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E-mail: Jack.Smith@cliftonlaboratories.com

I recently explained how I go about measuring receiver sensitivity and it occurred to me that others might find the description useful.

In the case at hand, the receiver to be measured is an Elecraft K2 transceiver, although that is not an important part of the technique, as a similar approach is usable with any CW/SSB receiver.

One note first, however. The K2 is a transceiver, which means that it can transmit back into your (probably) expensive signal generator. A few signal generators have protective circuitry that will save you the expense of repair, but most do not. So, before you connect your transceiver to your signal generator, disconnect its microphone and key to avoid accidental transmission.
 

We will make a very simple measurements--how much signal is required for a 20 dB signal-to-noise ratio at the headphone output. More precisely, we will measure the required signal for 20 dB (signal+noise)/noise ratio, but with a target of 20 dB, the difference between S/N and (S+N)/N isn't too great and we will call the result the 20 dB S/N sensitivity.
 

The required equipment is:

The illustration below shows the test setup. The headphones let you listen to the receiver audio at the same time you measure the level.

Before you connect your K2 transceiver to the signal generator, please re-read the note at the top of the page about first rendering your transceiver unable to transmit. You will be irked, to say the least, if the attenuators in your signal generator are smoked by a brief burst of power out of your transceiver.

MDS: 28 MHz
     Pre-amp off: -125 dBm
     Pre-amp on: -133 dBm

I've recorded the values to the nearest 0.1 dB, but I would be surprised if the measurements are accurate to ±1 dB, as I have not used other equipment to calibrate the signal generator output and because there is considerable room for error in estimating the average noise level as the meter readings bounce around.
 

First, my measured MDS levels are reasonably close to Elecraft's published specification of -130 and -135 dBm, for pre-amp off and on, respectively. I probably could have reduced the generator output a bit more with the preamp off and still had a copyable result.

Next, we note that although the preamplifier has 10 dB gain, it only changes the 20 dB sensitivity figure by at most 3 dB. Why is that? The reason is that the K2's 20 dB SNR sensitivity is determined by the receiver's noise figure. In simplistic terms, when the preamplifier is turned on, it increases the noise and signal both, as well as adding its own internal noise to the mix. Now, if we look at the results and work through the math behind the K2's front end design, we see that when the preamplifier is enabled, the composite noise figure improves several dB. This is because the composite noise figure of a multi-stage receiver is determined mostly by the first stage or two. With the preamplifier off, the SNR is determined chiefly by the losses in the K2's input filtering and the TUF-1 mixer loss and then the noise figure of the 2N5109 post-mixer amplifier. When the pre-amp is engaged, the noise figure is reduced to a value closer to the sum of the K2's input filtering and the pre-amp noise figure. (With only 10 dB pre-amp gain, later stages will still have some effect on the overall noise figure.)

With respect to the 20 dB S/N sensitivity, the measured parameters seem consistent and within range of a normal SSB receiver.

Stan, W5EWA, has provided measurements he made on his K2 following the procedure described at this page. Stan's data is with the preamplifier off. Except for 28 MHz, our readings agree reasonably well. The 6 dB difference at 28 MHz is more than can be attributed to calibration error.