PCGU1000, PCS500 and circuit analysis

I have used the PCS500 and K8016 together for circuit analysis for quite some time. This function is very usefull and similar instruments would cost a lot. I had problems with the K8016 that its output did not always follow the setup (some comms problem (I did disable the LPT plug and play)). I have now bought the PCGS1000 and is very glad to say that there is no such problem. I was however evaluating my new combination when I picked up the following problems.

In the 1st place a bode plot is normally defined as Gain = 20Log(Output amplitude/Input amplitude). This is especially true in terms of circuit analysis where the transfer function is independant of the input signal used. The PC lab2000SE software however gives a different answer for different input signal level. This is wrong since if you connect a short between the 2 scope signals it will sometimes tell you it has a gain of more, or less than 1.

The 2nd problem due to the measurement method is that when using automatic setup (and choosing AC as recommended) the scope AC transfer is actually measured (below 10Hz (I’m interrested in measurement between 1 and 100 Hz)). This is wrong since the measurement instrument should not introduce anything to the circuit under analysis.

The sollution is simply to calculated the gain from both input signals on the oscilloscope as is done for the phase measurement. I’m not sure if this is a new problem or not but I havent seen this before and I’ve just installed Version 3.08

I include the measurement results of a piece of coax (short) which should have Gain of 1 (0dB) and phase of 0 over the full frequency range :?

Hz Vrms dBV deg
1.00 0.472 -6.516 3.5
2.00 0.859 -1.323 2.7
3.00 1.131 1.070 2.8
4.00 1.307 2.328 2.7
5.00 1.434 3.131 2.4
6.00 1.512 3.590 2.3
7.00 1.568 3.909 2.1
8.00 1.610 4.138 1.6
9.00 1.635 4.271 1.7
10.0 1.659 4.394 1.2
20.0 1.729 4.757 0.6
30.0 1.745 4.835 -0.4
40.0 1.752 4.873 -0.6
50.0 1.758 4.898 0.0
60.0 1.760 4.911 -0.3
70.0 1.756 4.890 -0.4
80.0 1.758 4.899 -0.7
90.0 1.759 4.905 -1.0
100 1.760 4.912 0.0
200 1.767 4.945 -0.1
300 1.767 4.945 -0.2
400 1.764 4.928 -0.5
500 1.752 4.871 0.0
600 1.756 4.890 -0.3
700 1.758 4.900 -0.2
800 1.760 4.908 -0.5
900 1.754 4.879 -0.4
1000 1.760 4.910 -0.1

Thank you for your comments concerning the circuit analyser (Bode Plot) option of the PCSU1000/PCGU1000 oscilloscope generator combination.
You are right that also the input signal amplitude of the device under test should be measured for accurate Bode Plot measurement. To keep things easy this is not done in this case. The frequency response measurement is based on assumption that the input signal is constant on all the frequencies of interest. Normally this is the case due to the low (50 ohms) output impedance of the PCGU1000 function generator.

In your case the problem is the oscilloscope’s behaviour at the low frequencies when the AC coupling is selected.
This is a known issue and mentioned in the help file.

As a solution to this problem you may have to use the DC coupling of the oscilloscope and put an external series capacitor between the output of the circuit under analysis and the oscilloscope input. This capacitor decouples possible DC coming from the circuit under test.
To reduce the capacitor value you need to use you may use the scope probe setting x10. The input impedance is in this case 10Mega ohms and you may use a capacitor about 1uF to get proper measurement results for the phase and amplitude starting from 1Hz.