Computing stuff tied to the physical world

Function generator weirdness

In Hardware on Sep 10, 2012 at 00:01

Fighting my lab instruments… not good :(

The TG2511 Arbitrary Waveform Generator in my lab is very flexible, and does all the usual things one would expect of a modern signal generator, i.e. setting the shape, frequency, and amplitude of its output signal.

But there’s some weirdness w.r.t. the amplitude. The built-in impedance is quoted as being 50 Ω, no doubt the reasoning being that if you terminate the cable with 50 Ω as well, then a standard coax cable will have the least possible signal degradation (ringing, reflections, and such).

There is a setting to adjust for the assumed load, so that the instrument can adjust its amplitude accordingly. The default is to assume a 50 Ω load at the end of the cable:

DSC 3826

But this is where I get mightily confused. Here’s what I see with the scope set in high impedance mode:


That in itself is actually correct: the output amplitude is twice the expected value, since there is no 50 Ω terminating resistor. So let’s add it in (it’s an option inside the scope):


Huh? – Why isn’t the amplitude 3 Vpp, and ends up being only half that value?

But there’s something even stranger, and far more inconvenient about the TG2511. After a few minutes, I often get a message about the output signal being overloaded:

DSC 3827

Doesn’t make sense. I’m never loading the output with more than 50 Ω, i.e. well within specs. Sometimes this happens even when I completely disconnect the load. Even more suspect: the green “OUTPUT” doesn’t always match what’s happening. Sometimes the light is off, yet the output signal is still present!

In fact, I get this screen in the most unexpected situations. Maybe the TG2511’s output stage is broken? Yuck.

I have contacted the supplier for advice.

  1. My scope does not have a 50ohm option so I use a BNC Tee that has a 50ohm terminator plugged into one leg. Something to try if you can make that kind of connection – of course set the scope to high impedence for that test.

  2. So, it is behaving like either 1) the TG2511’s output impedance is 150 ohms (a strange number indeed), or 2) the TG2511 is correct (50 ohms) and the scope has an input impedance of 16.7 ohms.

    I would first simply measure the input impedance of the scope with an ohmmeter. Then, as Mark suggests, put known resistances in parallel. At these frequencies, of course, it doesn’t need to be done with BNCs — I’d just test quickly with clip leads.

  3. The output stage is effectively a 1 Watt broadband RF amplifier with a compensated response flat from near DC to at least ~250Mhz to render faithfully the non-sinudoidal waveforms with up to 25Mhz fundamental. When the load is mismatched (open circuit is almost as bad as short circuit), the power pushed out comes reflecting back into the final amplifier and ends up as extra heat there.

    There will be some basic thermal protect for the output stage – note the manual puts a limit of 60 seconds into a short circuit. Possibly this part of the circuitry is playing games with false triggers. The output stage itself seems to be generating clean waveforms.

    As for the odd readings – I guess it is time for Occam’s Razor. If the observed trace is exactly half of the expected with the 50Ω end loading, then the generator end has also halved its output. This will happen if the LOAD option is inadvertently toggled from ON to OFF.

    The UI is less than clear – the set point output V is tweaked behind the scenes to allow for 50Ω load present (generate 2V to see 1V at the far end) or 50Ω load absent (generate 1V to see 1V at the far end).

    • If the TG2511 is like my HP33120A (and I’m guessing it is), then the output is independent of the “load” (i.e., “assumed termination”) setting.

      The device output always is a voltage source through 50 ohm resistor. When requested, the displayed amplitude is adjusted according to the assumed impedance (typically 50 ohms, but it seems user-adjustable in this unit, not like the HP33120A). On the HP, toggling the terminator setting has no effect on the actual output voltage — only on the displayed value (behaviour that bugs me, actually)

      There is no “reflected” power here. However, if operated into a short, the output stage will see twice as much current as at its rated load impedance, and that poor 50 ohm source resistor will see FOUR times its usual power. Worst-case (10 VDC), that would be 2 watts.

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