Mixers
| Mixers | |
|---|---|
| What it does | Combines two signals to produce new signals at their sum and difference frequencies |
| Inputs | Signal (RF) and local oscillator (LO) |
| Main outputs | f1 + f2 and f1 − f2 |
| Common types | Single-balanced, double-balanced (diode ring), Gilbert cell, quadrature (I/Q) |
| In amateur radio | |
| Main uses | Superheterodyne receivers, SSB generation and detection, transverters |
| Common problems | Image response, spurious products, intermodulation |
| Related terms | Heterodyne, IF, product detector, balanced modulator |
A mixer is a circuit that combines two signals of different frequencies and produces new signals at their sum and their difference. The process is called heterodyning, and it is how nearly every radio moves signals from one frequency to another. A superheterodyne receiver uses a mixer to convert incoming signals to a fixed intermediate frequency, a single sideband transmitter uses mixers to create its signal and move it to the operating frequency, and a transverter uses one to put a radio on a band it was never designed for.
Sum and difference
When two signals pass together through a circuit whose output is not simply proportional to its input, called a nonlinear circuit, they interact. If one signal is at frequency f1 and the other at f2, the output contains:
- f1 + f2, the sum
- f1 − f2, the difference
- the original f1 and f2, unless the mixer is designed to cancel them
- weaker products such as harmonics and combinations like 2 × f1 − f2
For example, a 14.2 MHz signal mixed with a 5.2 MHz local oscillator produces 19.4 MHz and 9.0 MHz. A filter after the mixer keeps the one product that is wanted and rejects the rest.
An ideal mixer multiplies its two inputs, and multiplying two sine waves yields exactly the sum and difference. Whatever information the signal carries, such as speech or Morse, comes through at the new frequency unchanged. The one twist is that when the output is the local oscillator frequency minus the signal frequency, the spectrum is flipped, so upper sideband comes out as lower sideband.
Balanced mixers
A simple mixer, such as a single diode or transistor, passes both inputs through to its output along with the new products, and they must be filtered out afterward. Balanced mixers cancel them instead.
- A single-balanced mixer cancels one input, usually the local oscillator, at its output.
- A double-balanced mixer cancels both inputs, leaving mainly the sum and difference. The classic form is the diode ring: four diodes connected in a loop between two small wideband transformers.
- The Gilbert cell is an active double-balanced mixer built from transistors. The NE602 chip and its close relatives, such as the SA602 and SA612, combine a Gilbert cell with an oscillator and appear in many simple QRP kits.
Balance is also what makes SSB possible. In a balanced modulator, audio is mixed with a carrier oscillator, the carrier cancels, and only the two sidebands remain; a filter then removes one of them. In a receiver, a product detector does the reverse, mixing the signal at the IF with a beat frequency oscillator (BFO) to recover audio from SSB or CW.
Image frequencies
A receiver with a 9 MHz IF and its local oscillator at 23.2 MHz receives 14.2 MHz (23.2 − 14.2 = 9), but it also receives 32.2 MHz (32.2 − 23.2 = 9). The unwanted frequency is the image. It always lies twice the IF away from the wanted signal, on the other side of the local oscillator.
Once an image signal reaches the mixer, nothing afterward can remove it, so the cure is filtering ahead of the mixer. That is why receivers have tuned or switched band-pass filters at their inputs, and why a high IF, which puts the image far away, makes the job easier. Many HF transceivers convert incoming signals up to a first IF well above the HF range for exactly this reason.
Mixers in radios
- Superheterodyne receivers convert every incoming signal to one fixed IF, where sharp filters and most of the gain need to be designed only once.
- Transmitters often generate SSB at a fixed IF and mix it with a variable oscillator to reach the operating frequency. A classic homebrew design pairs a 9 MHz SSB generator with a VFO tuning 5.0 to 5.5 MHz. The sum covers 14.0 to 14.5 MHz on 20 meters, and the difference covers 3.5 to 4.0 MHz on 80 meters.
- Direct-conversion receivers put the local oscillator right on the signal frequency, so the difference lands directly at audio. They are simple, but hear signals on both sides of the oscillator at once.
- Software-defined radios often use a quadrature (I/Q) mixer, a pair of mixers fed with oscillator signals 90 degrees apart, which lets the software separate the two sides and reject the image.
- Transverters use a fixed local oscillator. A 1296 MHz transverter with a 144 MHz IF uses an oscillator at 1152 MHz, since 1152 + 144 = 1296.
Overload and intermodulation
Driven too hard by strong signals, a mixer generates unwanted products, including intermodulation: two strong signals at f1 and f2 create new ones at 2 × f1 − f2 and 2 × f2 − f1, which can land on an otherwise clear frequency. A mixer's ability to handle strong signals is a large part of receiver performance, and switching in an attenuator or switching off a preamplifier often makes these phantom signals disappear.