[ARC5] RF amp tubes...

[Richard Knoppow]

>
> While that is still the way used for AM the current method 
> for CW is MDS which is the minimum level a signal can be 
> heard above the noise by 3dB.
>
> Signal leakage of the generator, cables, and other 
> accessories used in the test are hard to reduce to an 
> acceptable level in the average basement shop with what is 
> generally used. Even a 8640B requires regular attention to 
> details.
>
> Carl
> KM1H
>
     Some receivers specify a ratio of the signal with BFO 
on to BFO off for CW. This is generally lower than the 
modulation method but also there may be a bandwidth 
difference. Bandwidth is all important and makes the usual 
Snr measurements invalid for comparing different types of 
receivers.  The most reliable method is noise factor.  A 
comparison of the level of noise from a calibrated source to 
the noise with just a terminatation.  Since the same signal 
path exists for both the ratio has some meaning.  Of course, 
this method can be criticized because it assumes the 
predominant system noise comes from the front end where in 
practical receivers there are other contributions such as 
mixers but, in general it works well. Typical communications 
receivers have NF on the order of 6 to 10 db. i.e., the 
noise above the theoretical minimum for a noise-free 
receiver. At some point it becomes very difficult to reduce 
NF and, since even at 10 meters there is significant noise 
from the antenna, it may be of no importance.  However, 
noise from natural sources tends to fall off above about 20 
Mhz front end noise becomes of increasing importance as one 
goes to the VHF and higher ranges. I am sure Carl knows this 
stuff and am not lecturing him.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com