Most HP 8924C units have an electronic input module. The electronic input module has electronic switching (vs. mechanical relays) in the output step attenuator to lower the chance of a failure. This also causes the RF output level below 30 MHz to be less than what the display indicates. Because of several variables, the chart could have 2-3 dB of uncertainty and is intended only to help the user be in the "ballpark". The chart below shows how much a typical HP 8924C's output level is attenuated at frequencies below 30 MHz. The chart measures the discrepancy in output level at -121 dBm (0.2 uV) for frequencies below 10 MHz. The discrepancies increase as much as 5 dB when measured at higher output levels. Discrepancies also vary as much as 1 dB from unit to unit. From 10 to 30 MHz, the error should be below 0.5 dB.
Frequency HP 8924C actual RF level output offset from displayed RF level output at -121 dBm
400 KHz -26 dB
600 KHz -21 dB
800 KHz -19 dB
1.0 MHz -16.5 dB
1.9 MHz -7.5 dB
2.0 MHz -7.0 dB
3.0 MHz -3.0 dB
3.8 MHz -2.9 dB
4.0 MHz -2.7 dB
5.0 MHz -2.6 dB
6.0 MHz -1.9 dB
7.0 MHz -1.1 dB
8.0 MHz -0.8 dB
9.0 MHz -0.7 dB
10 MHz -0.6 dB
Also to be noted is the HP 8924C Spectrum Analyzer level will be low at 6 MHz and below. The factory specification for the 8924C Spectrum Analyzer has a level slop of 2-3 dB. The typical 8924C spectrum analyzer shows levels about 1 dB low. at all frequencies. Technical personal at HP indicated this as normal with the 8924C. Below 6 MHz, whatever error the spectrum analyzer already has at higher frequencies is increased by the amounts indicated below. This information was measured from 2 HP 8924C units and most other should be in this ballpark.
Frequency Error from displayed level (displayed level will be this amount off from actual signal)
400 KHz -10 dB
600 KHz -7 dB
800 KHz -5.7 dB
1.0 MHz -4.7 dB
2.0 MHz -2.4 dB
3.0 MHz -2.1 dB
4.0 MHz -1.5 dB
5.0 MHz -1.0 dB
6.0 MHz -0.3 dB