why is "high capacitance for high frequency work" listed as a con? isn't it a pro of x10 probes that they offer a wider bandwidth and are better for high frequency than x1 probes?
@19:30 -20:30 talking about the 3ft piece of coax having 100pF capacitance which at 1GHz represents 1.6 Ohm shunt impedance
- NO. At RF frequencies coax is a transmission line and when terminated to a matched load (50 Ohm in this case on the scope input) will only have resistive losses in the wire , which were specified right there in the datasheet that you showed , in the next table right below the capacitance/ft data. That datasheet specified only 1 db of loss at 1GHz for 1m length of the coax used..
If coax cables had THAT low shunt capacitance at RF, they would be useless at high frequency, which is not the case and is an exact opposite.
The 21:1 probe is made using 1kOhm external resistor at the test point and 50 Ohm terminator either switched on on the scope input or using an external termination resistor at the scope input connector if the scope does not have 50 Ohm input feature . Otherwise it will not have 21:1 voltage divider ratio. (1000+50)/50=21. You may be confused because cheap and some mid-range scopes do not have built-in 50 Ohm input impedance feature and only have 1M input. For such scopes you have to use an external 50 Ohm terminator at the scope input.
@19:30 -20:30 talking about the 3ft piece of coax having 100pF capacitance which at 1GHz represents 1.6 Ohm shunt impedance
- NO. At RF frequencies coax is a transmission line and when terminated to a matched load (50 Ohm in this case on the scope input) will only have resistive losses in the wire , which were specified right there in the datasheet that you showed , in the next table right below the capacitance/ft data. That datasheet specified only 1 db of loss at 1GHz for 1m length of the coax used..
If coax cables had THAT low shunt capacitance at RF, they would be useless at high frequency, which is not the case and is an exact opposite.
@19:30 -20:30 talking about the 3ft piece of coax having 100pF capacitance which at 1GHz represents 1.6 Ohm shunt impedance
- NO. At RF frequencies coax is a transmission line and when terminated to a matched load (50 Ohm in this case on the scope input) will only have resistive losses in the wire , which were specified right there in the datasheet that you showed , in the next table right below the capacitance/ft data. That datasheet specified only 1 db of loss at 1GHz for 1m length of the coax used..
If coax cables had THAT low shunt capacitance at RF, they would be useless at high frequency, which is not the case and is an exact opposite.Added to that is that both sides of the coax need to see an impedance of 50 Ohms. Just putting 1k Ohm in series is not going to cut it. Terminating the end with 50 Ohm is only helping to get rid of reflections.
@19:30 -20:30 talking about the 3ft piece of coax having 100pF capacitance which at 1GHz represents 1.6 Ohm shunt impedance
- NO. At RF frequencies coax is a transmission line and when terminated to a matched load (50 Ohm in this case on the scope input) will only have resistive losses in the wire , which were specified right there in the datasheet that you showed , in the next table right below the capacitance/ft data. That datasheet specified only 1 db of loss at 1GHz for 1m length of the coax used..
If coax cables had THAT low shunt capacitance at RF, they would be useless at high frequency, which is not the case and is an exact opposite.Added to that is that both sides of the coax need to see an impedance of 50 Ohms. Just putting 1k Ohm in series is not going to cut it. Terminating the end with 50 Ohm is only helping to get rid of reflections.
Er, no.
I suggest you understand section 3 of
http://www.davmar.org/TE/TekConcepts/TekProbeCircuits.pdf
What is the scope, the scope's input impedance[1], the source, the source waveform, the cable length, the probing technique?
At these speeds the details matter.
[1] many are 50ohms//20pF, which causes problems.
Here is an excerpt on a home made 21:1 probe from "High Speed Digital Design, A Handbook of Black Magic" by Johnson/Graham.
The 1kOhm series resistor is not to "isolate high capacitance", but to reduce the rise time degradation L/R due to inductance of the sense loop, see the attached picture. With a 1kOhm resistor the ratio L/R is much smaller.
What is the scope, the scope's input impedance[1], the source, the source waveform, the cable length, the probing technique?
