Modulator in present day 50kW AM transmitter - Page 1
Forum section: electronics / rf-microwave

author: Circlotron
author_rank: Super Contributor
timestamp: June 04, 2021, 05:52:42 am
content: In my part of the country there is a 50kW AM transmitter at 621kHz on the dial. Decades ago it would likely have had a series modulator of at least 25kW audio capacity, a serious bit of gear. Is that system still ever used nowadays? And what about short wave transmitters with hundreds of kW? Must have been quite something to build a decent sounding audio amplifier big enough to 100% modulate those monsters.

author: radiolistener
author_rank: Super Contributor
timestamp: June 04, 2021, 01:11:23 pm
content: For 25 kW and bellow solid state transmitters are available:https://ampegon.com/wp-content/uploads/2019/12/Leaflet_Solid-State_Shortwave_Transmitter.pdfFor power more than 25 kW tube transmitters are used:https://ampegon.com/wp-content/uploads/2019/12/Leaflet_500kW_Shortwave_Transmitter_TSW2500.pdfhttps://www.nautel.com/content/user_files/2020/01/NX400-spec-sheet.pdfDRM broadcasting needs pretty high requirements for transmitter linearity and continuous power, so old AM transmitters from the previous century usually is not compatible with DRM transmission and needs to be upgraded. So, there is a reason to replace old transmitter with a new one which supports DRM and many other modern features, include better AM broadcasting quality.

author: ocw
author_rank: Regular Contributor
timestamp: June 04, 2021, 02:13:15 pm
content: Most present day 50 kW AM transmitters use some form of digital modulation. Gates early version of this was their PDM, pulse duration modulation. That is discussed athttps://www.steampoweredradio.com/pdf/gates%20harris/manuals/Gates%20PDM%20Transmitters_%20High%20Level%20Plate%20Modulation%20without%20Modulation%20Transformers%20or%20Reactors%20by%20Hilmer%20Swanson.pdfUpgraded versions of this modulation is used with solid-state transmitters. Nautel has six and nine phase Direct Digital Modulation available in their solid state transmitters which operate at up to 2,000 kW. See:https://www.nautel.com/content/user_files/2018/07/NX-Series-brochure-1.pdf

author: Circlotron
author_rank: Super Contributor
timestamp: June 05, 2021, 12:32:45 am
content: Wow! They are some serious bits of gear. And I made a mistake. By “series modulation” I meant having the secondary of a modulation transformer in series with the HT supply of the final RF stage, not the kind of setup where the final stage is fed by a cathode follower that has audio on its grid. I think I should have said plate or anode modulation. Those big battleships must have been quite impressive back in the day.

author: TheMG
author_rank: Frequent Contributor
timestamp: June 05, 2021, 05:11:26 pm
content: Modern LW/MW (AM band) transmitters are a completely different beast.Firstly, they are not at all linear amplifiers by any means. They use MOSFET finals switched at the carrier frequency, and an output LPF to strip out the harmonics resulting in a sine wave output.Modulation is done by a switching regulator (supplying power to the finals), controlled by a PDM (pretty much like PWM) signal.Modern transmitters use a very modular approach. Instead of a big DC power supply with a big modulation driver, each individual power amplifier module typically contains its own AC-DC SMPS, modulator, and final stage. This makes the design very scalable from the manufacturer's perspective: if you want a higher power transmitter, simply add more power modules. The RF output from all the power modules is combined to achieve the total output power.The modularity also increases reliability and means there are very few single points of failure. A failure of a module will simply result in a slight reduction in output power, and failed modules can be hot-swapped.Since everything uses switching technology (SMPS power supply, switching modulator, and switching finals), the overall efficiency of this type of transmitter can be very high, roughly around 90%, which is a very big deal when it comes to the cost to operate the transmitter, and is why the vast majority of high-power broadcasters have upgraded from older, much less efficient transmitter technologies. If you're throwing away 40% or more of your electricity as heat (which is pretty typical for older transmitter technologies), when you're transmitting tens or hundreds of kilowatts 24/7, it adds up real fast and a new transmitter pays for itself pretty quick!

author: TheMG
author_rank: Frequent Contributor
timestamp: June 05, 2021, 05:49:34 pm
content: As for shortwave, things are a bit different. The frequency begins to be high enough that switching losses become an issue, enough so that it is actually challenging for a solid state transmitter design to compete in terms of efficiency and cost with a tube final, so tube transmitters are still the most common when it comes to high power shortwave broadcast.From what I can find, some of the more modern shortwave transmitters use a class D (PDM) plate modulator, usually with a single tube, and this approach avoids the need for a big bulky modulation transformer since the PDM approach allows both the audio and DC level combined to be output from the modulator to the plate of the final RF output tube. This, as well as the final output tube, are indeed quite large and usually water-cooled.All this considered I find it quite impressive that these newer tube transmitters can manage to achieve overall efficiency around 70% or slightly more.