How to make better external 50ohm temination if scope have not internal 50ohm impedance matched inputs?
... if scope have not internal 50ohm impedance matched inputs?
@rf-loop: I checked the Tek 2465 schematics, 50 ohm coupling only consists in switching in a 50 ohm resistor in parallel to the 1 M resistor. The only additional trick is to put a thermal sensor on the 50 ohm resistor to check for channel overload.
Anyway, 50 ohm // 1 Mohm = 49.9975 ohm
@tinhead:
From which model is this picture from? From all the front-end pictures I have seen so far,
I never observed the ability to get a 50 ohm input termination!
Even worse: in most of these scopes, there is no real GND coupling and the Rigol does not even have AC coupling for trigger...
@tinhead: GND coupling should actually isolate completely the DUT from the front-end! At least, this is what I observed in ALL other scopes before. But of course, this costs one additional relay
I wouldn't leave this unpowered DSO connected to my DUT, if I were you
I completely agree with you rf-loop, of course, a schematic doesn't tell you the whole story
EEs at Tek, HP and LeCroy really knew their job: even 20 years later, these scopes are still nice pieces of jewelry!
To give you an idea of what these scopes are made of, please look at this very interesting April 1986 issue of the HP Journal, almost dedicated to "Digitizing Oscilloscopes", and to the HP5411OD in particular.
But we should compare what is comparable: these were x$10,000 scopes, vs. $300 ones! These old scopes were built around hybrid circuits with coupled microstrip lines and delay lines, our cheap DSOs are made up of 6-layer FR4 with stock components...
So for sure, you can't expect the same performance from both. Even if progress has made things cheaper, RF rules have not changed
To give you an idea of what these scopes are made of, please look at this very interesting April 1986 issue of the HP Journal, almost dedicated to "Digitizing Oscilloscopes", and to the HP5411OD in particular.
When I use only CH1 and cross 200ns time base boundary it switches probably to interlaced mode (my guess, I hear relay click) and I get a little bit different readings. See attached images. The change is subtle but noticable. Is this a normal behavior?
Hello everyone,
Just joined up.
I came across this thread looking for info on the Hantek DSO5202B which I subsequently purchased from an ebay seller.
The CRO works really well but I have struck a few issues with this CRO which I wonder if other people have found. I have sent this info off to Hantek but have not heard back from them. The firmware installed is later than what is on the website.
For instance, AC coupling of the input channels inserts a high pass filter that peaks at 100 Hz and rolls off below that. So when you display a 100 Hz sinewave and switch from DC to AC coupling the signal amplitude actually increases !! Ideally the AC coupling should be flat down to 10Hz and roll off below that without any peaking otherwise this is causing an error in the readings.
Also frequency measurement on the bottom of the screen does not correlate with the menu because the bottom screen measurement decimal places are always zero. ie frequency shown in measurement menu is 105.3Hz and frequency shown at the bottom of the screen is 105.00 Hz.
regards
Hello everyone,
Just joined up.
I came across this thread looking for info on the Hantek DSO5202B which I subsequently purchased from an ebay seller.
The CRO works really well but I have struck a few issues with this CRO which I wonder if other people have found. I have sent this info off to Hantek but have not heard back from them. The firmware installed is later than what is on the website.
For instance, AC coupling of the input channels inserts a high pass filter that peaks at 100 Hz and rolls off below that. So when you display a 100 Hz sinewave and switch from DC to AC coupling the signal amplitude actually increases !! Ideally the AC coupling should be flat down to 10Hz and roll off below that without any peaking otherwise this is causing an error in the readings.
Also frequency measurement on the bottom of the screen does not correlate with the menu because the bottom screen measurement decimal places are always zero. ie frequency shown in measurement menu is 105.3Hz and frequency shown at the bottom of the screen is 105.00 Hz.
regards
I test with HW7 DSO5102B (in this case very same as 5202B)
AC mode (Why?)
If compare to 1kHz sine signal level (set in calculations as 0dB reference)
Measured directly without probe.
Including old Wavetek flatness errors (yes but this my Wavetek is carefully adjusted so that it is surprising flat level - error is significantly below +-0.1dB in this area)
CH1, AC, 500mV/div
signal level 6div p-p @ 0dB
-6dB 10Hz
-3dB 17Hz
0dB 41Hz
+1dB 80Hz
0dB 350Hz - 1kHz
----------
About frequency counter.
Measure menu freq are _calculated_ from ADC captured data.
With low freq it may sometimes be better and more accurate than HW counter. Specially if use averaging.
HW counter work totally different. It counts events in trig line. It is event counter. It counts how many events in time window.
