[Amps] Why hasn't solid state replaced tubes?

[Ian White, G3SEK]

Gary Schafer wrote:
>
>
>Ian White, G3SEK wrote:
>> Gary Schafer wrote:
>>
>>>
>>> We are not talking about precise measurements here but getting in 
>>>the  ball park as to how bad a signal is by an easy method.
>>>
>> Forgive me, but you're still missing the point: measuring low-order 
>>IMD  simply doesn't tell us everything we need to know - not even 
>>approximately.
>>  On the air, there is a vital difference between these two cases:
>>  Transmitter #1, whose higher-order IMDs progressively reduce in 
>>amplitude with frequency offset, and go down rapidly so you can 
>>comfortably listen to weak signals only a few kHz away;
>>  Transmitter #2, which has almost exactly the same levels of 3rds and 
>>5ths, but whose higher-order IMDs reduce only gradually - or even 
>>worse,  only come down to a "plateau" level, and then audible splatter 
>>( =  high-order IMD on speech peaks) continues around that same level 
>>for  100kHz or more, on both sides of the main signal.
>>  You can live on the bands with TX #1, but not with TX #2... but a 
>>simple  measurement of low-order IMD can *not* tell which is which. 
>>Since we're  interested in high-order IMD, we need to  measure it 
>>directly.
>>  Some of us have done these measurements, with both spectrum 
>>analysers  and receivers, and seen the difference. More to the point, 
>>most band  users have heard the difference. Therefore we need a lab 
>>test that will  *show* us that difference.
>>
>>> Any method that uses voice as a signal source is not going to be 
>>>very  precise anyway.
>>>
>> With a standard voice signal that is digitally recorded and 
>>reproduced,  combined the peak-hold method of measurement, the results 
>>can be highly  repeatable. (Those two features of the test should not 
>>be separated.)
>>
>
>Are you telling me that a signal that is causing splatter out 10 to 20 
>kc or even 100 kc is going to be clean on the opposite side band?
>
>Please reread the first sentence of post.

I have. The key point that you are still missing is that your logic does 
not work in reverse!

If a signal is not "clean" on the opposite sideband, you cannot predict 
whether it will be causing significant splatter out to 10, 20 or 100kHz. 
You do not know what the relationship between low-order and high-order 
IMD levels is for that particular transmitter.

The only way to tell what the high-order IMD levels are is to tune out 
there and look for them.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek