Issues with HP4396B Network Analyzer

[twomilimeter]

HP4396B Network Analyzer

Hi,

I am just beginning to use a network analyzer.
The model is HP 4396B network/spectrum analyzer.
I started by trying to measure the input power at the R and B channel by changing the RF power source.
The RF source, R and B channel are connected to a two way power splitter. The setup is shown below.

I set the "meas" button to B to measure the power on the B channel and "meas" to R to measure the input power to the R channel.
The relationship of the output power from the RF source with the measured input power to the B channel are shown below.

I am a little confuse here. Why does the measured power at the R or B channel is larger than the RF power source?
For e.g., when the RF power source was set to -10dBm, I am measuring -15dB. Is -15dB the input power at the R or B channel or I have mistakenly understand the unit?

Secondly, I try to measure the response of a RC circuit with 50 ohm input and output terminated with R = 100ohm and C = 10pF. The 3dB roll off should be ~1.5MHz.
I set the "meas" channel to "B/R" and later to "B".
The setup, measured gain and phase are as shown below.
**broken link removed**
RC Setup

Gain with "meas" set to "B"

Phase with "meas" set to "B"

Why does the phase always change from -180 to 180? What does the phase indicate when the "meas" is set to "B/R" and "B" only?

Kindly advise.

Thank you very much.

Regards,
2mm

 

[biff44]

Re: HP4396B Network Analyzer

I am not sure I follow what you are saying. A network analyzer measures "relative" powers, not absolute powers. So, you seldom set the controls of it to measure only "B". You would instead set it to measure the ratio B/R, in dB's. That is usually the more useful measurement. Lets say you had a 3 dB attenuator pad, and wanted to measure its loss precisely. You would do the following:
1) set the network analyzer to measure B/R, and to sweep over the desired frequency range
2) connect the cables as in your first drawing
3) "Calibrate" the network analyzer by saving the sweep pattern of B/R. After "calibrating", your network analyzer display should show a straight line at 0 dB over the entire frequency range.
4) now install the device to be measured between the splitter and the B input. The display will now show the insertion loss of the attenuator pad as the frequency is swept. I assure you, you will not see the attenuator have any "gain"! The above 3 dB attenuator example should show 3.1 db loss at one frequency, 2.9 dB at another frequency, etc.

As far as the jump in phase from -180 to +180, it is just a mathematical quirk of the network analyzer. It is incapable of displaying -181 degrees, so as you are sweeping frequency and the meausrement phase goes from -179 to -180 to -181 degrees, you will instead see -179, -180, +179 degrees.

Rich

 

[twomilimeter]

Re: HP4396B Network Analyzer

Dear Rich,

Thanks for the information.
Could I confirm that if we select "B/R", and by doing a frequency sweep, are we actually measuring the power transfer function of the circuit with reference to R?
How about the measured phase? What does it physically mean?
Lastly, I was connecting the network analyzer to a RC low pass filter and the phase angle should only reach -90 degree. However, it appears to change linearly with frequencies as shown in the phase plot. Could this be due to delay in the cable?

Thank you very much.

Regards,
Twomilimeter