Explanation of VFO as one part of the the tuning variables in mixers

Question

I do apologize if this has been answer previously, but the term "VFO" seems to have become overloaded over the years.

I think I have a handle on the notion of multiple VFOs on device for the purpose of managing traffic on a DX, or when following the convention of splits for repeaters. That is, the definition of the initialism I more or less understand. Perhaps it is how the concept is applied in some mixer applications is where my confusion lies.

What I'm grappling with right now is how and why a VFO might be used (automatically or manually) with a "tuner" in analog mixers.

From an SDR perspective, I note that many software front-ends make the distinction between the VFO frequency and the tuned frequency. For someone who grew up just spinning a dial (attached to something related to a VFO, I assume) it was confusing to me that interacting with this software sometimes assumes you want to tune an offset to the VFO, rather than going to the frequency. Based on the comments posted for and about this software, apparently I am not alone.

But there is a separate distinction made in some online documents related to quadature mixing in VFO tuning. I'm interested in any discussion that involves using the two frequencies in signal processing rather than specifically for radio operation.

Can someone explain to me, as one would a child first learning about FFTs, this use of a VFO and an offset tuning for solving problems in certain mixers? Links to other related Q&A is fine if I missed something in my searches.

Answer 1

There are many kinds of radio receiver architectures, but a very common one is a superheterodyne, or "superhet". In a superhet, the tuned frequency can be the sum (or difference) of multiple oscillators, one of them often a VFO (an oscillator variable in frequency by changing a variable capacitor, or perhaps a PLL voltage). In a common type of single conversion superhet, the tuned frequency is the sum of an IF frequency (say 455 kHz) and the VFO frequency. So the VFO frequency (as measured by say an oscilloscope or frequency counter) is not the frequency indicated on the dial, or the carrier frequency of a tuned-to AM station.

An RTL-SDR, as typically used by many SDR applications, is actually a double or triple conversion superhet, with one VFO (actually variable) digitally synthesized in the tuner chip for heterodyning down to an IF, another digitally synthesized fixed frequency quadrature oscillator for heterodyning IF samples down to baseband IQ for transfer over a USB-port protocol, and yet another software synthesized oscillator (often variable, so yet another VFO) inside the SDR application to add or remove a tuned offset.

With SDR software that shows a spectrum and waterfall, the center of the entire available FFT waterfall is often at the hardware "VFO" frequency (which may be a single oscillator, or a sum/combination of more than one). But one can still use the SDR software demodulate a signal with a frequency above or below the center of the waterfall by IQ heterodyning with yet another offset oscillator (software VFO). The frequency indicated by the SDR software might be the arithmetic sum (or difference, depending on the signs of the frequencies) of the multiple VFOs.

The reasons for doing this are many.

One reason is so you can see an entire band, but listen to various signals within that band without shifting the entire waterfall. So one can listen to signals lower or higher in either the CW or SSB portions of a band while still looking at both across the entire waterfall, perhaps centered between them at a "VFO" frequency.

Another reason (for many RTL-SDR, mcHF/RS918, and SDRPlay devices), is that there is an IQ imbalance or offset in the initial quadrature heterodyne to IQ signals, which distorts the signal at the center of the waterfall (baseband frequency of zero), but less so for signal offset from the IQ VFO frequency. So the tuned, indicated, or "listened to" frequency is often offset by some amount (maybe a dozen kHz or so) from the VFO frequency.

In an mcHF transceiver, the offset is a configuration option, usually set at 12 kHz. So the indicated dial frequency is really 12 kHz above or below the actual VFO (a physical signal you can monitor with an oscilloscope), the quadrature signal fed to the Tayloe IQ mixer. The firmware later removes this offset by IQ software heterodyne before DSP demodulation at baseband.

In a direct sampling SDR (Elecraft K4 or Hermes Lite 2, et.al.), the first VFO is usually implemented as a digitally synthesized signal inside an FPGA, for down conversion before sample rate reduction to a sample rate more suitable for an affordable PC or embedded processor. SDR software can use this directly, or use one or more additional software oscillators for offset or multi-signal monitoring.

Answer 2

The more general term is LO - the Local Oscillator that goes into one side of every mixer.

In analogue radio you will have several (very different) LOs used to mix the signal down from RF to where you can hear it.
An SDR is quite similar to start with, but usually the hardware ends with an ADC sampling up to 1 MHz of spectrum. This is then mixed with yet another LO, in software, and the final audio products extracted. So you can tune the receiver both by adjusting the first physical oscilator in the receive chain, or the last digital oscillator.

While the FFT is useful for plotting the spectrum, it's not used directly in the receive chain. The digital process is called DDC, digital downconversion, and it's similar to the analogue process, with some new features along the way.

VFO is to me a physical radio term - I think of them as an adjustable LO that you can touch.
They might be connected to the dial or stored in memory, the radio might switch between two for Tx and Rx split, etc. They may map directly to physical oscillators, or the radio may cheat and (for example) adjust only the digital LOs in software for small movements, and the hardware ones only to change band. The end result is the same - a hand-adjustable frequency on a physical box radio. In an all-software radio there isn't really a VFO, unless it implements a virtual front panel in its user interface.

Answer 3

I'm not sure what you mean by "solving problems in certain mixers", but when you tune a superheterodyne radio, you're adjusting the frequency of the Local Oscillator (LO), which is an input to the first mixer in the signal chain. I presume that you're talking about a superhet, because that's the only kind of radio with an actual analog mixer. The frequency of the LO is not the frequency displayed on the front panel of the radio. The displayed frequency is meant to be the frequency of the received signal (or its suppressed carrier) when the signal is tuned correctly.

In the old days of purely analog superhets, having two VFOs meant the radio had two different LOs, that is two different oscillators, which used to be quite a fancy feature. Now that modern radios use digital frequency synthesis for oscillators, a VFO is just a number stored digitally somewhere, and advertising that a radio has multiple VFOs doesn't mean very much. (Now the fancy feature is having more than one receiver in the same transceiver, so the operator can listen to more than one frequency simultaneously.)

Know that for a receiver that is correctly tuned to a single-sideband (SSB) signal, the displayed frequency is the frequency of the suppressed carrier, which is not the same frequency as the center of the signal when viewed in the frequency domain (spectrum analyzer display, waterfall display, panadapter, etc).

For a Morse code receiver, an offset frequency is defined. (In modern radios the offset frequency is usually set by the operator via the settings menu; in older radios, the designers typically picked the offset frequency at design time.) When a Morse code signal is tuned so that its received audio signal in the speaker or headphones is at the same frequency as the offset frequency, then the frequency displayed by the radio is the frequency of the signal.