Part 3 - The IF Strip and AGC System

by Larry Gadallah

The First IF Strip:

The first IF strip's job is to take the signals provided by the first mixer and filter, amplify, and limit them. Generally this is the first place where selectivity is provided, usually by means of a "window" filter. This filter typically has a bandwidth of 40 to 100 khz. This is followed by several gain stages, and if RF- derived AGC is used, there is a circuit to determine the amplitude of signal presented to the first IF stage as well as systems to control the gain of the IF strip based on this information.

Image Rejection and Up Conversion

In modern receivers, "up conversion" is generally used. This means that the intermediate frequency (IF) is much higher than the highest received frequency. The reason for this is to provide improved image rejection. Superheterodyne receivers have been plagued since their invention by image reception. This is because the output of mixers, inherently non-linear devices, contain not only the desired combination of signal and local oscillator (LO) frequencies, but all possible sums and differences of these frequencies. In older receivers that used a 455 khz first IF, this led to situations where the receiver would respond to signals that were both 455 khz above and below the LO frequency, where the desired behavior was to have reception of only one of these signals. This characteristic of mixers cannot be eliminated, so the only solution was to apply heavy filtering of the image frequency (twice the IF frequency removed from the desired signal) before the first mixer. In cases like the previous one this was very difficult since the image frequency was separated by a very small amount from the desired signal frequency, and if filtering was used it had to be elaborate and expensive. The use of up conversion simplifies this filtering since the image frequency is greatly separated from the signal frequency. This means that a receiver which receives up to 30 Mhz might use a first IF of 70 Mhz. Such a receiver, tuned to 30 Mhz, with the LO set to 100 Mhz (since 100 - 30 = 70) has an image frequency of 170 Mhz (since 170 - 100 = 70). Eliminating the image frequency here is simple, requiring a low pass filter at perhaps 35 or 40 Mhz.

Impedance Matching

As was mentioned in the previous article about the first mixer, careful impedance matching to the first mixer is critical to preserve the dynamic range of the first mixer. In order to accomplish this, often a active impedance matching stage is the first circuit in the first IF strip. This circuit ensures that the wildly varying impedance of the crystal window filter would not be seen by the first mixer stage. All the comments made about the need for high dynamic range and low noise figure stages in the front-end apply to this stage also, since it is the last wideband stage in the receiver and in general, the AGC system does not protect it from the vast dynamic range of signals found in the front end. The use of modern CATV bipolar transistors and noiseless feedback techniques are also recommended for this stage.

Window Filters

The spectrum of the signal provided at the output of the first mixer is extremely wide. It is generally only limited by the bandwidth of the front-end stages. In modern receivers the front-end bandwidth is often an octave (2:1 range) or more.

The IF stages could never handle the number and range of signals in this spectrum so the receiver bandwidth must be constrained here. In older receivers, this was done using critically coupled IF transformers, but for modern receivers this is not practical due to the very high IFs used. For these receivers a crystal lattice filter is used, and it provides higher performance and better stability than critically coupled transformers. The latest technology in this area is the surface acoustic wave (SAW) filter. These piezoelectric devices provide similar or better performance than crystal filters at lower prices. For the homebrew receiver, many filters used for satellite TV IF strips (70 Mhz) are appropriate and available.

Gain Distribution

The IF stages usually provide most (80-90 db) of the overall receiver gain (120- 140 db). Since the dynamic range of modern receivers often exceeds 100 db, there needs to be some mechanism for changing the gain of the IF stages from 90 db to -10 db. This mechanism is the automatic gain control (AGC) system. The critical questions to be answered in designing an AGC system for a receiver are: Where is the signal to be sampled to determine its strength, and where is gain control going to be applied? Typically the front-end and first mixer stages have a fixed gain, so a logical place to sample signal strength is after the window filter of the first IF stage. This is a compromise, because strong signals outside the first IF "window" can overload RF stages in the receiver, but the alternative of allowing the AGC system to see more spectrum around the signal of interest leads to AGC system "pumping" where strong adjacent signals modulate the receiver gain (and the desired signal). The best solution found so far to this dilemma is the double loop AGC system, where a second, wideband AGC loop is added, and it controls the gain of the front-end. This additional AGC system ensures that all the signals adjacent to the desired signal are within the range that the first IF stages and the main AGC loop can handle. Normally the control of front-end gain is accomplished with PIN diode attenuators or switched resistive attenuators. Motorola, Plessey, and other semiconductor manufacturers provide integrated circuits (ICs) which provide many of these functions in small, easy to use packages.