[R-390] R-390A sensitivity measurements

[Bob Camp]

Hi

The only odd thing being done here is to terminate the generator with 
an external 50 ohm load. Here's what's going on put in fairly simple 
terms to keep the math from overloading my brain ...

1) The generator is set up to deliver 2 volts into an open circuit load
2) You terminate the generator in 50 ohms.
3) There is 1 volt on the output of the generator (since it's a 50 ohm 
generator).
4) The impedance at the output is now 25 ohms (50 in parallel with 50 = 
25)

Since the generator is calibrated in terms of a 50 ohm load the 
generator is "set to 1 volt" in all the examples above. A lot of this 
does not make a lot of sense, but that's the way it's all defined.

A normal sensitivity test is done with the generator only terminated by 
the impedance of the radio you are testing. If you have a radio that 
looks like an open circuit then it gets 2 volts. If the radio looks 
like 50 ohms it gets 1 volt. This is also the way a 50 ohm antenna 
would drive the radio.

If you want to have a 125 ohm generator then you need to put a 75 ohm 
resistor in series with a 50 ohm generator. Because of the way levels 
are defined you don't have to change the voltage level when you do 
this. If you first load the generator in 50 ohms you cut the output 
voltage in half and must figure that into what you are doing.

I know this all is a bit whacked, but that's the way RF is defined ....

	Take Care!

		Bob Camp
		KB8TQ




On Mar 1, 2005, at 9:31 PM, DJED1 at aol.com wrote:

> I tried making some sensitivity measurements on my receiver, in which 
> the IF
> gain was reduced per Chuck Rippel's suggestion.   I tried several 
> different
> scenarios:
> First, per the MIL spec, sensitivity is measured with an 8 Kc 
> bandwidth and
> adjusted for a 10 dB change in output by switching the modulation on 
> and off
> (30%, 400 cycles).   They also call for a resistor in series with the 
> radio
> input.   I couldn't see how this made sense, so I followed the 
> directions in the
> URM-25 manual.  It calls for a series or parallel resistor such that 
> the load
> the generator see is 50 ohms.   I know both the URM-25 and the new HP 
> need to
> be terminated in 50 ohms to provide the voltage indicated on their 
> meter, so I
> put an 82 ohm resistor in parallel with the nominal 125 ohm receiver 
> input,
> resulting in a 50-ohm load.   The results were interesting:
> Using the method of modulation on and off gave a sensitivity of 1.9
> microvolts.   Not the result you often hear discussed with these 
> radios, but credible
> for a radio where the specification is 3.3 yo 4.4 microvolts.
> I then tried the method I used in the past, of turning the modulated 
> carrier
> on and off, and using a 4 Kc bandwidth.   Big difference-   0.23 
> microvolts.
> I tend to like this method because it seems more representative of an 
> actual
> signal.
> Finally, considering that we listen to CW and SSB as well as AM, I 
> measured
> the sensitivity with the BFO on and a 2 Kc bandwidth.   Got about the 
> same
> result- 0.22 microvolts.   I then reduced the signal level until the 
> signal was
> just detectable in a 1 Kc bandwidth and got down around 0.01 
> microvolts.
> That's in the ballpark of the -143dBm noise floor that is quoted by 
> some for the
> radio.
> All in all it was an interesting exercise.   The measurements made 
> with the
> specified method explain the specification of 3.3 microvolts, while I 
> believe
> the method of switching the carrier on and off is more realistic.   
> Definitely,
> the CW measurement is more representative of real world conditions.   
> And I
> think 1/4 microvolt is adequate for almost all application in which I 
> would use
> the receiver.   I'd be interested in hearing from anyone who has 
> duplicated
> the test setup and gotten results better than mine.
> (Incidentally, I did check the generator for leakage prior to making 
> the
> measurements.   Detected a very low level with the receiver connected 
> to a wire
> near the generator, but nothing when connected to the generator by a 
> shielded
> cable.)
> Ed
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