Variable Square Wave to Sine Wave
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From: Tim Wescott on 8 Mar 2010 14:03 FyberOptic wrote: > Hiya folks. I've been interested in trying to build myself a radio > receiver for various bands, primarily as a learning experience. The > thing is, though, I've heard that it's kind of hard to build a stable > VFO above 50mhz. One of the bands I'd eventually like to receive is 2 > meters (144-148mhz). Might be fun to receive television audio, as > well. Not to mention, standard FM. > > Since I'm more familiar with digital electronic components rather than > analog, I've seen that there are programmable frequency oscillator > ICs, which can be set via a microcontroller. This sounds like an easy > solution for tuning, and would open the door to setting station > presets and the like as well. The problem is, I'm fairly sure that > these would output a square wave. > > I've read about how you can basically use a low-pass filter to tune > out harmonics of a square wave and get a sine wave output. But from > the way I understand it, an RC filter would just filter out a specific > frequency, defeating the purpose of the variable oscillator. > > So that's pretty much my question: How does one turn a variable > square wave into a sine wave? > > Or, alternatively, what are more reliable ways to produce higher > frequency sine waves to begin with? Though I'm still interested in > the former question as well, for curiosity's sake. Most modern mixers act as switches driven by the local oscillator signal. So even if you made a nice sinusoidal signal out of your local oscillator, it'd be effectively squared up inside the mixer. So use the chip, and don't worry about the square wave output -- it's not only not a problem, for some mixers it's desirable. _Do_ concern yourself with phase noise and frequency stability -- local oscillators for weak signal work have a lot of requirements imposed on them that just don't apply to -- or don't matter for -- oscillators for digital circuits. Were it me I'd just make sure that the LO is on it's own board, with connectors to make it reasonably easy to swap out. Then I'd proceed from there. Get a copy of the ARRL Handbook -- it addresses a lot of these issues for you. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: osr on 8 Mar 2010 14:18 One, google "softrock radio" Two, with all due respect to my learned colleague, instead the of ARRL handbook, try Experimental Methods in Radio Frequency Design, by Campbell, and Hayward, its more geared to modern, simple, home made gear then the ARRL handbook is , sadly... then look at http://www.kangaus.com/2m_converter.htm then http://www.pongrance.com/ the old kit Steve
From: Tim Wescott on 8 Mar 2010 18:53 osr(a)uakron.edu wrote: > One, google "softrock radio" > > Two, with all due respect to my learned colleague, instead the of ARRL > handbook, try > > Experimental Methods in Radio Frequency Design, by Campbell, and > Hayward, its more geared to modern, simple, home made gear then the > ARRL handbook is , sadly... Is it? Sad about the Handbook, but at least there's something out there. When I'm feeling wordy I recommend the latest Handbook, plus one from the 50s, 60s, or 70s. If you don't mind toobs, there's a _lot_ of good stuff in those older book! I like Hayward's "Radio Frequency Design", but it's a text for a 4th-year college class, and requires that you have strong analog chops to just pick it up and read. > > then look at > http://www.kangaus.com/2m_converter.htm > > then > http://www.pongrance.com/ > > the old kit > > Steve -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: Bitrex on 8 Mar 2010 23:47 Tim Wescott wrote: > osr(a)uakron.edu wrote: >> One, google "softrock radio" >> >> Two, with all due respect to my learned colleague, instead the of ARRL >> handbook, try >> >> Experimental Methods in Radio Frequency Design, by Campbell, and >> Hayward, its more geared to modern, simple, home made gear then the >> ARRL handbook is , sadly... > > Is it? Sad about the Handbook, but at least there's something out there. > > When I'm feeling wordy I recommend the latest Handbook, plus one from > the 50s, 60s, or 70s. If you don't mind toobs, there's a _lot_ of good > stuff in those older book! > > I like Hayward's "Radio Frequency Design", but it's a text for a > 4th-year college class, and requires that you have strong analog chops > to just pick it up and read. >> >> then look at >> http://www.kangaus.com/2m_converter.htm >> >> then >> http://www.pongrance.com/ >> >> the old kit >> >> Steve > > Hayward's "Introduction to Radio Frequency Design" was the first book on RF design I ever bought a few years back...I bought it because it had "Introduction" in the title... O_O
From: oopere on 9 Mar 2010 02:38
FyberOptic wrote: > Hiya folks. I've been interested in trying to build myself a radio > receiver for various bands, primarily as a learning experience. The > thing is, though, I've heard that it's kind of hard to build a stable > VFO above 50mhz. One of the bands I'd eventually like to receive is 2 > meters (144-148mhz). Might be fun to receive television audio, as > well. Not to mention, standard FM. > > Since I'm more familiar with digital electronic components rather than > analog, I've seen that there are programmable frequency oscillator > ICs, which can be set via a microcontroller. This sounds like an easy > solution for tuning, and would open the door to setting station > presets and the like as well. The problem is, I'm fairly sure that > these would output a square wave. > > I've read about how you can basically use a low-pass filter to tune > out harmonics of a square wave and get a sine wave output. But from > the way I understand it, an RC filter would just filter out a specific > frequency, defeating the purpose of the variable oscillator. > > So that's pretty much my question: How does one turn a variable > square wave into a sine wave? > > Or, alternatively, what are more reliable ways to produce higher > frequency sine waves to begin with? Though I'm still interested in > the former question as well, for curiosity's sake. > > Thanks! > > - Jeff > > To produce "higher frequency sine waves" (RF) you make use of a VCO. You may design your own (perhaps Colpitts) VCO or you may buy a canned one frome Minicircuits or similar. To reliably adjust the oscillation frequency you phase-lock your VCO to a stable crystal reference. This is achieved by a PLL (Phase-Locked Loop) which is offered by different vendors. A microcontroller is used to program some registers of the PLL to achieve the desired frequency. Commercially available PLLs clearly state the signal level that is required from your (sinusoidal) VCO. So, the whole thing looks like this: XTAL--------PLL-->Filter-->VCO-->Power splitter---> synthesized sine ^ ^ | | | | | �-----------------------� uController-� Once you become familiar with this you may start looking at spurs, jitter, etc. For "lower" frequencies, direct digital synthesis (DDS) may also work. Pere |