You are right, we aren't in agreement on the use of the resolution BW to measure low level signals.
This is an excerpt from my 1971 HP text "Electronic Measurements and Instrumentation", chapter 16-8 :
"Sensitivity The ability of the swept superheterodyne spectrum
analyzer to measure small signals is determined by its own internally
generated noise. Typical noise figures vary from 25 dB at low frequency
to 40 dB at 12 GHz. The internally generated noise referred to the
spectrum-analyzer input exceeds basic thermal noise by these noise
figures. At room temperature, the thermal-noise power spectral density
4 X 10- 9 mW/MHz, or -114 dBm in a 1-MHz bandwidth. The avail-
able thermal noise power, in watts,
Ph = kTB (16-8-1)
where B = bandwidth of system
k = Boltzmann's constant, 1.38 X 10- 23 W-sec/°K
T = absolute temperature, °K
The noise on the spectrum-analyzer display is that contained only within
the passband of its intermediate-frequency filter. Although the spectrum
analyzer covers a wide frequency range (by sweeping) it is a narrow-band
instrument and therefore very sensitive to continuous-wave signals.
Noise power is proportional to bandwidth, and so the highest sensitivity to
continuous-wave signals is obtained by using the narrowest bandwidths.
Figure 16-27 shows the levels of thermal noise for various bandwidths at
room temperature.
From the noise figure and the thermal noise level in various band-
widths, the ability of the spectrum analyzer to measure small signals can be
determined" (the nomograph).....
Your cobra has the same sensitivity as the TRC then (.25 uV for SSB and 0.5 uV for AM). Working to 50 Ohms, 0.7uV is equivalent to -110dBm, which is close to receive threshold for S/N of 10dB. Have you actually meseasured the noise floor of your radio at I/F with your analyzer, say right before the detector diodes, while it is close to threshold ? Also, when you lower your res BW by a factor of 10, does your displayed noise floor go down 10dB? Mine does.
Time to sweep a scan width is = proportional to the scan width in Hz / the square of the resolution BW in Hz.
Anyway, I'm still making measurements, including the AF signal to noise ratio at the speaker at various input levels (with the C/N at the IF), and a look at the 10 mHz LO (PLL) spectral noise, which ends up in the IF chain, where I'm taking the C/N measurements.
The TRC is a good radio, but its limitations include no preselector and a noisy LO. Not having a preselector like every common FM tuner in existance does (analog ones) results in amplification by TR5 of the whole CB band (500 KHz).
Still a nice rig. I sometimes can clearly make out SSB phone from Ireland, England, Germany, France, Spain in the morning hours, with the 1/2 wave vertical dipole, on Ch 38 LSB.
