Re: PM demodulation

On Fri, 05 Nov 2004 18:31:35 +0800, Steve Underwood <steveu@dis.org>
wrote:

>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.

I was actually thinking of TWTs when I said solid state ... [is] more
reliable.

I guess we have had different experiences.

Regards,
Allan