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Old 8th June 2012, 01:14 PM   #1
PChi is offline PChi  United Kingdom
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Default High frequency sine wave generator.

Has anyone any suggestions for a 'high' frequency sine wave generator circuit?
Variable frequency, 100 kHz to 10 MHz approximately.
Output level not critical, ideally variable up to say 1 V rms.
Distortion not critical, ideally < 1% total harmonic distortion.
Cheap.

I can't think of any options other than spending money on a function generator or arbitrary wave form generator.

I have prototyped an Electrostatic Direct Drive amplifier and have tested the open loop gain to 100 kHz but I really want to test the open loop gain and phase response at higher frequencies. I have thrown together an operational amplifier based Wein bridge but it's not great and only gets up to around 1 MHz. I have searched for function generator circuits without success apart from the obsolete Maxim and Intersil offerings. Designing and building a function generator that can reach 10 MHz with a reasonable sine wave doesn't seem to be easy.
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Old 8th June 2012, 01:59 PM   #2
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One easy way, although I donīt know if can be a solution for your request may be, to make an RC oscillator using a 74HC04, or any similar. Then, pass the squeare output by a LC multi-section to get a sinusoidal waveform, such filter mus be a couple decades below the RC frequency. The trouble that it is difficult to sweep, because the signal level will vary with frequency.

A best variant may be a PLL with a 4046, and then filter in the same way as above.
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Old 8th June 2012, 02:12 PM   #3
DF96 is online now DF96  England
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RF sig gen? Otherwise known as VFO plus buffer.
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Old 8th June 2012, 02:31 PM   #4
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Quote:
Originally Posted by DF96 View Post
RF sig gen? Otherwise known as VFO plus buffer.
Warning that not all RF signal generators are sine wave output.
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Old 8th June 2012, 03:00 PM   #5
DF96 is online now DF96  England
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True. Cheaper ones often just take the raw output from an oscillator and buffer it a little. One option would be to use a 2-gang tuning capacitor, with switched inductors. One gang for an oscillator, the other for a tuned buffer. That would reduce harmonics, provided voltage levels were correct (use AGC of some kind).
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Old 8th June 2012, 03:16 PM   #6
wahab is offline wahab  Algeria
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A MAX038 from Maxim should do it up to 20Mhz with various output functions
at ultra low cost.

http://datasheets.maxim-ic.com/en/ds/MAX038.pdf
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Old 8th June 2012, 03:21 PM   #7
tomchr is offline tomchr  United States
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HP3312. You can pick them up for less than $100 on ePay. $100~200 for a calibrated/tested model. It goes to 13 MHz and offers the usual sine shaper <1 % THD you find in function generators. Lowest frequency is in the mHz range. I forget if it's 10 mHz or 100 mHz.

~Tom
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Old 8th June 2012, 03:28 PM   #8
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Simplest way is with a VariCap diode. Use two oscillators into a mixer and use the beat frequency as the output.
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Old 8th June 2012, 04:08 PM   #9
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Katie: I triyed this way, but didnīt give good results.
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Old 8th June 2012, 04:32 PM   #10
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Wavetek made various function generators like the 185 that went to 5 MHz, and more that went higher. Download a manual with schematics to see how they did it. IIRC, they used current sources to make simple discrete oscillators that performed very well, then the usual rounding circuits to make the sine waves with about .2% THD. No doubt modern 1-chip solutions would get you there quicker, but the discrete solutions are simple and use no special parts. Also look at the ARRL handbook for various oscillator solutions.
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