I'm not pessimistic, I'm just cognizant of the fact that they are designed for very low impedance sources at high frequencies and a compensated probe to get the correct frequency response. A cheap scope simply can't be tested as a separate system from it's probe unless you are willing to build specialized circuitry, inline termination is not the same as having an input designed to have 50 Ohm impedance over the device's usable frequency range ... the probe and it's compensation are integral parts of what makes up the frequency response.
- The Oscilloscope Vendor needs to specify system bandwidth for me (specific probe+scope), since the inverse RMS formula* for Gaussian scopes does not apply to Flat systems!
- If my signal rise time is significantly faster than my system rise time (scope+probe), I can estimate the signal rise time by solving for t_r(signal) in
t_r(measured)=SQRT(t_r(signal)^2+r_t(system)^2)
But is this valid for the Flat response scope too?
It is worth noting that lower frequency oscilloscopes, at least Tektronix ones with low frequency extending into the 100s of MHz, were specified for bandwidth *at the probe tip* with specific exceptions. This is from an older (paper!) version of "ABC's of Probes" by Tektronix which is available online as part of Linear Technology application note 47:
Most manufacturers of general-purpose oscilloscopes that include standard accessory probes in the package, promise and deliver the advertised scope bandwidth at the probe tip.
For example, the Tektronix 2465B 400 MHz Portable Oscilloscope and its standard accessory P6137 Passive Probes deliver 400 MHz (-3db) at the probe tip.
However, not all high performance scopes can offer this feature, even when used with their recommended passive probes. For example, the Tektronix 11A32 400 MHz plug-in has a system bandwidth of 300 MHz when used with its recommend P6134 passive probe. This is simply because even the highest impedance passive probes are limited to about 300 to 350 MHz, while still meeting their other specifications.This makes calculating the system bandwidth from the root sum of the squares of the specified probe and oscilloscope bandwidth marginal at best and impossible at worst and cheap oscilloscope manufacturers and marketing droids are hardly going to make it easier.
It is worth mentioning that the standard probe tip measurement uses a coaxial probe tip connection, 50 ohm signal source, and 50 ohm termination so the probe tip sees 25 ohms in a coaxial environment which is hardly representative of actual use.
- The Flat scope has less sampling alias errors, and also requires a less minimum sampling rate than a Gaussian response scope to reconstruct a signal for the same bandwidth
The only aliasing errors I run across are those caused by nonlinearities in the digitizer itself which mix the incoming signal with the sampling frequency producing products above the Nyquist frequency. Interleaved ADCs are especially prone to this problem as Agilent discusses in one of their application notes. Equivalent time sampling if available largely negates this issue simply by supporting a sampling rate so high that aliasing becomes a non-problem even with a Gaussian response.
This is my first flat response scope, so I'm wondering how much of those gotchas I will run into still. It also makes me wish I had not sold the Tek7104 after all, it would be good to have something to compare against.
I am inclined to think that an old style sampling oscilloscope is an even better choice in this case although they lack a Gaussian response as well. They have the advantage of predictable frequency response and are my go-to tool for calibrating the pulse generators used to calibrate the transient response on analog oscilloscopes.
My fastest analog non-sampling oscilloscope is a 500 MHz 7904 and my fastest vertical amplifiers for it are only 400 MHz but wow, it sure works well and especially so with lower bandwidth vertical amplifiers. It is difficult to appreciate how visually clear a high acceleration potential oscilloscope CRT is when used at low bandwidths until you see one. Just that by itself is enough reason to use a 500 MHz CRT oscilloscope in a low bandwidth application and may explain much of the nostalgia for the 2465 series of oscilloscopes.
I'm really worried by the fact that I will see overshoot where a Gaussian scope would not.
Like, is this signal really looking like this? How do I know it is my circuit and not the scope behaving differently from what I would expect.
What if I am specifically interested in measuring overshoot, I guess I better make sure my signals aren't too fast?
Other than that, I think this means that for in-band signals, this scope will serve me extremely well.
I started having the same concern after evaluating a couple of low end and high end DSOs which all displayed this problem in one form or another and I suspect oscilloscopes which have "upgradable" bandwidths will suffer from this to an even greater extent. I would much rather have good transient response which I cannot fix than better anti-aliasing since that has been a solved problem, at least for those who understood it, until recently.
On one hand, their anti-aliasing prevents aliasing which was not a problem before. On the other hand, the same anti-aliasing messes up the transient response. I am failing to see an advantage here.
If I get around to it this weekend, I will post the measured frequency response of my 2232 in DSO mode using an SG503 leveled sine wave oscillator. I have faster DSOs I could test but lack a faster leveled signal source.
Home
Help
Search
Login
Register
