Allan Herriman wrote:>On Fri, 05 Nov 2004 10:11:08 +0800, Steve Underwood <steveu@dis.org> >wrote: > > > >>Allan Herriman wrote: >> >> >> >>>All "solid state" finals for broadcast FM have been around since the >>>'80s. At the time, they were better in terms of reliability, but not >>>initial purchase price. >>> >>>I don't know how the comparison stands today. >>> >>>Regards, >>>Allan >>> >>> >>> >>> >>I said big FM transmitters. 100kW ERP from a silicon final stage? I >>haven't seem that used >> >> > >You're right. At 100kW, valves (tubes) still rule. Can't license >anything like that around here though. > >Here's a 40kW solid state job from Harris: >http://www.broadcast.harris.com/product_portfolio/product_details.asp?sku=WWWZDD40CD >I think that's about as powerful as they get. > >That one sounds pretty impressive for a solid state machine.> > >>Up to a couple of kW things are OK, but you need to stack too many well >>matched transistors to keep scaling things. Minor mismatches cause a lot >>of stress, and reliability tends to be rather poor. >> >> > >Interesting. Usually solid state transmitters are used because they >are *more* reliable (= more cost effective in the long term) that the >valve units they replace. >That was certainly the case for the microwave SSPAs produced by a >company I used to work for. > >I believe the latest satellites still use small TWTs, because semiconductor amps cannot match their reliability. I don't know how much radiation tolerance might come into that, though. Big thermionic devices are usually very reliable. The heaters wear, but in a more predictable way than consumer devices used to. If swappped at the appropriate times, unexpected failures seem pretty infrequent. Repairing big devices is a fairly low cost activity. When I worked in radar, the TWTs were generally the most reliable part of the system. Regards, Steve
Re: PM demodulation
Started by ●November 5, 2004
