[Drake] Re: Couple of questions

[Barry L. Ornitz]

Martin Sole, HS0ZED, asked:

> Interesting question time. The Drake manual states 10dB 
> signal to noise at 0.25uV. Is that 0.25uV p.d. or e.m.f? 
> I think Americans use terms like soft and hard micovolts, 
> never sure which is which. I understand p.d. (potential 
> difference) being the voltage across one of two loads,
> 50 ohms from the receiver and 50 ohms from the sig gen, 
> with e.m.f being open circuit volts from the generator 
> with no additional external load.

I agree with Jim Garland, W8ZR, about this being the voltage, 
measured across the antenna terminals, from a 50 ohm source.  
You should note that the receiver rarely ever produces a 50 
ohm resistive load.

I too have never heard of the terms "Hard" of "Soft" 
microvolts.  As Jim again noted, people are often confused by 
the output of signal generators.  Rarely, if ever, is the 
output voltage actually measured in a signal generator.  
Typically the voltage is measured upstream of the calibrated 
attenuator.  The output meter (and attenuator) calibration 
take into consideration the fact that the output should be 
properly terminated into 50 ohms.

Since receivers rarely present a matched 50 ohm input 
impedance, an attenuator is often added between the signal 
generator output and the receiver.  A six dB attenuator is 
commonly used.  This provides a reasonable 50 ohm match to the 
generator independent of the actual receiver impedance.  Of 
course, this added attenuation must be accounted for when 
doing the actual receiver sensitivity test.

        73,  Barry L. Ornitz     WA4VZQ     ornitz@tricon.net