–1 dB compression point

An amplifier produces an output signal that has a higher amplitude than the input signal. The transfer function of the amplifier (indeed, any circuit with output and input) is the ratio OUT/IN, so for the power amplification of a receiver RF amplifier it is Po/Pin (or, in terms of voltage, Vo/Vin).

Any real amplifier will saturate given a strong enough input signal (see Fig. 3.16). The dotted line represents the theoretical output level for all values of input signal (the slope of the line represents the gain of the amplifier).

As the amplifier saturates (solid line), however, the actual gain begins to depart from the theoretical at some level of input signal. The –1 dB compression point is that output level at which the actual gain departs from the theoretical gain by –1 dB.

The –1 dB compression point is important when considering either the RF amplifier ahead of the mixer (if any), or any outboard preamplifiers that are used. The –1dB compression point is the point at which signal distortion becomes a serious problem. Harmonics and intermodulation are generated at high levels when an amplifier goes into compression.

Third-order intercept point
It can be claimed that the third-order intercept point (TOIP) is the single most important specification of a receiver’s dynamic performance because it predicts the performance as regards intermodulation, cross-modulation and blocking desensitization.

Third-order (and higher) intermodulation products (IP) are normally very weak, and don’t exceed the receiver noise floor when the receiver is operating in the linear region. As input signal levels increase, forcing the front-end of the receiver toward the saturated nonlinear region, the IP emerge from the noise and begin to cause problems.

When this happens, new spurious signals appear on the band and self-generated interference begins to arise. Look again at Fig. 3.16. The dotted gain line continuing above the saturation region shows the theoretical output that would be produced if the gain did not clip.

It is the nature of third-order products in the output signal to emerge from the noise at a certain input level, and increase as the cube of the input level. Thus, the third-order line increases 3 dB for every 1 dB increase in the response to the fundamental signal.

Although the output response of the third-order line saturates similarly to that of the fundamental signal, the gain line can be continued to a point where it intersects the gain line of the fundamental signal. This point is the third-order intercept point (TOIP).

Interestingly, one receiver feature that can help reduce IP levels is the use of a front-end attenuator (or input attenuator). In the presence of strong signals even a few dB of input attenuation is often enough to drop the IPs back into the noise, while afflicting the desired signals only a small amount.

Other effects that reduce the overload caused by a strong signal also help. Situations arise where the apparent third-order performance of a receiver improves dramatically when a lower gain antenna is used.

This effect can be easily demonstrated using a spectrum analyser for the receiver. This instrument is a swept frequency receiver that displays an output on an oscilloscope screen that is amplitude-vs-frequency, so a single signal shows as a spike. In one test, a local VHF band repeater came on the air every few seconds, and one could observe the second- and third-order IPs along with the fundamental repeater signal.

There were also other strong signals on the air, but just outside the band. Inserting a 6 dB barrel attenuator in the input line eliminated the IP products, showing just the actual signals. Rotating a directional antenna away from the direction of the interfering signal will also accomplish this effect in many cases.

Preamplifiers are popular receiver accessories, but can often reduce rather than enhance performance. Two problems commonly occur (assuming the preamp is a low noise device). The best known problem is that the preamp amplifies noise as much as signals, and while it makes the signal louder it also makes the noise louder by the same amount.

Since it’s the signal-to-noise ratio that is important, this does not improve the situation. Indeed, if the preamp is itself noisy, it will deteriorate the SNR. The other problem is less well known, but potentially more devastating. If the increased signal levels applied to the receiver push the receiver into non-linearity, then IPs will emerge.

When evaluating receivers, a TOIP of +5 to +20 dBm is excellent performance, while up to +27 dBm is relatively easily achievable, and +35 dBm has been achieved with good design; anything greater than +50 dBm is close to miraculous (but attainable).

Receivers are still regarded as good performers in the 0 to +5 dBm range, and middling performers in the –10 to 0 dBm range. Anything below –10 dBm is not usually acceptable. A general rule is to buy the best third-order intercept performance that you can afford, especially if there are strong signal sources in your vicinity.

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