Part 2 - The First Mixer & Local Oscillator

by Larry Gadallah

The First Mixer:

The first mixer's job is to take the RF signals provided by the front end and to shift them in frequency for presentation to the first intermediate frequency (1st IF) amplifiers, filters, and finally the detectors. The mixer must handle the full range of signals presented to the receiver; both weak and strong signals at the extremes of the receiver's frequency coverage. Mixers are three-port devices, meaning that they are devices which have have three inputs or outputs. The first mixer in a receiver has RF signals from the front end and the local oscillator signal (LO) as inputs and the 1st IF signal as an output.

Linearity & Dynamic Range

More than any other single stage in a receiver, the first mixer determines the dynamic range of a receiver. Older semiconductor receivers that used early designs of transistorized mixers typically exhibited horrible dynamic range and collapsed under practical signal conditions. For modern high-performance receivers, the first mixer design is critical.

There are two main types of mixers: passive and active. The simpler and cheaper of these is the passive design, which is usually implemented using a set of precision matched hot carrier or microwave diodes (called a diode ring). These are specially chosen to exhibit nearly perfect diode characteristics at high frequencies. Diode rings have an advantage in that they are inherently broadband devices. One of the main disadvantages of them is that they require a high level of drive power from the local oscillator to obtain good dynamic range performance..

Currently, active mixers can provide the highest dynamic range available, using VMOS field effect transistors (FETs). These VMOS devices work extremely well because they are excellent switching devices at high frequencies. A significant disadvantage they have is that they require a high drive voltage from the local oscillator. Typically, this means that tuned circuits must be used between the VMOS devices and the local oscillator. This severely restricts the bandwidth of the mixer and causes difficulties with impedance matching. Nonetheless, modern mixer designs using VMOS FETs can attain dynamic range figures of 120 to 140 dB, as compared to figures like 100 to 110 dB for the best amateur radio equipment available commercially today.

Impedance Matching

More early semiconductor mixers suffered from poor performance due to impedance mismatching more than any other factor. In general, if a mixer's inputs and outputs are not properly terminated over its operating range, the mixer's dynamic range and linearity will be severely limited. This is one reason why passive diode ring mixers are much simpler to build than active mixers: These circuits are broadband by their nature and present a more constant impedance across their operating range than do active mixers.

Attenuation/Gain

Typically, active mixers provide a slight gain at the expense of added noise. Passive mixers generally contribute less noise but also exhibit a slight attenuation. In modern receiver designs, these factors are normally irrelevant since most of the receiver's gain should be provided in the IF stages rather than at the front end and any loss in the mixer circuit can easily be recovered in the following stages.

For a homebrew receiver, the passive diode ring mixer is the easiest design to build. Often these can be purchased preassembled. Active mixers can provide superior performance to diode ring mixers, but they require specialized equipment to assemble and test. Perhaps preassembled VMOS FET mixers will become available in the future at which point they would become the superior choice..

The Local Oscillator:

The Local Oscillator, or LO, is the main unit controlled by a receiver's tuning controls. It determines what signal from the front end will fall within the IF strip's passband. The purity of the signal produced by the LO is very important, because any noise in the LO output will be combined with signals from the front end at the appropriate frequencies and will end up in the IF passband.

Analog VFOs

The LO is generally required to be a variable frequency oscillator, and it is desirable for it to tune over as wide a range as possible. Older sets mainly used mechanical variable capacitors as the tuning elements. These had a number of disadvantages, often including poor temperature stability and linearity. Better sets, such as those built by Drake or Collins often had permeability tuned oscillators or PTOs. These are generally held to be the best mechanical tuning devices built for use in variable frequency oscillators (VFOs). They were stable, exhibited a linear mechanical vs.inductance characteristic, and allowed high resolution mechanical tuning dials to be built.

Digital VFOs (Synthesizers)

Today, most sets use phase locked loop frequency synthesizers (PLLs) for their LOs. The PLL is a well known circuit, and it offers some key advantages, including stability, precision, and digital control. However, it also exhibits some crippling disadvantages: PLLs with high divide ratios exhibit large amounts of noise near their output frequencies and have a limited frequency resolution (typically 100 Hz in modern radios). In order to reduce this noise to acceptable levels, some very complex circuitry is being used. This complex circuitry contributes a great deal to the cost of high-performance receivers today.

There is a new breed of frequency synthesizer beginning to appear in consumer electronics: The direct digital synthesis (DDS) frequency synthesizer. The design of these units is not new, what is new is the VLSI integrated circuits that make DDS synthesizers practical to build today. The DDS has been in use in very exotic (and expensive) military and aerospace equipment for some time, but now some companies are offering DDS chipsets for a couple of hundred dollars. This is an excellent price for the qualities that DDS synthesizers have to offer: low noise (if used correctly), almost infinite frequency resolution (1 Hz is quite easy to achieve), and extremely fast frequency lock (measured in nanoseconds). For the amateur receiver builder, these chips are the answer to synthesizer design, which would be a very major deterrent to receiver building otherwise.

For a homebrew receiver, a DDS chipset provides a solution to what would likely otherwise be the most difficult portion of the receiver design.

The next installment will be about the first IF stage.