We’re in the middle of a 3 part series on RF Performance with Ian Graham, Principal Engineer for the Systems Engineering group. In the first video, Ian defined the different specifications for RF Performance, such as reliability vs cost, the minimum acceptable performance by the regulatory authorities, and how Tait exceeds these levels of performance.
Today we’ve got video two of the series, where Ian explains the key RF specifications for transmitters. In this video, Ian delves into adjacent channel power and how sideband noise can affect neighboring receivers, preventing distant site communication.
Ian also explains how you can recognize a good transmitter from a bad one by the specifications, and the numbers that demonstrate why Tait is excellent in this area.
Evan: All right, so thanks, Ian, now we’ll talk about some of the key RF specifications for the transmitter and how they’re measured. Sound good?
Ian: Fine, no problems. So performance specifications for a transmitter are just separated into desired and undesired aspects. So in other words, what comes out at the desired frequency versus what else comes out that you really wish didn’t. The major desired specification obviously for a transmitter is the actual output power and the frequency that you want. That’s what’s really conveying the information. So this is the level of RF power you have transmitted at the desired frequency, usually measured in Watts although sometimes dBm. Tait Base stations, mobiles and portables are designed to transmit particular levels of RF power. A higher number is better, generally the more power generated the further away the signal can be picked up by the receiver.
But typical output power levels of Tait radios, say for base stations, it’ll 50 Watts or 100 Watts and then for the terminals, mobiles, the standard was 25 Watts and more recently, we’ve done higher power ones of 40 Watts, 50 Watts and for a portable, it can be anywhere between 3 and 5 Watts depending on the band.
Evan: So if my knowledge is right, that means that the base station can transmit a lot further than a mobile, and a mobile can transmit a lot further than a portable, correct?
Ian: That’s the idea, yes.
Evan: So you talked about, this is obviously something that is desired, what are some of the undesired things that come along with them?
Ian: Radio transmitters always emit smaller amounts of energy at frequencies other than the desired one. So if you start thinking of where some of the energy is, some of the energy that’s emitted is in the adjacent channel. So the channel right next door to the one you’re actually transmitting on. Transmitters produce small amounts of RF power outside the channel they’re operating on. The amount of RF power produced in the neighboring channel is called adjacent channel power. So it’s usually quoted in dBc, which simply means, dBs below the power generated under the wanted channel. For that, a larger number is best, so 65 dBc is better than 60 dBc for example. And a transmitter with lower adjacent channel power will allow a radio operating on an adjacent channel to get closer to the site before it causes interference. In short, transmitters that produce lower adjacent channel power contribute to a more reliable robust system.
Evan: Okay, so talk to me about sideband or wideband noise Ian, what’s that about?
Ian: Okay, well if you look further out on the adjacent channel, transmitters continue to produce small amounts of RF power well beyond the adjacent channel. Sideband noise is actually the power produced by the transmitter at large offsets from the operating frequency. It’s usually quoted in dBc per Hertz for specific frequency offsets, and again, a lower number is better. -160 dBc per Hertz, which is roughly what a Tait base station will achieve, is better than -150 dBc per Hertz for example.
So at a radio site, sideband noise from co-located transmitters can actually deafen the receivers operating nearby, even though they’re quite widely separated in frequency. So that means distant terminals would be unable to reach the site preventing communication. Site transmitters with better sideband noise performance require the addition of fewer expensive large cavity filters to attenuate the noise below the level of where it would deafen the neighboring receivers. So that results in better coverage and less sites required to cover the area and reduces the system size and cost.
Evan: Okay, well next we’ll take a look at the receiver’s RF Performance Advantage.
