Oscilloscope tester like Hameg HZ60

[morecat_lab]

Hi,

I built a very simple oscilloscope tester based on Arduion, which you might be interest.

https://github.com/kuwatay/simpleDDS

Two channel sin wave (software DDS) with phase adjustment.
  --> This function is useful for the test of XY.
Two channel square wave (hardware timer, 8MHz to 1Hz)
Two channel square wave (software-based, sync with sin wave)

It even has OLED and rotary encoder for user interface.

-Yoshi

[cosenmarco]

Hi morecat_lab, thanks for posting. It looks really nice. It seems the ATMega8, for example, has comparable characteristics with the PICs in terms of Timer+PWM operation for the generation of the scaled clock signal. Also the price is comparable.

I think we should stick to a specific device and I don't want to start a discussion PIC vs AVR because there's already enough.
So I'm going to choose PIC because I have a programmer, I already did a PIC project and we have already a working program thanks to Ian.M which does what we need.

So I'd like to fix some points that will let us choose the best PIC device for the job:
- Voltage source: 4XAA batteries. The PIC has to turn on / off all other devices (adjustable voltage reference and output stage) due to sleep mode.
- Clock 8Mhz crystal-controlled (so we don't necessarily need a LF PIC variant)
- 16 bit Timer with PWM
- 14 pins

Does it make sense? Did I forget something?

[David Hess]

Two channel sin wave (software DDS) with phase adjustment.
  --> This function is useful for the test of XY.

That is pretty cool.  Tektronix liked to specify horizontal bandwidth by both the -3dB point and the point where the phase mismatch became 2 degrees.  To measure the later, you could set the phase adjustment to -2 degrees and then increase the frequency until the diagonal line is straight.

Commonly the 2 degree point happened at 35 or 50 kHz.  Some oscilloscopes had an option available for "phase correction" which could extend this to 1 or 2 MHz depending on the -3dB bandwidth of the horizontal amplifier.

If you wanted higher XY bandwidth, then you needed sampling oscilloscope, digital storage oscilloscope, or an unusual oscilloscope like a 7104.

[Ian.M]

So I'd like to fix some points that will let us choose the best PIC device for the job:
- Voltage source: 4XAA batteries. The PIC has to turn on / off all other devices (adjustable voltage reference and output stage) due to sleep mode.
- Clock 8Mhz crystal-controlled (so we don't necessarily need a LF PIC variant)
- 16 bit Timer with PWM
- 14 pins

Does it make sense? Did I forget something?

You wont find that combo in the 'classic' midrange family, and the code is set up for one odd-numbered 16 bit timer for output compare, + one even numbered (not Timer 0) 8 bit timer  with period register for the PWM.   Those features come with the plain (E)CCP modules that do max. 10 bit PWM not the 16 bit dedicated PWM modules found in many newer devices.  I'd need to completely rewrite the code to support the newer 16 bit PWM modules, but I don't have any PIC16F1xxx parts in stock and don't trust the simulator's 16 bit PWM support.  If we do go 3.3V with a PIC16(L)F1xxx part, choosing one with a CCP or ECCP module rather than a 16 bit PWM module will minimise the code changes required. 

I think the easy option is to keep things much as they are, run the PIC at between 4.5V and 5.5V for faster output transitions and use the level translation capable gates I linked to for the output stage, running from an adjustable rail to set the amplitude.

In another thread:

An alternative approach is:

• one "section" consisting of a 74lvc1g14 + 143ohm series resistor to give an output of 0->1.5V to 0->5V depending on Vcc, with an output impedance of ~150ohms
• three of those sections in parallel, to give an output impedance of ~50ohms
• driven by whatever signal is available, including a simple RC relaxation oscillator at a suitable frequency

That certainly gives a clean signal with <1ns transition time into 50ohms. I can't directly measure any faster with my 350MHz scopes. However, indirect frequency-domain measurements indicate a transition times of ~650ps, and a quick test by someone else (i.e. not verified by me) indicated ~300ps rise/fall times.

SN74LVC1G14 Single Schmitt-Trigger Inverter

That could do pretty well for the output stage.  Run the 74LVC1G14 gates near 5V Vdd for speed, and rework the HZ-60's attenuator chain to look like a 50R load and have 50R output impedance.  That could give you 2.5V, 0.25V and 0.05V unloaded or 1.25V, 0.125V and 25mV into 50R.   Drop the 74LVC1G14 supply by about a volt by switching in a different resistor in the regulator feedback network, and you could also get 2.0V, 0.20V and 0.04V unloaded or 1.0V, 0.10V and 20mV into 50R from the same three BNC sockets.

