Dynamic range
What is meant by the blocking dynamic range of a receiver?
The difference in dB between the noise floor and the level of an incoming signal that will cause 1 dB of gain compression.
Two ends of the same scale. The noise floor is the weakest signal you can hear; the compression point is the strongest you can tolerate before the receiver stops behaving linearly. Dynamic range is the span between them, and it is what determines whether a receiver copes with a band that has both.
Which of the following describes problems caused by poor dynamic range in a receiver?
Spurious signals caused by cross modulation and desensitization from strong adjacent signals
What is the term for the reduction in receiver sensitivity caused by a strong signal near the received frequency?
Desensitization.
The strong signal is not on your frequency; it merely pushes the front end into compression, and everything gets quieter. Recognisable because signals fade in step with someone else’s transmissions.
Which of the following reduces the likelihood of receiver desensitization?
Insert attenuation before the first RF stage.
What is the purpose of the preselector in a communications receiver?
To increase the rejection of signals outside the band being received
Two approaches: reduce everything (attenuation), or reject what is out of band (preselector). Attenuation is blunt and free on the low bands; a preselector is selective and costs nothing in sensitivity within the band.
Third-order intercept
What causes intermodulation in an electronic circuit?
Nonlinear circuits or devices.
Why are odd-order intermodulation products, created within a receiver, of particular interest compared to other products?
Odd-order products of two signals in the band being received are also likely to be within the band
The General result: odd-order products land near their parents, where filtering cannot remove them. Which is why the specification that matters is:
What does a third-order intercept level of 40 dBm mean with respect to receiver performance?
A pair of 40 dBm input signals will theoretically generate a third-order intermodulation product that has the same output amplitude as either of the input signals
IP3 is an extrapolated figure — no receiver survives +40 dBm at its input. Third- order products rise 3 dB for every 1 dB the inputs rise, so the two lines converge at a theoretical intercept point, and where that point falls characterises linearity in one number.
Higher IP3 is better, and together with the noise floor it defines the receiver’s usable range. It is the number to compare when two receivers claim identical sensitivity.
Repeater intermodulation
What creates intermodulation interference between two repeaters in close proximity?
The output signals mix in the final amplifier of one or both transmitters.
Two transmitters on one hilltop. Each one’s signal reaches the other’s output stage, where the amplifier — a non-linear device operating at high level — mixes them and re-radiates the products. Neither operator is doing anything wrong.
Which of the following is used to reduce or eliminate intermodulation interference in a repeater caused by a nearby transmitter?
A properly terminated circulator at the output of the repeater’s transmitter
A circulator passes energy one way round and dumps anything arriving the other way into a terminating load. Your output leaves; the neighbour’s signal is absorbed instead of reaching your final amplifier. Hence the “circulator loss” line item in the ERP calculations from the antennas lesson — it is there for this reason.
Link budgets, at Extra level
Two worked questions, both applying the decibel addition from the foundations track.
What is the received signal level with a transmit power of 10 W (+40 dBm), a system antenna gain of 12 dBi, and a path loss of 103 dB?
40 + 12 − 103 = −51 dBm.
What is the link margin with a transmit power of 10 W (+40 dBm), a system antenna gain of 12 dBi, a path loss of 120 dB, and a receiver minimum discernible signal of −80 dBm?
Received = 40 + 12 − 120 = −68 dBm. Margin = −68 − (−80) = +8 dB.
The method never varies: add gains, subtract losses, then compare with what the receiver needs. The difference is the margin, and it is how much fading the link survives.
Check yourself
- Two receivers both have a −130 dBm noise floor. One has IP3 of +10 dBm, the other +30 dBm. Which do you want in a contest?
- Your repeater develops interference whenever a nearby repeater transmits. What is happening and what is the fix?
- Transmit +37 dBm, antenna gain 10 dBi, path loss 130 dB, receiver MDS −90 dBm. Margin?
Answers
- +30 dBm. Same sensitivity, far better tolerance of strong signals — which is what a crowded band demands.
- Intermodulation — the neighbour’s signal reaching your final amplifier and mixing there. Fit a properly terminated circulator at your transmitter output.
- Received = 37 + 10 − 130 = −83 dBm. Margin = −83 − (−90) = +7 dB.