Reply by Steve Underwood●November 5, 20042004-11-05
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