[electrolust]

As far as I understand the specs I'm looking for are:
- Precision square wave both in frequency and in voltage for various frequencies
- Voltage precision 1% (or better) for 2.5V, 250mV, 25mV adjustable through a trimmer
- Frequency accuracy: as good as the crystal oscillator
- Rise time: within 3ns (possibly within 1 ns) for the 25mV 50Ohm-terminated output
- Budget within 80€ ~ 90$ for PCB + components

How about http://www.ebay.com/itm/CALIBRATION-AND-TESTING-DEVICE-FOR-TEKTRONIX-AND-ALL-QUALITY-OSCILLOSCOPES-/132283680426?

Half the precision (2%), double your budget ($200), and 10x the rise time (30nS).  Regardless of not hitting your target specs, it's probably a reasonable calibrator for your scope, and at just 2x your budget (but completely packaged and tested) it's a bargain.  IIRC he used to be asking more on the order of $400.

[2N3055]

An alternative approach is:
SN74LVC1G14 Single Schmitt-Trigger Inverter

Funny, I ordered few today  , to experiment..

In meantime I have to figure out original AC problem David mentioned.. Probably I will use LVC version anyway, but it made me curious...

Regards,

Sinisa

[David Hess]

In meantime I have to figure out original AC problem David mentioned.. Probably I will use LVC version anyway, but it made me curious...

Which AC problem is that?  Controlling ground and supply pin bounce to produce a clean output?

If you are replicating the original Hameg design with resistive dividers to produce different output voltages, then that is going to make very fast clean edges somewhere between difficult to impossible.  I would settle for clean at the expense of speed so it can be used to check probe and divider compensation and then add a special fixed level output with the fastest possible edge.

One thing to consider here is actually verifying that a fast edge is clean.  For that either a high bandwidth oscilloscope that you trust or a sampling oscilloscope is required.  The designs I suggested have the best chance of achieving a fast clean edge without undo hardship.

[2N3055]

Which AC problem is that?  Controlling ground and supply pin bounce to produce a clean output?

Dual slope on the edge that you noticed.. Out of curiosity, i would like to understand why is there.

As I said, to try one chip inexpensive solution, I already ordered few 74LVC14 single gates... They are fast, SOT23, minimal parasitics...

I agree with you, clean, well defined edge, is more important than speed...

I like  your proposal for bipolar cascode and fast small signal schottky diode cutoff into single 50 terminator right on the BNC.
I'm studying schematics you showed....

[David Hess]

Dual slope on the edge that you noticed.. Out of curiosity, i would like to understand why is there.

I like the MOSFET drain-source saturation explanation.

As I said, to try one chip inexpensive solution, I already ordered few 74LVC14 single gates... They are fast, SOT23, minimal parasitics...

LVC/ALVC was one of the candidates I considered.  I do not remember what 5 volt logic I considered.  I looked specifically for tristate outputs which can be configured to operate as N or P channel open drain outputs like the 74LVC125.

I agree with you, clean, well defined edge, is more important than speed...

For checking compensation and vertical calibration, a surprisingly slow edge speed and low fidelity can be tolerated so that part of the design is trivial.  Some precision pulse generators with linear (class AB) outputs are more than fast enough.

High fidelity fast edges are a whole different matter requiring careful layout and special parts.  The fast edge outputs on the PG506 have the termination resistors built into the BNC connectors.

I like  your proposal for bipolar cascode and fast small signal schottky diode cutoff into single 50 terminator right on the BNC.
I'm studying schematics you showed....

The problem is that it will require more than a 3 volts.  It might just be possible on 5 volts but higher would be better.

Below are a photographs of the construction of the PG506 fast outputs to show what can work and the full schematic.  Tektronix managed it with air wired through hole parts so I think a surface mount implementation can do better.  Those little resistors are 1/8 watt carbon composition which have low parasitics except of course for their lead length.  Some of the complexity is to make up for the non-ideal construction.

Things to take special note of are how the BNC connector attaches to the printed circuit board almost forming a coplanar waveguide and the high performance 50 ohm termination which is built into the end of the BNC connector in the form of a disk.  A simple design will use a 50 ohm transmission line to the BNC and a 50 ohm termination probably made from a pair of 100 ohm resistors.

[cosenmarco]

@electrolust: I thought about getting myself one. Seems a nice device. Rise time is <= 30ns and fall time <= 50ns. Functionality would be harder to replicate than the Hameg for my pet project.

So, just to clarify how I intend to use the PIC, I created a block diagram which will show the different components and the voltages involved.

If I understand correctly the Schmitt Triggers like the buffers / inverters we were considering (eg. SN74LVC1G17) and  can accept input up to 6V independently of the supply voltage. This doesn't hold true for the 74AC14. Would a resistor divider be a possible solution to bring the input in the range 0->Vcc (3Vpp from max 6.4Vpp)?

