Rohde & Schwarz 100W UKW FM broadcast amplifier

[Da_Stier]

Hi all,

I recently got a 100W FM broadcast amplifier module from Rohde & Schwarz.
I didn't get any documentation with it and the internet doesn't seem to know about this module at all after a quick search.

So I thought, it might be interesting to show the module and what I had to do to get it going.
This is the complete module, as I got it:



It has a quite large heatsink / cooling channel piece attached with two 60mm fans providing airflow.



Now for the "reverse engineering" or in this case either a chain of measurements and tests to see how it works.
I figured out that the power connector with 3 positive and 3 negative wires connects directly to the drain / collector of the main output device. (I didn't know yet what technology it uses but by the looks of it, it might be bipolar). Also it ONLY connects to that.



The big resistors going over the device are more common on BJT devices so to see what I dealt with, I desoldered them to take a look at the device.
Surprisingly it was a BLF278 (a very common VHF power transistor) which is a MOS device.
I didn't think too much about the exact circuit yet but the big resistors might actually be a rather crude way of biasing it.



This information is very important, since it tells me which voltage I need to apply.
BJTs run mostly on 13V, while MOS or LDMOS devices run on either 27V or 50V.
This one is a 50V transistor but it will work fine with 48V as well.

The driver is a BGY33, with the BGY series being a very common 20W power module which is available for different frequencies / bands.
These modules are made to run directly from 12V without any external biasing which makes them easy to use.
I traced back one of the 12V lines to a supply rail which is interestingly supplied via an IDC ribbon cable connector.



The second 12V supply of the driver module is switched by some logic.
Since I didn't feel like getting through all of that and building my own controller, I just traced it back enough to see how it is switched.
It is switched by a small BJT, which means that I can just backfeed the 12V into it.

So I added a small bodgewire to get the driver going.



I also attached a 12V supply wire since I didn't feel like making a ribbon cable for it.
Since the case has a hole pattern on the sides, it makes it easy to run the wire out of the unit without any modifications.



And that was about it, just like this the amplifier was amplifying once again.
With 5,5dBm of input power @100MHz, it produces a very stable 105W on the output.



To do so, it uses 2.57A @ 12V and 6.5A @ 48V, meaning it uses a total of 342.8W.
This gives it an efficiency of around 30%, which is about what I would expect.

By driving it with 13dBm on the input (and saturating it), it produces about 230W and is propably well into class AB or maybe even class C, since the efficiency goes up to right around 50%.



Greetings,
Michael

[Mads Barnkob]

Very nice reverse-engineering, where knowledge and experience does more than trying to learn the entire circuit

The power section does seem overly crude, is this just common for the age or it or does the bandwidth tell you that it wasn't supposed to be so clean or narrow?

Pretty powerful amplifier, nice job overall!

[Da_Stier]

Thanks Mads,

as you say, it isn't always about knowing every bit of circuitry and every value but much more about understanding how to get it going or exploiting it. 

I think the biasing is mainly a band thing. Most lower frequency stuff (like shortwave and maybe up to 70MHZ) I have seen uses bipolar devices and the techniques common for them. This means a simple big bias resistor, maybe some feedback resistor to get a flatter respone over a wider bandwith and often even a diode simply glued to the power transistor to help with thermal stability. This carries over to more modern MOS devices as well, I guess the designers just stay true to the stuff that worked in the past.

On the other hand most microwave amplifiers (in the GHz range) can't really do this and use a Lambda/4 transmission line to apply the gate bias. Since the DC end has capacitors on it which are a RF short, the gate of the LDMOS sees it as an open, minimizing the effect on your cirucuit.

The FM broadcast band is kind of in between the frequencies that can still use the "crude" stuff when done right and needing the microwave treetment. So I guess it depends on the engineer developing a circuit, which technique will be used.



Greetings,
Michael