At these speeds the details matter.
[1] many are 50ohms//20pF, which causes problems.This is measured using the calibrator output on a Lecroy Wavepro 7300A set to 50 Ohm with full bandwidth. The coax cable (about 50cm) is soldered onto an SMA (with 1k series resistor) which is connected to the calibrator output (BNC). Lead lenghts about 2cm. The test with the Tektronix P6156 probe (with 20x attenuator) is done with a BNC adapter on the probe in order to connect to the calibrator output. No ground leads or ground spring:
The P6156 is basically a 1k Ohm resistor in series with a coax cable so it is no surprise the result is the same. Edit: I see something went wrong with the inline images which I fixed now.
Use with 1 Megohm Input Oscilloscopes
High resistance inputs require external 50 W terminations (Tektronix part number
011-0049-01). Introduction of a termination will result in a slight signal
reflection in the system (appearing at twice the cable delay time) due to the shunt
capacitance of the oscilloscope in parallel with the 50 W termination. To reduce
the effects of this reflection, add a 2X attenuator between the probe connector
and the 50 W termination. This will decrease the reflection by a factor of four
while increasing the attenuation by a factor of two. For sine wave measurements,
the 2X attenuator is recommended to minimize standing waves.
What is the scope, the scope's input impedance[1], the source, the source waveform, the cable length, the probing technique?
At these speeds the details matter.
[1] many are 50ohms//20pF, which causes problems.This is measured using the calibrator output on a Lecroy Wavepro 7300A set to 50 Ohm with full bandwidth. The coax cable (about 50cm) is soldered onto an SMA (with 1k series resistor) which is connected to the calibrator output (BNC). Lead lenghts about 2cm. The test with the Tektronix P6156 probe (with 20x attenuator) is done with a BNC adapter on the probe in order to connect to the calibrator output. No ground leads or ground spring:
The P6156 is basically a 1k Ohm resistor in series with a coax cable so it is no surprise the result is the same. Edit: I see something went wrong with the inline images which I fixed now.
It sounds like the 50ohm at the source is connected calout-50ohmToGnd-1kohm-cable-scope. In that case the effect you see without the source 50ohm might be due to the cal out behaving properly when correctly loaded.
Alternatively, I suspect the scope isn't 50ohm input but is really 50ohm//15pF. In that case you will see the effect stated in the P6156 manual, viz:
What is the scope, the scope's input impedance[1], the source, the source waveform, the cable length, the probing technique?
At these speeds the details matter.
[1] many are 50ohms//20pF, which causes problems.This is measured using the calibrator output on a Lecroy Wavepro 7300A set to 50 Ohm with full bandwidth. The coax cable (about 50cm) is soldered onto an SMA (with 1k series resistor) which is connected to the calibrator output (BNC). Lead lenghts about 2cm. The test with the Tektronix P6156 probe (with 20x attenuator) is done with a BNC adapter on the probe in order to connect to the calibrator output. No ground leads or ground spring:
The P6156 is basically a 1k Ohm resistor in series with a coax cable so it is no surprise the result is the same. Edit: I see something went wrong with the inline images which I fixed now.
It sounds like the 50ohm at the source is connected calout-50ohmToGnd-1kohm-cable-scope. In that case the effect you see without the source 50ohm might be due to the cal out behaving properly when correctly loaded.No because with the second test with the coax, the cal output still sees a 1k Ohm resistor at it's output and yet the resulting signal looks fine.
QuoteAlternatively, I suspect the scope isn't 50ohm input but is really 50ohm//15pF. In that case you will see the effect stated in the P6156 manual, viz:No; I have used a 3GHz scope to do the test. I'm damn sure it has proper 50 Ohm inputs! Please check the specs before making these kind of statements.
I suggest you to do a simulation with transmission lines in Spice.
Alternatively, I suspect the scope isn't 50ohm input but is really 50ohm//15pF. In that case you will see the effect stated in the P6156 manual, viz:No; I have used a 3GHz scope to do the test. I'm damn sure it has proper 50 Ohm inputs! Please check the specs before making these kind of statements.