Understanding broadband silicon tuners for broadband cable and TV apps: A Tutorial

The tutorial explores the basics of a typical RF front end found in consumer products by explaining the operation of the fundamental elements of the RF front end: mixers, filters and amplifiers. The tutorial also presents key performance specifications used in this market.

After reading this tutorial, RF system designers will be equipped to assess manufacturers’ data sheet claims, understand the advantages of silicon tuners over legacy approaches and consider how silicon tuners can be used in their applications.

Introduction
In recent years, advances in silicon technology have enabled the creation of cost-effective silicon tuners that can replace the old “can” tuners. With silicon tuners, designers can integrate high-performance, low-power tuners directly onto the main board.

Why Tuners?
Typical broadband cable systems enable consumers to view a multitude of channels. In North America, each channel is 6 MHz wide spread across the 67-860 MHz frequency range. The job of a tuner is to select a single channel from the available channels. The tuner must be agile, so that different channels can be selected as the user’s preferences change. After all, nobody wants a multimedia device that only receives one channel.

Of course, each channel carries useful information, which is modulated onto the channel before transmission. Not only must a tuner select the desired frequency from the available channels, it must not damage the modulated signal while making this selection.

There are various methods used to select an individual channel from the available channels. All of these methods ultimately result in filtering out the energy from the undesired channels, leaving only the energy from the desired channel. Furthermore, the tuner shifts the carrier frequency used to transmit the energy to a fixed frequency used by the receiver’s processing circuits. A tuner’s performance is judged on how well it filters undesired energy, how well it passes the modulated signal, how much power it uses, how easy it is to build, and how much it costs.

Frequency Shifting with Mixers
The essence of a tuner is to select energy from the desired portion of the tuner’s input RF range and convert this energy to a fixed frequency output. The fixed frequency output of the tuner is passed to the input of the demodulator. The demodulator then recovers the information and passes it on the circuits that provide the information to the user (i.e. displaying a television picture and audio). Because the tuner accomplishes all of the frequency shifting prior to the demodulator, the demodulator needs to only work with one input frequency. This fixed frequency is called the “intermediate frequency” or IF.

The frequency conversion from a selectable RF input to a fixed IF is done with mixers, amplifiers and filters. How these components are put together determines the tuner’s characteristics. Tuners with a single mixer are called “single conversion,” while tuners with two mixers are call “dual conversion.” The mixer is the element of a tuner that converts one frequency to another. A mixer is a special type of amplifier with two inputs and one output. Because of intentional non-linear design of the amplifier, the output signal contains two signals whose frequencies are the sum and difference of the input frequencies. It is these sum and difference signals that make an amplifier a mixer capable of shifting a signal’s frequency.

Mixer Math
The two inputs to a mixer are commonly RF and Local Oscillator (LO). The RF input from the antenna or cable contains a large number of input channels or ichannels while the LO is a single-frequency sin wave. The LO is generated from a tunable circuit, usually a PLL, and is used to select the desired channel. The output of the mixer is the IF described above.

The mixer output can be expressed mathematically in the frequency domain as follows:

Defitions:
fRF = The frequency of the desired channel in the input spectrum.
fLO = The frequency of the local oscillator.
fIF = The desired intermediate frequency of the system.

Then, the spectral content of the mixer’s output is made up of the following two elements:

fRF + fLO
|fRF - fLO|

The choice of which output to use for the IF is up to the system designer. In most cases, the difference component is used, as this results in an output that is of lower frequency than either the RF and LO frequencies.

Now, let’s use an example to explore the properties of the mixer. To determine the frequencies used in a mixer, design decisions must be made. The first decision is the IF output of the tuner or mixer. Often, this is a choice dictated by the selection of the demodulator. In the US, a common demodulator input frequency is 43.75MHz.

The next design decision is whether the mixer will use high-side injection (an LO above the RF) or low-side injection (an LO below the RF). For this example, we’ll assume high-side injection. The designer must decide whether to use the sum output or the difference output; as previously stated, almost all designs use the difference output.

Finally, the frequency of the desired channel must be known. Of course, this is a user selection. For this example, we’ll pick 350 MHz.

Now that the IF and RF frequencies are known, it is a simple matter to calculate the LO that will tune the desired channel. The equation that applies to this example is:

For the curious, the negative result inside the absolute value means that a spectral inversion occurs. This is handled by the demodulator.

With an LO of 306.25 MHz, the input frequency of 350 MHz is shifted to an IF of 43.75 MHz. This type of conversion, where a high frequency is shifted to a lower frequency is called a “down-conversion.” Since only on mixer is used, this is a single down conversion tuner.

“Can” tuners are single down conversion tuners. As we will see, there are significant design issues with this type of tuner when used in broadband applications.

The Image Problem
Continuing with the example above, consider what happens when RF energy is present at the input to the mixer at 262.50 MHz. fLO remains set at 306.25 MHz. Just as above, fIF frequency is computed using equation (1), which gives a result of 43.75 MHz. This is the same IF output as for the desired channel at 350 MHz! This means that if input energy is present at both 262.50 MHz and 350 MHz, the output of the mixer will contain energy from both frequencies. This signal will not be useable by the demodulator.

Both RF frequencies result in the same IF output because of the absolute value function in equation (1). When fLO is fixed, there are two values for fRF that result in the same output. One value is a positive result from the subtraction while the other is a negative result. Because the absolute value function allows two values that produce the same result, there are two inputs to the mixer that produce the same result. In tuners, the undesired value is called the “image’ frequency.

It is important to note that the image frequency is not created within the mixer or other tuner components. It is simply the other value that results in the same IF when calculating the result of the absolute value function. The equation to calculate the image frequency can be determined to be:

fIMAGE = 2 * fLO - fRF (3)

Whenever the difference output from the mixer is used, there will always be two input frequencies that result in the same IF output. One frequency is the desired RF frequency while the other called the image frequency. To ensure that the system works properly, the energy at the image frequency must be prevented from reaching the input of the mixer by using a filter.

Image Filters
Now, consider the impact of image frequencies when a single conversion tuner is used in a application, such as a broadcast or cable TV system, with channels spread across a wide band. For almost all of the channels, the image frequency will fall within another channel. This means that the filter that is used to remove image energy for the desired channel is filtering a currently undesired channel. If the user later decides to tune to this undesired channel, a fixed filter would prevent viewing of the channel.

The common solution to this problem is to use tunable pre-filters in front of the mixer. These filters are called tracking filters, because the filter’s pass-band tracks the tuned channel. Because consumer grade tracking filters are limited to bands smaller than the receiver’s full input spectrum, several tracking filters are used in front of the tuner. Now, not only must the tracking filter be tuned, the correct tracking filter must be selected for the desired band. In addition, tracking filters require manual calibration during manufacture, are sensitive to vibration and temperature, and suffer performance degradation with age.

The bottom line is that tracking filters are not compatible with today’s designs, which demand minimal manufacturing steps and are expected to meet performance requirements for the life of the product.