like asking 'What is the difference between a Car and a Vehicle?' - One is a subset of the other, so you can't really speak of 'differences'.

Dave Tweed
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Joren Vaes
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"Frequency synthesizer" is just a more general term than "PLL". Every PLL is a frequency synthesizer but not vice versa.

A PLL is one common type of several possible types of frequency synthesizers. Often frequency synthesizers are PLLs but not every frequency synthesizer is a PLL.

An example for a frequency synthesizer that is not a PLL would be a Direct Digital Synthesizer (DDS).

Curd
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  • Thanks for your answer. It's helpful to me. Happy to chat with you. – wu yi May 06 '19 at 11:25
  • A clock recovery circuit contains a PLL but is not a frequency synthesizer. – TimWescott May 06 '19 at 15:25
  • @TimWescott: 1. Nobody said that a clock recovery circuit **is** a frequency synthesizer. 2. Still you could consider the PLL contained in the clock recovery circuit as a frequency synthesizer. In that case the clock recovery circuit **contains** a frequency synthesizer. – Curd May 06 '19 at 16:15
  • "Every PLL is a frequency synthesizer" -- I believe this statement is misleading. – TimWescott May 06 '19 at 17:23
  • @TimWescott: In the most general sense, any oscillator is a frequency synthesizer, and since a PLL contains an oscillator, the statement is true. – Dave Tweed May 07 '19 at 20:21
  • A PLL is still a frequency synthesizer even if the *useful output* is the loop error rather than the synthesized frequency – Chris Stratton May 07 '19 at 20:37
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examine the block diagram of a PLL

schematic

simulate this circuit – Schematic created using CircuitLab

and we find the LoopFilter output may be useful for extracting Frequency Modulation.

On the other hand I've seen Frequency Synthesizers, in the 130--180MHz range, have voltage inputs for sonar information from submarine-tracking piezo sensors. The sonar voltage was inserted into the LoopFilter voltage of the embedded PLL.

analogsystemsrf
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  • Your answer is tightly connected to practical applications. Leave me to think over when I am to involve in deeply. Thanks for your detailed answer! – wu yi May 06 '19 at 11:34
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    This reply does not answer the question to me. It does not compare PLLs with Frequency Synthesizers or explain what (if) there is a difference. I also don't know what that loop-filter-output-being-usefull-for-FM-modulation has to do with the discussion - while being true, there are a lot more reasons to have that filter there (stability, phase noise, suppressing spurs) etc.... – Joren Vaes May 06 '19 at 11:51
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    I'm not sure why OP selects this as his accepted answer, all I see is some random facts which do not answer the question at all – Bimpelrekkie May 06 '19 at 12:21
  • 130-180MHz for sonar? – RoyC May 06 '19 at 14:11
  • Yeap. With 125KHz channel spacings. Allocated channel was programmed into the sonabouy by IR photodiodes, before being dropped into the ocean. – analogsystemsrf May 07 '19 at 05:29