E4D: Receiver Characteristics

E4D covers the receiver characteristics that determine how well a receiver handles strong signals while still copying weak ones: blocking dynamic range, intermodulation distortion and its causes, cross modulation, desensitization, the third-order intercept point, the preselector, link margin calculation, received signal level calculation, and converting dBm to power.

These concepts are central to understanding why receivers fail in the presence of strong signals and how to prevent or mitigate those failures.

Blocking Dynamic Range

The blocking dynamic range of a receiver is the difference in dB between the noise floor and the level of an incoming signal that will cause 1 dB of gain compression in the receiver. When a very strong signal enters a receiver, it can saturate the front-end amplifiers and mixers, reducing their gain. The 1 dB compression point defines when this gain reduction becomes significant. Blocking dynamic range measures how far above the noise floor a signal must be before it starts compressing the receiver's gain.

This is distinct from intermodulation dynamic range (which relates to the third-order intercept). Blocking dynamic range is specifically about the noise floor to the 1 dB gain compression threshold.

Effects of Poor Dynamic Range

Poor dynamic range in a receiver causes spurious signals resulting from cross modulation and desensitization from strong adjacent signals. When a receiver's dynamic range is insufficient, strong signals from nearby stations cause the receiver's nonlinear stages to generate mixing products (cross modulation) and reduce gain (desensitization) — both of which appear as interference that may be confused for actual received signals or that mask weak desired signals.

What Causes Intermodulation

Intermodulation in an electronic circuit is caused by nonlinear circuits or devices. When a circuit element has a nonlinear transfer characteristic (output is not a linear function of input), two or more signals passing through it produce sum and difference frequencies — intermodulation products — that were not present in the original signals. All real amplifiers and mixers are somewhat nonlinear, and strong signals drive them further into their nonlinear operating regions.

Negative feedback reduces intermodulation in amplifiers. Lack of neutralization and positive feedback cause oscillation. Neither of these is the fundamental cause of intermodulation — nonlinearity is.

Intermodulation Between Repeaters

Intermodulation interference between two repeaters in close proximity is created when the output signals of the two repeaters mix in the final amplifier of one or both transmitters. When the transmitters are co-located on the same tower or site, the strong output from one transmitter can reach the final amplifier stage of the other transmitter through the antenna system, where it mixes with the local transmitter's output in the nonlinear final stage to generate intermodulation products.

Circulator for IMD Suppression

A properly terminated circulator at the output of a repeater's transmitter reduces or eliminates intermodulation interference caused by a nearby transmitter. A circulator is a three-port device that passes RF in one direction around its ports while isolating signals in the reverse direction. When installed at the transmitter output with a 50-ohm termination on the third port, it provides high isolation between the transmitter output and the antenna port — preventing a nearby transmitter's signal from reaching the final amplifier through the antenna connection. A band-pass filter in the feed line between transmitter and receiver does not solve intermodulation at the transmitter output. Class C and Class D amplifiers address efficiency, not intermodulation suppression.

Desensitization

Desensitization is the term for the reduction in receiver sensitivity caused by a strong signal near the received frequency. When a strong off-channel signal enters the receiver's front end, it can drive the amplifiers into their nonlinear region or even into saturation, reducing the gain available for the desired weak signal. The result is that the receiver becomes less sensitive — it requires a stronger desired signal than normal to produce a usable output.

Reducing the likelihood of desensitization is accomplished by inserting attenuation before the first RF stage. Adding 10 or 20 dB of attenuation reduces the level of all incoming signals — including the strong interfering signal — before it reaches the sensitive amplifier stages, keeping the front end operating in its linear region.

Preselector Function

The purpose of a preselector in a communications receiver is to increase the rejection of signals outside the band being received. A preselector is a tunable bandpass filter placed before the first amplifier or mixer. By passing only signals within the desired band and attenuating everything outside, the preselector prevents out-of-band signals from reaching the mixer, where they could otherwise generate spurious responses or cause intermodulation. Unlike a fixed filter, a preselector can be tuned to track the receiver's tuning.

Third-Order Intercept Point

The third-order intercept (IP3) is a figure of merit for receiver linearity. It is the extrapolated input signal level at which third-order intermodulation products would theoretically equal the output amplitude of the desired signals. A third-order intercept level of 40 dBm means that a pair of 40 dBm input signals will theoretically generate a third-order intermodulation product with the same output amplitude as either of the input signals.

In practice, the receiver clips or compresses well before actually reaching this level — the IP3 is an extrapolated intersection of two theoretical lines. Higher IP3 means better linearity and higher dynamic range. The IP3 is not a threshold below which no IMD occurs, and it does not mean that the receiver tolerates signals up to 40 dB above the noise floor without producing IMD.

Odd-Order Intermodulation Products

Odd-order intermodulation products (third-order, fifth-order, seventh-order, etc.) of two signals in the band being received are also likely to be within the band — this is why they are of particular interest. For two signals at frequencies f1 and f2, the third-order products fall at 2f1 − f2 and 2f2 − f1. When f1 and f2 are close together (as they would be if both are in the amateur band being received), these third-order products also fall within the same band. Even-order products (2f1, 2f2, f1 + f2) fall far from the original signals and are easily filtered. Odd-order products cannot be filtered away because they fall within the passband.

Link Margin Calculation

Link margin is the difference between the received signal level and the minimum signal level required for reliable communication (MDS plus any required SNR margin).

A positive link margin means the link has sufficient signal for reliable communication with margin to spare. A negative link margin means the link is insufficient.

Received Signal Level

Received signal level is calculated by starting with transmit power and adding gains while subtracting losses along the path.

Converting dBm to Power

A receiver minimum discernible signal of −100 dBm represents 0.1 picowatts. Converting dBm to watts: −100 dBm = 10^(−100/10) milliwatts = 10^(−10) milliwatts = 10^(−13) watts = 0.1 × 10^(−12) watts = 0.1 picowatts.

E4D Practice Questions