When constructing a new radio, what should your step resolution be and what is considered too much in resolution in frequency tuning and transmitting?

I'm talking about capacitor type tunning.

Normal radios go in steps of .5 before going to the next band:

  • 100.0
  • 100.5
  • Etc, Etc.

Capacitor types allow you get more inbetween which offers a better resolution:

  • 100.1
  • 100.2
  • 100.3
  • 100.4
  • 100.5
  • $\begingroup$ But that restriction is typically broken up by fine-tuning knobs or just automatic frequency-correcting measures (e.g. signal-locking PLLs). Are you asking us what's sensible of "user experience", or in terms of "technical realization of tuning"? PS: the tuning resolution has practically nothing to do with whether you go for capacitor-based tuning or build a good tuner with discrete-frequency, locked clocks, so "Capacitor … allows … better resolution" isn't correct. $\endgroup$ – Marcus Müller Dec 9 '19 at 20:09

I'm assuming you're building a digitally-controlled tuner.

In general, there is no such thing as too much resolution. Think first about usability — choose an encoder (assuming you are using a knob for a physical tuning control) which has a high counts-per-revolution value (but not so high that you're entering the realm of overly expensive parts, whatever that means to you), and then let the user adjust the ratio of encoder steps to frequency. (The Elecraft KX3, for example, has both a permanent speed setting and a front-panel control for quickly switching between coarse or fine tuning.)

Fine steps are most obviously valuable in a SSB receiver — the closer to exact frequency, even within a few Hz, the better the audio can sound. But even for other modes that are not so frequency-sensitive, in a crowded band it can be valuable to tweak your receive tuning to help filter out an unwanted adjacent signal.

For VHF/UHF FM operation (among others), you should offer the option of tuning in discrete steps, which is useful for quickly finding well-known channel frequencies that are “round numbers”. But you should provide an option as to what the tuning step is because it may vary depending on local practice. Furthermore, such a tuning step should definitely not be a single encoder step, because that would be much too fast, unless this is the only mode of operation your transceiver supports and the encoder is detented (clicky).

(I plan to update this answer with some more concrete numbers based on the transceivers I have available, but right now they're not in front of me.)

  • $\begingroup$ I fogot to add "capacitor tunning" $\endgroup$ – Ben Madison Dec 9 '19 at 17:44
  • $\begingroup$ @BenMadison If you mean using a variable capacitor for tuning, then I don't know what a "step" would be in this context; can you edit your question to explain further? $\endgroup$ – Kevin Reid AG6YO Dec 9 '19 at 17:48

what is considered too much in resolution in frequency tuning and transmitting?

Nothing. You physically can't get arbitrary exact frequency within finite time (Heisenberg won't let you), but that's far from where you are.

On the contrary: since oscillators aren't perfect and have frequency error, you'd always want to be able to tune. There's no downside to that.

The question what's practical when someone turns a knob is a different one: If I was to build a radio, there'd be numeric buttons to key in the frequency I want. I'd simply stop typing digits when I think I'd be right.

Normal radios go in steps of .5 before going to the next band:

.5 of what unit?

Anyway, I don't think that claim is true. Radios have very different ways of selecting frequencies. For some applications, a 75 kHz channel raster is right. For others some other.

Capacitor types allow you get more inbetween which offers a better resolution

That's simply not true. Fractional frequency synthesizers have a very high resolution, and the remaining offset is trivially compensatable with numerically controlled oscillators of various architectures.

An electrically tunable capacitor only has as many settings as the DAC controlling it has possible output values. It's still a relatively inaccurate way of generating frequencies, so although you might think you have a high resolution, you actually just got many steps of relatively random frequencies.

A mechanically tunable capacitor is typically hard to actually tune very reproducibly and exact.

For these technical reasons, modern radio devices don't use undisciplined capacitor-based synthesizers at all – instead, they use the aforementioned fractional frequency synths (which can, in some instances, include an electronically tunable capacitor, but not necessarily).

So, wrong claim: If you actually want good resolution, then you need good accuracy, otherwise your resolution is largely meaningless. You get excellent accuracy with things that are not tunable capacitors.


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