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I would like to know how a digital AM radio does the tuning without having any variable capacitor gang that is usually used in any analog radio circuit.

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    $\begingroup$ Hey, you said you wanted to start looking through your questions and accepting answers, a week ago! You still haven't done so, so now we're at 15 questions you asked, 24 answers and 0 accepted answers. Please fix that. Ryuji and Mike both put in work in their answers, and they are both enlightening, and they answer your question. You should accept one! (and do the same for your other question where applicable) $\endgroup$ Commented Nov 5, 2024 at 17:51
  • $\begingroup$ I don't see any ACCEPT button to do that. $\endgroup$ Commented Nov 8, 2024 at 12:19
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    $\begingroup$ It is there, please read the site tour. See the green checkmark right there. $\endgroup$ Commented Nov 8, 2024 at 13:04
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    $\begingroup$ Here's an even more detailed pictured guide $\endgroup$ Commented Nov 8, 2024 at 13:05
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    $\begingroup$ Hi! Have you had problems following the guide? Marking questions as answered is really important to this community, because it allows future readers to see which things have actually been competently answered. That's why I'm "pestering" you about this. $\endgroup$ Commented Nov 9, 2024 at 17:29

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There are a few different ways to do it, but study Si4734 for example.

https://www.skyworksinc.com/-/media/SkyWorks/SL/documents/public/data-shorts/Si4734-35-short.pdf

It has internal variable frequency oscillator, whose frequency can be varied numerically by the small built-in processor, and a variable capacitor (varicap diode) whose capacitance can be varied by the bias voltage (which is also controlled numerically by the built-in microprocessor).

Also, there are digitally controlled capacitor bank ICs. Something like this:

https://www.littelfuse.com/media?resourcetype=datasheets&itemid=ec36d614-0353-42c4-810c-9703b6289f08&filename=ncd2400m

In the classic superhet MW AM radio receivers, the tuning of the loopstick is used to increase the signal voltage to feed to the first transistor, which became a formulaic circuit in the days of vacuum tubes and early transistors (high noise, low gain, expensive). But that is less important these days because low noise amplifiers can be built cheaply inside an IC. In other words, the tuning of a loopstick could be replaced by an untuned or poorly tuned circuit and a high-impedance low-noise amplifier. The rest of the radio (demodulation, de-emphasis, etc.) are all done numerically in DSP, often called by a broad term software-defined radio.

No consumer-grade software defined broadcast radio receiver sets using direct sampling DSP are known at this time. Direct sampling architecture is a lot more computationally burdensome, consumes greater supply power, and requires more expensive chips, so IF DSP architecture is commonly used. In IF DSP radios, the RF front end must be done in a similar way to the traditional all-analog radio.

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  • $\begingroup$ I have ACCEPTED this answer but I would like to know whether is it possible to use a programmable capacitor independently without using any MCU? $\endgroup$ Commented Nov 8, 2024 at 12:28
  • $\begingroup$ I don't see my answer accepted. You need to click on the green check mark. You could ask a separate question, specifically asking for a digitally controlled capacitor bank. I'd be happy to answer that. $\endgroup$ Commented Nov 8, 2024 at 13:53
  • $\begingroup$ Also, instead of asking for a specific programmable capacitor bank, you might want to tell what you are trying to accomplish or what design outline you are considering and ask for possible solutions. Frame your question carefully with enough context and background information. @user2166694 $\endgroup$ Commented Nov 8, 2024 at 14:01
  • $\begingroup$ i need to make a universal loop antenna module using such a programmable capacitor. So that I would able to connect any kind of loop antenna with any inductance with the universal capacitor. $\endgroup$ Commented Nov 12, 2024 at 10:40
  • $\begingroup$ @user2166694 what kind of size range? where and how you plan to deploy the loop? Provide some outline of your possibility. $\endgroup$ Commented Nov 12, 2024 at 10:58
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It sounds like you are talking about an SDR, Software Defined Radio.

Most of the components that you see in an analog radio such as:

Tuning capacitors; fixed capacitors; resistors; inductors; IF cans; mixers, etc. are in the software —a complex computer program— running on a small embedded computer in the radio.

https://en.m.wikipedia.org/wiki/Software-defined_radio :

A basic SDR system may consist of a computer equipped with a sound card, or other analog-to-digital converter, preceded by some form of RF front end. Significant amounts of signal processing are handed over to the general-purpose processor, rather than being done in special-purpose hardware (electronic circuits). Such a design produces a radio which can receive and transmit widely different radio protocols (sometimes referred to as waveforms) based solely on the software used.

SDR block diagram

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    $\begingroup$ There may be direct sampling radio receiver sets for consumers, but do you know? I think most are IF DSP radios and they still require RF front-end circuit in a somewhat simplified (and integrated) way. $\endgroup$ Commented Nov 5, 2024 at 13:29
  • $\begingroup$ @RyujiAB1WX Yes, exactly. For example, I use an RTL-SDR and Gqrx software on my computer. I also have a Softrock SDR for HF and Linrad. $\endgroup$ Commented Nov 5, 2024 at 13:36
  • $\begingroup$ But for consumer radio receiver sets, the total power consumption is also a factor, and IF DSP (with fairly low IF) has the advantage. I actually do not know of direct sampling consumer radio today. So, the RF circuits cannot be replaced entirely by the software... yet. $\endgroup$ Commented Nov 5, 2024 at 13:40
  • $\begingroup$ Agreeing with Ryuji: For a consumer device, the cost difference between "no LO, but adjustable/selectable filters to suppress blockers, or a large-dynamic range signal chain" and "more components, but fixed IF filters and an easy AGC + cheap ADC" needs to be accounted for – and I don't think the fact that LOs are easier to tune than filters are will change anytime soon. $\endgroup$ Commented Nov 5, 2024 at 23:00

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