Screnscope teardown.

[SeanB]

This arrived today. I was looking for a new scope and this was both in my price range and locally available.

Initial testing says basic functions work ( powered up and nagged about not having a mouse) and then it was do like Dave time.........

Nice board, easy to get to and only minor production line modifications visible, with most pots sealed.

Only con was the supplied power supply, no AUS converter here, and no other way other than to bodge it, so used a spare one I had to hand instead.

Will definitely be exploring more over the weekend, after reading the on line manual more thoroughly.

[SeanB]

Finally set up on the table/workbench, and did a test with my oldish GW function generator. 2MHz square wave not looking so good at full amplitude here.

[cliffyk]

It looks like an 80ns or so risetime, do you know the function generators's specs?  If 2MHz is near it's upper limit then its squarewave output may look like that.

Here's a 2 MHz squarewave from an old Leader 1300s, its top frequency.  The function generator has a 600? output and which was properly terminated at the 'scope, a 300MHz Lecroy 9450A--so we can be pretty sure this is what the generator's output looks like:

[amspire]

Set Ch3 to Cal, and with the probe at x10, check the Cal waveform on Ch3 to see if the probes are properly compensated. I think they do calibrate them before delivery, so they should be fine. Otherwise check the manual for instructions.

The risetime of the Leader 1300 is "less then 100nS", so I think your waveform is fine.

Richard.

[cliffyk]

<snip>

The risetime of the Leader 1300 is "less then 100nS", so I think your waveform is fine.

Richard.

I am sorry for the confusion, I did not mean to imply I thought there anything wrong with my 1300s, in fact it is in top shape calibrated just a couple months ago. 

The captured display was provided as an example of what a square wave from a less than precision instrument, pushed to its highest output frequency could be expected to look like.  The waveform I captured was not probed, but a direct impedance matched connection from the generator's output to the 'scope's input...

[alm]

The function generator has a 600? output and which was properly terminated at the 'scope, a 300MHz Lecroy 9450A

The waveform I captured was not probed, but a direct impedance matched connection from the generator's output to the 'scope's input...

Where did you get a 600 ohm cable?

[cliffyk]

The function generator has a 600? output and which was properly terminated at the 'scope, a 300MHz Lecroy 9450A

The waveform I captured was not probed, but a direct impedance matched connection from the generator's output to the 'scope's input...

Where did you get a 600 ohm cable?

You don't need 600? ohm cable when the cable is only 0.5m long, that's 1/240th wavelength at 2MHz and still only 1/12th wavelength at 42Mhz, the 19th harmonic.  I used a Pomona 600? pass through terminator at the generator output with a 0.5m 50? interconnect to the 'scopes' 1M? input.

For reference here's the same generator output captured with a properly compensated 300MHz Lecroy X10 probe:



And here are the two traces overlaid, Memory C is the X10 probed capture...

[amspire]

I am sorry for the confusion, I did not mean to imply I thought there anything wrong with my 1300s, in fact it is in top shape calibrated just a couple months ago. 

I did get your post confused for SeanB's post. For some reason, I thought it was SeanB with the Leader generator instead of the GW generator.

Anyway, I agree with you. A 600ohm cable is not needed to correctly terminate the Leader generator to the scope.

Richard.

[ejeffrey]

You don't need 600? ohm cable when the cable is only 0.5m long, that's 1/240th wavelength at 2MHz and still only 1/12th wavelength at 42Mhz, the 19th harmonic.  I used a Pomona 600? pass through terminator at the generator output with a 0.5m 50? interconnect to the 'scopes' 1M? input.

A .5 meter length of 50 ohm coax cable is 50 picofarads which is 1600 ohms reactance at 2 MHz.  At the 3rd harmonic, the reactance is less than 600 ohm.  This will definitely cause attenuation and phase shifts of the harmonics of the square wave.  You need a low capacitance cable or a proper probe.

[SeanB]

Yes, the GW is at it's max frequency and it's max amplitude. The probe is a x1 probe, so the 600R output would not be any issue. Drop down to 1MHz and the square wave is square, with very little overshoot.

[vk6zgo]

You don't need 600? ohm cable when the cable is only 0.5m long, that's 1/240th wavelength at 2MHz and still only 1/12th wavelength at 42Mhz, the 19th harmonic.  I used a Pomona 600? pass through terminator at the generator output with a 0.5m 50? interconnect to the 'scopes' 1M? input.

A .5 meter length of 50 ohm coax cable is 50 picofarads which is 1600 ohms reactance at 2 MHz.  At the 3rd harmonic, the reactance is less than 600 ohm.  This will definitely cause attenuation and phase shifts of the harmonics of the square wave.  You need a low capacitance cable or a proper probe.

Or a 600:50 Ohm pad & a 50ohm termination at the 'scope end.

[SeanB]

Was looking today at travel converters that have an AUS plug outlet, but they cost as much as the SMPS I have plugged into it, so one wall wart now not going to be used......

