LC circuit is the main element of every frequency generator, I intend to make an FM transmitter, but many opensource circuit use analog variable L or C part to set and adjust the target frequency My question is : is there any IC or digital circuit to set the LC parameter digitally
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2$\begingroup$ There are of course many digital FM transmitters, and likewise many methods they might use to set the output frequency. Can you explain what you are trying to accomplish? en.wikipedia.org/wiki/XY_problem $\endgroup$– Phil Frost - W8IICommented Mar 20, 2021 at 14:51
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$\begingroup$ Hello and welcome to ham.stackexchange.com! $\endgroup$– rclocher3Commented Mar 22, 2021 at 16:17
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3 Answers
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Perhaps you are thinking of a switched capacitor filter? Unfortunately, most switched capacitor IC components are (were) designed for audio frequencies, not VHF.
Another possibility is to use an LC tuned circuit containing a voltage controlled varicap diode capacitor, controlled by a (digital input) DAC. As per the earlier answer, you can use a use a programmable divider (or two) to compare and help lock a varicap LC oscillator frequency to a more stable fixed frequency reference.
If you want to select a frequency purely digitally, you can use a (expensive?) high-speed ADC to sample the RF (direct sampling of RF) and then hetrodyne, filter, and/or demodulate the desired frequencies using DSP hardware (FPGA) or DSP software. For a transmitter you can use a DAC to synthesize RF from samples generated digitally (hardware or software). Or you can use a digital frequency synthesizer IC (Si5351A et.al.) to mix your desired FM/VHF frequency up from baseband using a Tayloe mixer. Both of the above usually still require some sort of fixed frequency reference (TCXO, etc.). Open source SDR transceiver projects based on both of these methods exist.
Yet another option is to use a high speed digital timer, but vary the timer period at a very high rate for modulation. The PiFM project (available on GitHub) does this to produce broadcast FM frequency signals directly from a Raspberry Pi GPIO pin. All these digital RF outputs still need to be passively filtered to remove spurious out-of-band harmonics.
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$\begingroup$ thanks for the explanation, I was thinking of multiplexing between a group of varicap, diode, and oscillator component by a microcontroller to reach the designated frequency, but I'm not sure how much fine-tune can be reach $\endgroup$ Commented Mar 21, 2021 at 1:11
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Frequency setting and control can be achieved with a phase locked loop (PLL). The frequency of the oscillator is compared to a known and stable reference frequency. Look for further explanation with search terms: radio PLL, RF synthesizer and frequency stabilisation.
Digital control of frequency: fractional PLL and All Digital PLL.
See picture. Copyright? I am the source.
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$\begingroup$ thanks, @f-sessink , how good this IC is? is it possible to couple it with a high power gain antenna to extend the range? is there any other IC or open schematic board? $\endgroup$ Commented Mar 21, 2021 at 1:15
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$\begingroup$ No idea; this was just to elucidate that the hardware you are searching for does exist. Be aware that the module from Aliexpress is low power, low range law-prohibited inmost countries of the world. And use of the module requires a digital control word, or interfacing with a computer (non standard application). For a few dollars more you can have a transmitter with a display that indicates the frequency and does not require any control hardware. nl.aliexpress.com/… $\endgroup$– user16925Commented Mar 21, 2021 at 10:42
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A "simulated inductor" might work in your application:
You could adjust the simulated inductance by using an analog multiplexer IC and resistive ladder array IC to change the values of RL and R1.
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$\begingroup$ Does this work for an FM transmitter at 100 MHz? I don't think so.... $\endgroup$– user16925Commented Mar 21, 2021 at 12:50