[Drake] Couple of questions

[James C. Garland]

At 04:46 AM 1/8/2003, you wrote:
>  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.
>
>
>Martin, HS0ZED
>Bangkok

Hi Martin,

The 0.25uV receiver sensitivity referred to in the Drake manual refers to 
rms volts, measured across the antenna input terminals of the receiver with 
a 50 ohm antenna (or signal generator with 50 ohm output impedance) 
connected to the terminals.

I've never heard of "soft" and "hard" microvolts, so doubt that the words 
are in common usage in this country. Also, "EMF" and "potential difference" 
are customarily used in connection with d.c. voltages, such as produced by 
a battery or power supply. The terms pop up in beginning physics courses, 
but aren't used much any more in electronics. Unless explicitly stated 
otherwise, you can safely assume that any specified RF voltages are rms. If 
a specification pertains to peak-to-peak voltage, it will usually be stated 
as such.

Confusion often arises about the output of signal generators. As you 
correctly note, the open-circuit voltage from a signal generator is twice 
the voltage measured with a matched load connected. In other words, if a 50 
ohm signal generator has an output of 1V with no load (as measured, say, 
with an oscilloscope), then it will have an output of 1/2 V when a 50 ohm 
load is connected to it. The meter on a signal generator is always 
calibrated, assuming a matched load is connected. People often forget this 
fact and are puzzled when the voltage seems too high. I usually 
differentiate between the two cases by referring to "open-circuit voltage" 
when there is no load. Although you're technically correct by calling  the 
open circuit voltage an EMF,  I think that's likely to cause confusion, 
since EMF isn't any longer in common parlance.

73,
Jim Garland W8ZR