Wouldn't try to measure audio S/N with an RMS AC voltmeter. The LF 8556 section of the HP will do that though, with a 1 KHz test signal modulated on the carrier from the signal generator. That's a good test to add, along with the carrier to noise ratio tests. The diode switching noise in AM demodulation will make the net audio S/N worse than FM, but its a good test to add to the plan.
I plan to do that test before and after the diode swap, to get some numbers on the reduction of the audio noise floor.
I have already modeled the C/N improvement I predict to see with the RF amp swap (2sc999E). If I raise the sky temperature high enough (4000K or so), then the realized C/N improvement will be in the less than 1dB category. That's what the test is about, to put real numbers behind the mod, with a real environment. The stated sensitivity of the unit is 0.5uV for 10 dB audio S/N. THe are 10 and 12 meter SSB ham rigs that have twice and three times that sensitivity, and CB radios half as senstitive.
As far as the actual system noise temp (including the antenna-ground-sky), I plan to measure that noise power (relative change) with my current 2SC1730. That is, connected to a 50 ohm load, and then connected to the vertical 1/2 wave dipole, on a couple of different frequencies.
I am having trouble locating a real 2SC2999E currently, but will find one soon.
As far as results, they are relative, since the equipment and methods won't change, just a few components. So calibration and ageing are second order error factors. I'm intrested in 1st differences here.
