| Products > Test Equipment |
| > 1 GHz DIY differential probes |
| << < (12/35) > >> |
| nctnico:
--- Quote from: Marco on October 16, 2018, 08:24:17 am --- --- Quote from: tggzzz on October 15, 2018, 01:39:31 pm ---Or from a few bits of coax and a combiner, e.g. http://emcesd.com/pdf/cd94scr.pdf --- End quote --- More expensive than a bunch of BFU660F's though. How about something simple like this? Is the simulator really giving me that bad an impression of how it would work with a tight layout and some =<0204 components? R1=R2=0 is only useful for say 10s of mV of input, probably want to bias the bases to ~4V with opamps instead of resistor dividers, might want to clamp R5-R8 with Schottkys. PS. that Deltasense probe has nice specs, large voltage range and input impedance. --- End quote --- I don't think this configuration will work very well (probably due to the miller effect). I did find a fast opamp and I'm going to try a classic difference amplifier approach. After all the reason to use a differential (instrumentation) amplifier is to have extremely high input impedances on both inputs but in this case a few kilo Ohms is already OK. Perhaps a discrete solution specifically designed to act as a differential probe can be pushed to a higher bandwidth especially if it doesn't need to amplify. DC offset and temperature stability will be challenging nuts to crack though. |
| Marco:
There's not a whole lot of amplification in that circuit, 1.5 or so since the emitter resistance of RF transistors is rather high, so Miller capacitance isn't a big deal. The simulator says the circuit has >400 Ohm of input impedance at 1 GHz. When you increase R1&R2 the Miller capacitance is reduced further still. |
| dietert1:
Your difference stage (and whatever amplifier you make) will have an input capacitance of about 0.5 to 1 pF, which is much better than the usual passive probe and may be good enough for a 1 GHz probe. Yet if you want the 0.04 pF or 0.02 pF that cerebus proposed above and that Mr. Rosenkränzer implements in his probe, there will be a capacitive divider in front of the amplifier. This is also mentioned in the Keysight video. The difference stage always looses a factor 4, because you drop one of two output ports and you want to drive the line to the scope with 50 Ohm output impedance. In total you will have a factor 1:50 or 1:100. I think you really want some gain. If you put emitter followers in front of your difference stage this will get you roughly a factor 5 (BFU66F Cbe/ Ccb). Those who want DC to GHz might use THS4302 gain blocks. By the way, i received a LMH3401 evaluation board. That is an excellent difference stage, like +/- 1 dB up to 2.9 GHz. Image is boring. |
| dzseki:
--- Quote from: BravoV on October 16, 2018, 11:32:57 am --- --- Quote from: Mechatrommer on October 16, 2018, 11:09:31 am ---can we make 100fF? --- End quote --- Femto farad ? I'm guessing its not that easy, example of a 3 decades old, HP 54701A 2.5GHz active probe (not a differential probe) has 0.6pF (typical) with input resistance of 100K Ohm. --- End quote --- I have the HP 1120A which is only 500MHz active probe (not differential), it has 100kOhm||3pF input impedance in 1:1 mode and 1MOhm|| <1pF when using a divider (10:1 or 100:1). The point is this is a quite old stuff, appeared in one HP Journal from 1969 I think, the service manual do actually contains all the schematics, even the probe's hybrid, it might worth a look. |
| dietert1:
In my opinion that old technology is completely obsolete and its analysis a waste of time. With the THS4302 gain block i mentioned you can make a high impedance 1:1 probe with 0.4 pF input capacitance and a bandwidth of 2.4 GHz. |
| Navigation |
| Message Index |
| Next page |
| Previous page |