Becouse it is not as modern real timeintervall counters it resolution is highly dsependent about time window setting for counting. How long is gate open window what time it use for counting these pulses. (yes it is just pulse counter. Totally different as modern frequency calculating time interval counters as something like cheap HP53131 or similars. They can solve 9-10 or more decimals in one second.)
And why use AC coupling ? Simply because you may have a small AC signal superimposed on a large DC offset so you want to be able to get rid of the DC component otherwise it is not possible to view the small AC signal with maximum sensitivity.
regards
And why use AC coupling ? Simply because you may have a small AC signal superimposed on a large DC offset so you want to be able to get rid of the DC component otherwise it is not possible to view the small AC signal with maximum sensitivity.
regards
Yes, this is mostly nearly only reason.
This is not FW related thing. It is HW.
Analog front end is simple and cheap. It is just like compromise.
If we look example old expensive HP or Tektronix front ends they have lot of adjustments for corrections and corrections for corrections.. more accurate want, there is more small errors what need compensate with some adjustable or not adjustable circuit.
Specially I do not like this way they do AC/DC in front end. From designer I can ask why? Some reason? Any reason?
Or reason is becouse copymachine did not change it. It is problem with copy, you copy also mistakes or bad's.
Also there are all component tolerances. Tolerances may be sometimes in worst case compination and randomly sometimes best case. (example this peaking freq difference - but in this case you have real 5202B what have some different componet values in front end so it may also affect this difference.)
I do not like this front end. I can not understand this solution for AC/DC. (or maybe if really think every cent)
I have recommend to some my customer (who want more low cut off frequency) to use external DC block.
I make one short experience.
Scope input DC
Probe 1x
Measured
From 50ohm output terminated with 50ohm feed trough terminator.
For DC block: 100nF capacitor from terminator center to probe tip.
Response "flat" to around 20Hz
very slowly it satrt drop. 10Hz 3V-pp have drop to 2,9V
1,5Hz it have drop to 2,14V
Lack of time I did not test with 1uF
Probe 10x
as before, 100nF
in practice, nearly flat down to 1Hz.
This can give also idea how to measure extremely low frequencies over high DC. (1uF, 10uF and even more but be careful if voltage is high... becouse..if there is transient... well: after smoke come out from component, equipment do not work...smoke need put back agen)
In lab it is many times needed. DC block. (for common use it may be total block or sometimes only ground connecting "center" block.) For probe tip DC block, well, everyone can imagine how to do if need.
In some rare case I have use 100uF polypropylene capacitor with some Tektronix. (some reason need measure extremely low freq's over high DC)
I think this front-end design is not bad, especially for its price and using only stock components.
Regarding the LF loopback, the "corner" point at which it occurs is given by the capacitor in the loop and I don't remember which resistors around. But it occurs at some 100s Hz, and if position is well compensated, should be smooth to HF without too much hiccups. This shouldn't affect the frequency response above that.
In order to understand what's going on, we should put a probe on the AD8370 input (TP2_1), first, then a differential probe between the 2 test points at the LMH6552 output, and see what we get. I suspect that the varicap introduces non-linearities, but I am not sure.
Then what is displayed on the LCD, that's another story... The ADC clock seems to have a lot of jitter, so this is not good for ADC interlacing and may thus produce a lot of distortion, and running without any anti-aliasing filter too is a bad idea, as you may get additional distortion by spectrum folding at the sampling frequency...
I think this front-end design is not bad, especially for its price and using only stock components.
Regarding the LF loopback, the "corner" point at which it occurs is given by the capacitor in the loop and I don't remember which resistors around. But it occurs at some 100s Hz, and if position is well compensated, should be smooth to HF without too much hiccups. This shouldn't affect the frequency response above that.
In order to understand what's going on, we should put a probe on the AD8370 input (TP2_1), first, then a differential probe between the 2 test points at the LMH6552 output, and see what we get. I suspect that the varicap introduces non-linearities, but I am not sure.
Then what is displayed on the LCD, that's another story... The ADC clock seems to have a lot of jitter, so this is not good for ADC interlacing and may thus produce a lot of distortion, and running without any anti-aliasing filter too is a bad idea, as you may get additional distortion by spectrum folding at the sampling frequency...
do you have the schematic of the front end so I can see what you are talking about ?
regards
davidYou can find it as an attachment to this tinhead's post.
I can see the AC/DC relay and DC blocking cap but where is the peaking coming from at 100 Hz ??Please read again my reply above, when a speak about the corner frequency at some 100s of Hz. It looks like the transition from LF to HF is not well calibrated.Possibly need to simulate it in spice to see what is going on.Possible, but not easy... You must make provisions for stray capacitance due to component pins and also to the FR4 PCB (with permeability tolerance of around 50%), and circuit loop inductance and also mutual inductance between circuit loops
When you get a model that is close enough to reality, let me know, I will be very interested, and probably others too!