I am in the trying to design a sine wave generator. This generator should be able to sweep through a range of frequencies and the range would be 150 KHz to 80 MHz. the sweep rate is not more than 1.5*10^-3 decades/s.

There are different oscillator topoplogies, but I can't use, say a purely RC oscillator or a crystal oscillator as they offer a static method of generation (single frequency).

As I want a more dynamic design, I think I would need a varactor diode or a FET transistor in its resistive region, but as this is a new area for me, I don't know if the ranges of the mentioned devices would be sufficient. Or alternatively a simple potentiometer which one can manually rotate and change the output frequency, as the design is not focused on either automatic or manual change of frequencies.

I was hoping a more experienced individual could direct me towards the right topology to look at.

Any help is highly appreciated.

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    \$\begingroup\$ Are you happy to use DDS, like a analog.com/en/products/ad9913.html#product-overview for instance, or do you want something more steam-driven? \$\endgroup\$ – Neil_UK Jul 14 at 7:47
  • \$\begingroup\$ It is not a simple task to find an oscillator that can be swept in frequency without disturbing the oscillation condition. However, it is - for my opinion - nearly impossible to build a circuit that can be swept from 150kHz to 80 MHz. \$\endgroup\$ – LvW Jul 14 at 7:50
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    \$\begingroup\$ Agree, DDS is the way to go, like signal generators have been doing (since the post steam era). For "manually rotate" you can just buy a 100MHz sig gen and get the stability, versatility, and quality that comes with it. \$\endgroup\$ – P2000 Jul 14 at 7:55
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    \$\begingroup\$ @LvW It is far from impossible. I have an HP 8601 sweep generator built 40 years ago that can sweep from 100 kHz to 110 MHz. The trick, as Andy describes in his answer, is to use a fixed high frequency oscillator mixed with a high frequency voltage tuned oscillator (The HP uses a varactor diode). \$\endgroup\$ – Barry Jul 14 at 12:08
  • \$\begingroup\$ OK - if you refer to industrial products.....of course, there are such devices. However, the questioner says that he is "new in this area"....Do you think that a "newcomer" can build such a device? \$\endgroup\$ – LvW Jul 14 at 12:56

Consider building an oscillator that can be swept over a range from 80 MHz to 160 MHz (typically a Colpitts oscillator) and mix (using a 4 quadrant mixer) the output with a fixed frequency of 80 MHz. The difference frequency (wanted) can be extracted from the mix and will range from DC to 80 MHz. The sum product (unwanted) will range from 160 MHz to 240 MHz and can be removed with a fairly good low pass filter.

You can also use a variable oscillator frequency of 200 MHz to 280 MHz and mix with a fixed frequency of 200 MHz (for example). The difference will still range from DC to 80 MHz but the sum (unwanted) will be 400 MHz to 480 MHz and a little easier on the low pass filter design.

Stability control will be an issue but, if you use a decent phase-locked-loop and some digital control then it becomes more stable (just like how DDS works really).

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    \$\begingroup\$ This is the method used by HP in their purely analog 8601A sweep generator built many years ago. A 200 MHz crystal oscillator is mixed with a varactor tuned 200.1 to 310 MHz oscillator. This yields a continuous sweep from 0.1 to 110 MHz. \$\endgroup\$ – Barry Jul 14 at 12:11

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