[R-390] R-390A sensitivity measurements

[Bob Camp]

Hi

A lot of this is not a matter of what is right and wrong from 
theoretical standpoint. It's an issue of how things are defined. In the 
case of a receiver measurement you do not supply an external load, 
regardless of what the input impedance of the radio actually is. Fifty 
ohm radios rarely if ever present 50 ohms back to the generator. Since 
we don't know the impedance of the radio the decision was made long ago 
to define this measurement not in terms of the *actual* voltage at the 
input of the radio but instead to define it in terms of what the 
voltage *would be* if the radio was presenting 50 ohms.

One way of looking at this is to consider what we are trying to do. 
When we use the radio we hook it up to an antenna. Antennas are weird 
just like receivers, but let's ignore that for right now. With a bunch 
of power the antenna would put out 2 volts into an open circuit and 1 
volt into a fifty ohm load. By the way we define things we would call 
this a 1 volt signal from the antenna. If the receiver presents an open 
circuit then it gets to use 2 volts. If it presents a 50 ohm load it 
gets to use 1 volt. We test the radio the same way we use the radio.

If you are going to calibrate a scope then yes you need to remember to 
terminate the generator properly. Probably the biggest issue here is to 
check what the input capacitance of the scope looks like at the 
frequency you are using. A 15 pf scope capacitance can have a big 
effect at 400 MHz.

If you are going to use a 125 ohm antenna then by all means use a 125 
ohm generator. In fact if you use a balanced antenna then a balanced 
generator is a good idea. The radio should be aligned with the same 
source impedance as your best guess for the antenna. Again test the 
radio the way you are going to use the radio.

Hard core electrical engineering for breakfast - gotta love it !!!

	Take Care

		Bob Camp
		KB8TQ



On Mar 1, 2005, at 10:09 PM, DJED1 at aol.com wrote:

> Thanks for the note, Bob.  I went with the instructions in the URM-25 
> manual, which clearly intend that there be a 50 ohm load on the 
> generator for the output voltage to be equal to the meter reading.  I 
> was reminded of this when i put my scope on the HP generator to check 
> the output- way off until I put a 50 ohm termination on the etup, then 
> the generator output was right on.  So I'm convinced that the 
> generator needs to be terminated properly.  Of course this is not a 
> problem for 50 ohm receivers.  For the R-390, it troubles me that we 
> don't have an impedance match between the generator and the radio, but 
> we do know, I submit, the voltage across the receiver terminals.  If 
> you wanted to determine the power going into the receiver, I would use 
> the 125 ohm value and the measured voltage.  Probably the best way is 
> to devise a lossless transformer to go from 50 ohms to 125 ohms.  I 
> haven't tried to do that yet, but I'm working on it.
>  Regards, Ed