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Say I have an oscillator that needs to span between 1 GHz and 1 Mhz. Ill be controlling it with a microcontroller, probably an arduino. According to this, http://faculty.mu.edu.sa/public/uploads/1400394697.1447Ch16%20-%20Oscillators.pdf, there are 2 pieces to an oscillator, the amplifier and the feedback circuit, which actually produces the oscillations. I think I can use a crystal tank to span from 1MHz to 1GHz. Here are my questions:

  • Can you use a varactor diode in a tank circuit to change the frequency of oscilation using an arduino?
  • Can you use a varactor diode to change a crystal tank circuit's frequency from 1MHz to 1GHz?
  • Can a crystal circuit span between 1GHz and 1MHz using a varactor diode?

This is kind of hard to explain and I'm new to this so sorry if it is hard to understand. If you need clarification just ask. Thanks, Brody.

user2455722
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    Relays good in the 10's of MHz are expensive, at a GHz you would use solid state switching like PIN diodes. Generally you don't design a single oscillator for several decades span in range - more typically a wide range oscillator operates at an offset frequency so that its range is less than 2:1, and then subtracts out the offset. Even more typically, the application itself is offset. – Chris Stratton Nov 15 '15 at 02:27
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    going past 3:1 in a varactor tuner is going to be real hard, Television tuners broke the VHF band in half 45-100Mhz and 115-345 and then after that UHF was another 3:1 span – Jasen Слава Україні Nov 15 '15 at 03:34
  • Just get a PLL chip, possibly with an external divider. The ADF4351, for example, will do 35 MHz to 4.4 GHz, and it's programmable via SPI. You can extend the range down to 1 MHz with an external divider and RF switch. – alex.forencich Nov 15 '15 at 08:16
  • With a crystal, a varactor can only pull the frequency by a few hundred PPM tops : in other words, a few hundred Hz at 1 MHz. Some way short of 1 GHz... –  Nov 15 '15 at 11:30

3 Answers3

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You'll never get a what you want by pushing or pulling a crystal. I would not hesitate in going for a DDS system: -

enter image description here

You can buy a kit for this device too. There may be cheaper options in the DDS range that don't span up to 1 GHz but you can make a frequency doubler or tripler to cover the range.

Andy aka
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Yes, you can use a varactor to change an oscillator frequency, but the range is limited to a few % (depending on the Q of the oscillator and other performance parameters).

A varactor can't practically sweep between 1 MHz and 1 GHz.

For these applications, generally a single frequency oscillator is used with a PLL to generate the variable output frequency.

jp314
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  • Does that mean the frequency could only be changed in integers using a divide by n counter? Also, does more divide by n counters mean I can fine tune the frequency more? I'm getting this from here: https://www.techpowerup.com/articles/overclocking,/overclocking/voltmods/28 – user2455722 Nov 15 '15 at 03:58
  • No, a PLL doesn't have to use integers only. A Fractional-N PLL basically alternates between two (or more) integers -- imagine if it alternated between 10 and 11 with a 50 % duty cycle -- you'd get a divide by (about) 10.5. Using divide by 10 for 90 % of the time and divide by 11 10 % of the time, you'd get about divide by 10.1 effectively. – jp314 Nov 15 '15 at 20:46
  • Then, what are the benefits of using PLL over DDS and vice versa? – user2455722 Nov 16 '15 at 00:38
  • A PLL is frequently used to generate a frequency that is a specific relation to a reference frequency. The waveform is not a critical characteristic (generally in digital circuits it is a square wave). A DDS is a DAC driven by a counter and the whole purpose is to generate a specific waveform (usually a sinusoid). Of necessity (to get enough samples), the frequency is much lower than the step rate of the DAC. – jp314 Nov 17 '15 at 03:02
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look at making or buying* an oscillator that you can sweep between 500MHz and 1000MHz, then follow that with a programmable divider (eg upto divide by 512)

* programmable oscillators are common on computer motherboards so should be relatively easily had.

Nick Alexeev
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Jasen Слава Україні
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