Back in February, I built an active differential probe for the MakeMIT hackathon, but I never really got around to measuring it’s bandwidth, other than confirming that it passes an 1ns-risetime, 100kHz square wave with reasonable accuracy and estimating it’s upper cutoff frequency based on the risetime of the output signal versus the risetime of the input signal. Recently, however, I acquired a much faster signal generator than I had before, which allows me to make accurate measurements far into the GHz range, so I decided to get some better data on the probe.
The experimental setup is relatively simple: first, connect the signal generator directly to a 50Ω-terminated oscilloscope input and measure the signal amplitude as the sine source is stepped through all of the frequencies of interest (I measured 10 points per decade, with extra data points added later as I felt were necessary to smooth out the bode plot). It’s important to do this because the signal source will inevitably have some amplitude variation with frequency, and the scope has a limited bandwidth. I used a Tektronix 784D, which claims a bandwidth of 1GHz (I measured 970MHz — pretty close!) and I picked a signal level of +10dBm (0.707V RMS, 2V pk-pk) because this seemed like a reasonable input level for a high frequency measurement. It is noteworthy though that the probe has a very wide dynamic range and is capable of much higher differential input levels, but I don’t yet have a signal source that can do more than +10dBm at high frequencies.
Once this baseline is established, the next step is to insert the active probe into the circuit and repeat the frequency sweep; the signal source is connected to a 50Ω termination soldered directly to a free-floating female SMA jack, the active probe input is attached across the termination, and the active probe’s output is fed into a 50Ω-terminated oscilloscope input. At high frequencies, the input capacitance of the probe will load down the circuit under test much more than the input resistance, but at 300MHz (close to the measured -3dB point), with an input capacitance of at most 0.5pF per differential input, this impedance is over 1kΩ, so we can assume the termination is good. At 1GHz, this impedance drops to 318Ω, which will start to have some noticeable effect on the termination, but by this point the results of the measurement show that the gain has already rolled off by more than 10dB. Theoretically, it would be more correct to use exactly the same length of SMA cable in both the baseline measurement and the probe gain measurement, but I didn’t do this for lack of an SMA extender because the losses down 1.5′ of SMA cable at these frequencies and power levels should be negligible.
Dividing the signal amplitude with the probe in place by the signal amplitude with a direct feedthrough yields the gain of the probe, which is summarized in the following plot (from 1kHz to 1GHz):
The lower -3dB frequency occurs around 7kHz, while the upper -3dB frequency occurs around 350MHz. The original risetime measurement yielded an upper cutoff of around 400MHz, which is fairly close to the true value. Clearly this probe doesn’t achieve the 1GHz upper cutoff target, but it gets close enough to be useful! Some interesting features to note are that the midband gain is around -19dB, instead of the design target of -20dB, and there is a sharp +2dB resonant peak around 60MHz.
The increased midband gain suggests that there is extra gain somewhere in the circuit that wasn’t planned for. This, in addition to unaccounted-for high frequency loss mechanisms in the PCB and components could partially explain why the top-end gain rolls off at a lower frequency than predicted by the SPICE simulations. I suspect that the 60MHz resonance is a product of the parallel AC coupling capacitors I used between the input buffers and differential amplifier; the largest coupling capacitors (1uF) are placed furthest away from the signal path, giving them the greatest amount of parasitic inductance.
This seemed like a good idea at the time because the smallest capacitors, which pass the highest frequencies, can be closest to the signal path and therefore have the least parasitic inductance. Counter-intuitively, though, this results in the largest coupling capacitors’ self-resonances to occur at even lower frequencies than they normally would. It seems that the solution would either be to remove the largest AC coupling capacitors, which would result in a higher low-end cutoff, or rearrange the AC coupling capacitors to put the largest ones closer to the signal path, which would add some extra parasitic inductance into the high frequency path and reduce the high-end gain. Time to start switching around capacitors to try to make the resonance go away!
That’s all for now, after I experiment with the capacitor arrangement and try to get the gain closer to -20dB, I might send out for another revision of this board with a high frequency PCB substrate (ie. Rogers) and 4-layers. Hopefully the price doesn’t go through the roof (fingers crossed!).
You can read more about the active probe project here.