sensitivity

Measuring Receiver Sensitivity

There may be an occasion when its appropriate to measure and record receiver sensitivity in real terms using an accepted standard. For radio receivers, real term sensitivity is expressed as the value of a modulated RF voltage applied to the antenna input necessary to provide a 10db S/N + N figure. This means, what input voltage is required to raise audio output 10 db over the receiver noise floor.

Measuring the receiver sensitivity in the R390A is an easy, straight forward procedure. The receiver Line Level meter can even be used to help with the measurement. Here is the procedure:

1- Turn the receiver and signal generator on and allow them to warm up for 1 hour.

2- Put the receiver and generator on 4.8 mHz (or any other frequency of choice)

3- Set the initial generator output to 1uv and modulation level 1kc, 30% but do not connect the antenna input yet

4- Set the R390A Function switch to "MGC," Bandwidth to 4kc, RF gain full CW, BFO off.

5- Set the Line Meter range switch to -10 and carefully adjust the Line Gain control for a -10db indication on the Line Level meter.

6- Watch the Line Level meter and peak the Ant Trim on receiver noise alone; readjust the Line Gain control for a -10db indication on the Line Level meter. This level is now indicating a relative level of the receiver noise floor.

7- Connect the generator to the receiver antenna input (I use the "Balanced" input) and adjust the Kilocycle Change control to center the output of the generator in the receiver passband.

8- Adjust the RF output level of the generator to cause the Line Level meter on the R390A to read "0."

9- Note the output of the generator in microvolts. This value is the 10db S/N + N receiver sensitivity. It should be less than .5uv although the official specifications of the receiver call for 3 microvolts.

Once the receiver sensitivity has been established, you can then "massage" the IF Gain (usually by dropping in nearly all the way) and tube lineup for the best recovered audio. The technique to "massage" the If gain is covered in the Technical, IF Gain section of this site.

Optimizing the vacuum tube lineup in the signal part is another technique to maximize performace. Because changing them does not require re-alignment, an easy start is to work with the tubes in the signal path of the IF strip. These are the 5749W's IF amps, V-501, V-502 and V-503. The 6AK6 4th IF amp V-504 and the detector, V-506, a 5814A.

With your generator set up and the receiver at step 8 above, install a replacement tube at V-501, allow it to warm up about 5 minutes and note the indication of the Line Level Meter. If it climbs above 0db, the new tube has improved gain, if not, reinstall the old tube. Repeat the same steps for V502, V503, V-504 and V-506. You may "find" a few extra db in your spare tube stores.

Page prepared by R. Charles Rippel

E-mail to: Charles Rippel
Revision: 04 December 24, 2001
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