My signal generator drives a 50 ohm coaxial cable, terminated with a 50 ohm to ground at the input of a voltage amplifier.

However, when I look at my input signal over frequency, it doesn't behave as I expected.

When I remove the connection to the amplifier, then it is all fine. I get an almost a straight line, with some +/- 5 mdB curves. This leads me to conclude that the problem comes from the input impedance of the amplifer, changing with frequency, which again leads to an impedance mismatch of the cable.

The frequency range I am interested in goes up to 50 MHz, where there is already a change of 0.5 dB.

The cable has a length of 1 m. The gain of the amplifer is 10. The generator also has a 50 ohm series resistor (Rser.)

What can I do about that, or what is the general procedure for this kind of problem?

See the simulation in LTSpice.

enter image description here

Many thanks!

EDIT: The generator has a DC offset of 300 mV (see SINE(300m) and an AC amplitude of 250 mV.

The parameters for the cable are specified for 1 meter. The only thing to change is the parameter "len" for the length of the cable, the other parameters remains the same.

EDIT 2: See the following image: I reduced the length of the cable to 0.1 m. Of course, now at f=50 MHz, I only have approx 200 mdB of change. enter image description here

The next image shows how it behaves when the cable is disconnected from the Input of the Amplifier at a length of 1 m. Only a minimal change of some mdB, which are negligable enter image description here

  • \$\begingroup\$ Can you provide clearer screenshots? Change the colors for your plot and condense so that the text and components are legible. \$\endgroup\$
    – Groger
    Jul 21, 2020 at 11:47
  • \$\begingroup\$ Sorry Groger, is the image not clear when clicking on it? When I click on it, it is clear, sorry for that ! \$\endgroup\$
    – newstudent
    Jul 21, 2020 at 11:50

1 Answer 1


You are driving a signal down the transmission line that goes below 0 volts. 0 volts is also the most negative supply voltage to your op-amp therefore, you are likely to be causing the op-amp to exceed it's common-mode input range (-0.1 volts to Vcc+0.1 volts).

Of course, it's only a model so no real damage.

You are also pretty close to the limits from what you can expect with the model for your transmission line. It appears you are specifying "per metre" values for R, L, G and C and, at 50 MHz, free-space has a wavelength of 6 metres but probably only about 4 metres in a cable therefore, you are likely to be asking too much from the t-line model.

  • \$\begingroup\$ Andy aka: No, the Generator has a DC Offset of 300 mV! Thats LTSpice notation, its written in the brackets of SINE(). AC 200 mV is my AC Amplitude \$\endgroup\$
    – newstudent
    Jul 21, 2020 at 11:49
  • \$\begingroup\$ Then check your t-line - see my continued answer. \$\endgroup\$
    – Andy aka
    Jul 21, 2020 at 11:52
  • \$\begingroup\$ When I disonnect the Amplifier from the cable with its 50 Ohm, so that I only have my Generator, cable and 50 Ohm, then I have no problems. So the cable should not be the problem ?! \$\endgroup\$
    – newstudent
    Jul 21, 2020 at 11:55
  • \$\begingroup\$ It's worth a try - it could be that a problematically "lumped" cable terminated in the right resistance still looks good. Try dividing R, L and C by ten and see if it fixes things. Mess around with it is the easiest route then figure out why it works better. t-line models are a bit of a law unto themselves. \$\endgroup\$
    – Andy aka
    Jul 21, 2020 at 11:57
  • \$\begingroup\$ Please see my Edit on my first post. I added two images: one where I reduced the length of the cable to 0.1 m (of course, the change gets less), and one image with a length of 1 m Cable, but disconnected from the Amplifier, where it behaves as expected. \$\endgroup\$
    – newstudent
    Jul 21, 2020 at 12:14

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