Generators HiRes mode is for 1:10 probe input!
At 160MHz a typical x10 scope probe has an impedance of around 70 ohms - hardly 'high' so it will be about 3db down fed from a 50 ohm source.
I think the OP needs to get a bit of an rf clue.
The OP used a direct BNC connector, at least that is what he said. But you're correct, if a little harsh, in that if the scope wasn't set for 50ohms or didn't have a 50ohm pass through the 16pF of input capacitance would be looking like 60 ohms or so at 160MHz.
The software could compensate for the gain dropping with frequency but could only be for typical values so some devices may be incorrectly compensated (I'm just surmising).
It can be and probably is calibrated. The TG5011 for example has sine amplitude calibrations at 14 frequencies. 1 and 2 MHz then 5 to 40MHz in 5MHz steps then 43, 45, 47 and 50MHz.
The close spacing at the high end indicating there is more to be calibrated there.
The Rigol could have many more, the storage cost and time on an automated calibration set up is trivial.
If anyone is interested...
Here's a stepped sweep from the DG4162, 4 feet RG58, 30dB Minicircuits attenuator, Agilent N9320B 3 GHz Spectrum Analyzer.
The results look very good for the sine wave amplitude response for 1Vpp (+4dBm) output.
I guess that I have a good one (in the output amplitude regard).
No argument from me that Agilent and Tek are better but I don't own any to compare. I have noticed that the DG4162 seems to have sweet spot frequencies where the the spurious responses seen on the SA are very few. Probably more expensive instruments do better in this regard.
I think you get what you pay for and at least this unit is good enough for my needs.
Set the scope to peak detect acq mode and it'll run the ADC flat out all the time.
There is no change by using "peak detect" or "antialiasing"! Only the yellow color is more yellow ( not so light).
If anyone is interested...
Here's a stepped sweep from the DG4162, 4 feet RG58, 30dB Minicircuits attenuator, Agilent N9320B 3 GHz Spectrum Analyzer.
The results look very good for the sine wave amplitude response for 1Vpp (+4dBm) output.
I guess that I have a good one (in the output amplitude regard).
No argument from me that Agilent and Tek are better but I don't own any to compare. I have noticed that the DG4162 seems to have sweet spot frequencies where the the spurious responses seen on the SA are very few. Probably more expensive instruments do better in this regard.
I think you get what you pay for and at least this unit is good enough for my needs.
Those results are well within the spec of 0.8dB and as you say are good. Agilent, Tek etc are much more expensive and don't go to such high frequencies (unless you pay a lot more money) but they do allow the full 10Vpp over the whole (reduced range). They also have more memory and more flexible memory with variable sample clock (in some cases) and more sophisticated filtering available but if you don't need the extra facilities it is not worth paying for them.
My personal conundrum is deciding between the Rigol, an ex demo TTi TG5011 or an (very) old stock Tabor WW5061. They all have different pros and cons and I can't decide, I'm suffering paralysis by analysis.
Using the same RG58 cable and changing the generator to 50Ohm resistance (from High-Z) and including an in-line 50 Ohm terminator, gives me results well within the instrument's specifications.
Some clarity would be nice on this - I'm trying to "get an RF clue." I was under the impression that the High-Z function emulates a 50Ohm termination... Would anyone care to educate me on what my misunderstanding is?
Thanks to all those that have participated in the thread.
Using the same RG58 cable and changing the generator to 50Ohm resistance (from High-Z) and including an in-line 50 Ohm terminator, gives me results well within the instrument's specifications.
Some clarity would be nice on this - I'm trying to "get an RF clue." I was under the impression that the High-Z function emulates a 50Ohm termination... Would anyone care to educate me on what my misunderstanding is?
Thanks to all those that have participated in the thread.
As far as I understand it all the "High-Z mode" does in the DG4062 is adjust the amplitude value, it doesn't change the 50 ohm impedance of the generator. If you set High-Z mode and then ask for 2Vpp you'll get 2Vpp at the inside point of the 50ohm source impedance as the assumption is there is no voltage drop across the 50 ohms as it is connected to a High-Z that passes no current. If the mode is set to 50 ohms and you set 2Vpp then the voltage at the inside of the 50 ohm internal impedance is set to twice this (4Vpp) to allow for the fact that there is a 50 ohm load so the voltage at the output will be one half (half the voltage across the internal 50 ohms and half across the load 50 ohms).
When you set high Z mode the source impedance was still 50 ohms. You then connected it to the 1M/16pF input of your oscilloscope. The problem was the 16pF capacitance, at 1MHz the impedance has a magnitude of around 1k much higher than the internal 50 ohms so the measured voltage is close to the value you set of 2Vpp. At 160MHz the impedance has dropped to only about 60 ohms which is similar to the internal 50 ohms (they are different phases of course, one being real and the other capacitive) so the voltage will have almost halved.
With the 50 ohm pass through load the load starts at 50 ohms (the 1M + 16pF being high impedance) and at 160MHz it is down to about 1/sqrt(50^-2+62^-2) or about 39 ohms which is not that far off the original 50 ohms (I've somewhat simplified the fact that you have different phases, though I've allowed for it a bit in calculating the 39 ohms).
The most accurate results will be obtained with a scope with a proper 50ohm input impedance.
I've not touched on the effect of the cable. With the 50 ohm pass through the 50 ohm cable is terminated in something close to 50 ohms so is well matched so still looks like 50 ohms at the generator. Without the pass through the 1M/16pF impedance will be transformed by the cable but as the wavelength even at 160MHz is still quite long (1.875m in air, maybe one third of that in the cable depending on the dielectric constant) for most of the frequencies the unmatched cable will look like extra capacitance at the generator and will look inductive between the generator and the oscilloscope. It is complicated to work out the effect accurately, you could do it on a Smith chart. The point is that where the cable is not well matched it will have an effect, where it is well matched (50 ohm impedance each end) it should just introduce a slight time delay.
I hope all the above makes sense.
I hope all the above makes sense.
Very much so, thank you - I really appreciate the clarification.
The most accurate results will be obtained with a scope with a proper 50ohm input impedance.
Here is the same sweep (1...160 MHz) when DG4162 is connected directly with BNC cable to Tektronix TDS3032 scope 50 ohm input. DG4162 is in 50 ohm output mode.
The point is that where the cable is not well matched it will have an effect, where it is well matched (50 ohm impedance each end) it should just introduce a slight time delay.
This is the impedance at the end of of a 1.1m RG58 BNC cable connected to a DSO-X 3000 scope input. 200 ohm/div, 0 ohms at the bottom, 1 to 200MHz.
Then the same with the scope input at 50 ohms. 2 ohm/div, 50 ohms in the middle.
Rufus, that illustrates the point very well!
My background is in microwave engineering (monolithic circuits 10GHz and above), we tended to view any frequency below 1GHz as dc so I tend to forget that there is quite a difference between 1MHz and 200MHz!
Thank you for the visual representation - it lends further clarity. BTW, nice scope!
The point is that where the cable is not well matched it will have an effect, where it is well matched (50 ohm impedance each end) it should just introduce a slight time delay.
This is the impedance at the end of of a 1.1m RG58 BNC cable connected to a DSO-X 3000 scope input. 200 ohm/div, 0 ohms at the bottom, 1 to 200MHz.
Then the same with the scope input at 50 ohms. 2 ohm/div, 50 ohms in the middle.