[cliffyk]

You don't need 600? ohm cable when the cable is only 0.5m long, that's 1/240th wavelength at 2MHz and still only 1/12th wavelength at 42Mhz, the 19th harmonic.  I used a Pomona 600? pass through terminator at the generator output with a 0.5m 50? interconnect to the 'scopes' 1M? input.

A .5 meter length of 50 ohm coax cable is 50 picofarads which is 1600 ohms reactance at 2 MHz.  At the 3rd harmonic, the reactance is less than 600 ohm.  This will definitely cause attenuation and phase shifts of the harmonics of the square wave.  You need a low capacitance cable or a proper probe.

You are disregarding the low power level and short electrical length of the line.  Assuming a velocity factor of 0.70, at 42MHz the 0.5m physical line is only 1/10th of the wavelength (?).

As sort of an example of this, here are spectra (a 0 to 50MHz sweep) of the fastest rise time square wave signal I have currently available (2ns), at 1MHz from an upgraded/modified Heath 4501 oscilloscope calibrator, as recorded via a Rigol DSA815 spectrum analyzer:

direct coupled, 0.25m 50? cable:


less direct coupled and grossly mismatched (see below):


Not a whole lot of difference is there?

Here is the "less direct" coupling--10m of 50? cable, comprised of one 3m, three 2m, and one 1m length.  Joined with cheap BNC M-M couplers and three Mini-Circuits 75? directional couplers, with a 10k? IHF termination at the spectrum analyzer's 50? input:

spider web:


At low signal levels and relative lower frequencies coupling and impedance matching are far less important than in "real" transmission lines.
 

In the last, here's the spectra of the Leader 1300s squarewave at 2MHz ("reasonably" terminated, is that OK?):



Note the prevalence of even harmonic content, this is what rounds the edges--I suspect this is what a spectra of the OP's GW function generator would look like.

[alm]

I agree that impedance mismatches are most likely irrelevant for a relatively slow square wave like this. This is why I found the comment about matching 600 ohm termination odd. If the signal is electrically much longer than the cable, why worry about termination? If capacitive loading is an issue, then terminating a 50 ohm cable into 600 ohm is not going to look very resistive at any wavelength other than DC.

[ejeffrey]

You are disregarding the low power level and short electrical length of the line.  Assuming a velocity factor of 0.70, at 42MHz the 0.5m physical line is only 1/10th of the wavelength (?).

I am not disregarding it, it is simply irrelevant to my point.  This has nothing to do with transmission line behavior and is simply the effect of the cable capacitance.  A 600 ohm output driving 50 picofarad of cable will create a low-pass filter with a 3 dB point around 5 MHz.  If you don't see its filtering effect then you are incorrect about it being a 600 ohm source.

[cliffyk]

You are disregarding the low power level and short electrical length of the line.  Assuming a velocity factor of 0.70, at 42MHz the 0.5m physical line is only 1/10th of the wavelength (?).

I am not disregarding it, it is simply irrelevant to my point.  This has nothing to do with transmission line behavior and is simply the effect of the cable capacitance.  A 600 ohm output driving 50 picofarad of cable will create a low-pass filter with a 3 dB point around 5 MHz.  If you don't see its filtering effect then you are incorrect about it being a 600 ohm source.

Now I feel like a complete idiot--a 67 year old brain fart of the sort I having been having more of lately as diabetes catches up with my decadent lifestyle. 

I actually have two 1300s instruments, one with a completely savaged output board converted to 600? output, and other mods, for audio console work--very easy to do because of the separate design and implementation of the output panel.  It sits at the audio end of my bench right beneath a Kikusui ORC-11 low distortion generator (600? out) and on top of an old Krohn-Hite dual channel hi-low pass filter, also 600?.  All of which are instruments I have used most frequently in the last several years, in maintaining older sound studios here about.

However at 'tother end of the bench, the "RF" end, is a "stock" 1300s having a 50? output; that is the one I have been using for this series, with and without the 600? pass-through, just to make sure things were really screwed up--I just had "600?" hardwired into my brain when thinking "Leader 1300".

I.e. my eyes and brain were out of synchronization and neither side wanted to yield--thank you to those that pushed me through this roadblock, I really did used to know this stuff.

I apologise to the group--I really should just sell all this crap and sit on the front porch with a shawl draped about me like a proper retired person.

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FWIW here are some FFT analyses of a 500kHz squarewave from the 600? output 1300s (limited to 500kHz output because of the hacks) using both a terminated line and low capacitance (x10) probe; and also a time domain comparison of the two connections.  In both the Memory C trace is the "600? terminated" connection, the Memory D trace the low capacitance X10 probe.

I did these on my old Lecroy 9450A that I could operate in my sleep...

frequency domain:


Above I have used the cursors to mark the 15th harmonic at 7.5kHz, you can see the 6dB+ amplitude improvement in the "probed" trace (-17.5 in the "terminated" spectra vs. -11.5 dB in the "probed" spectra) as has been pointed out should be the case.

time domain:


Here you can see Memory C's (the "terminated" connection) slower rise time, more rounded top edge, and slower fall and roll-off--as compared to the probed connection shown in Memory D.

So, all it as it should be...

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I still think the OP is seeing the best his old GW function generator pushed to its max can put out...