[Ian.M]

A resistor divider should work, to get down from 5V logic to ~3.3V but you may need to fiddle with a 'gimmick' cap across the upper arm to compensate for the 74AC14 input capacitance to speed it up.  The problem is trimming the gimmick cap - a x10 probe gives you about 15pF||10Meg, most of the 15pF being stray capacitance to the probe body shield, which will swamp the input capacitance you are trying to trim for.  To do it right you really need a 2pF input capacitance active probe.

[David Hess]

The 74AC14 has Schmitt trigger inputs so compensation should not be required at the input for a fast output edge.  But it does not hurt and the capacitor value can be calculated from the divider ratio and datasheet specification for input capacitance plus an estimate for circuit capacitance.  Leave a spot for the capacitor in the layout if you think it may be necessary.

If you want to find out, test it with the resistive divider and just watch the output edge to see if it matters.  There is no need to probe the 74AC14 input.

[David Hess]

Here is an MTS529 portable oscilloscope tester that I have.  It uses an ECL output stage to generate a 300mV edge into 50 ohms with a roughly 350 picosecond rise and fall .  It lacks the pulse fidelity of the PG506 fast rise and fall outputs (1) but is good enough to test bandwidth and verify proper operation of an oscilloscope up to 500 MHz.

The construction shows the problem.  An attempt was made to maintain a 50 ohm environment but the layout is marginal and the loop area of that connection to the BNC connector spoils the potential performance.

As shown on my 2232, the response looks very good ... too good and better than the performance of my slightly slower PG506.  The reason for this was that the transient response of this particular 2232 was calibrated against this MTS529 because at the time it was the best source that I had and I never recalibrated it against the PG506 when I acquired it later.

(1) You will have to take my word for this for now as I do not have any photographs of its pulse response but it displays a typical "peaked" response with some ringing.  It is not terrible but it is not as clean as the PG506.

[cosenmarco]

Hi, I worked on a preliminary version of the schematic.
To-the-point feedback would be much appreciated.   

[Ian.M]

I can see a few issues:

* you've got a 100K + 330K potential divider across the +6V rail and Gnd so that's a permanent 14uA drain and the resulting Thévenin equivalent resistance of 77K is way out of spec for input impedance for correct PIC ADC operation, so at the minimum you need a 0.1uF cap at the SENSE ADC pin to lower the source impedance   To reduce the power down (Sleep) quiescent current, you may prefer to scale the divider resistors up into the Megs or 10s of Megs range and add a micropower OPAMP to buffer it for the ADC.

* the 4.5V tap on the battery will be a PITA.  Ditch it, use a low current 5V micropower LDO to power the PIC off the 6V rail (with the full expectation the 5V will droop before the battery is dead), ditch the  Battery/DC switch and use the switch contact on the DC jack to isolate the battery instead.

* You need to ground all unused inputs of the 74AC14 so they don't toggle due to capacitive coupling inside the chip and glitch the output edge.

* If you ever need ON and Error LEDs lit simultaneously they'll need seperate anode resistors rather than a shared cathode resistor.

* As the PIC and the 74AC14 are running at different Vdd voltages, you'll need a series resistor in the Signal line to limit current into its input protection diodes to no more than 10% of the permitted maximum with a small capacitor across it (say 50pF) so you still get fast edges.

[cosenmarco]

you've got a 100K + 330K potential divider across the +6V rail and Gnd so that's a permanent 14uA drain

Yeah, I wanted to avoid quiescent current aside the PIC and the regulator and this is a mistake.

and the resulting Thévenin equivalent resistance of 77K is way out of spec for input impedance for correct PIC ADC operation

I wanted to use the comparator against the internal reference voltage of 1.024V to sense the battery voltage using the resistor divider which should give me a bit less than 1.024V when rail is at ~4V

To reduce the power down (Sleep) quiescent current, you may prefer to scale the divider resistors up into the Megs or 10s of Megs range and add a micropower OPAMP to buffer it for the ADC.

If I do a divider with 3.3M / 1.2M it will consume 1.44µA. I think if I use the hysteresis of the comparator there is no need for another external opamp, right?

the 4.5V tap on the battery will be a PITA

My thinking was (1) to avoid regulator drop when PIC was off and (2) be able to use as much battery life as possible. What would cause my PITA with this setup? Also the switch may be handy if I know I will store the device for very long time completely cutting the battery line.

You need to ground all unused inputs of the 74AC14 so they don't toggle due to capacitive coupling inside the chip and glitch the output edge.

will do! thanks.

If you ever need ON and Error LEDs lit simultaneously they'll need seperate anode resistors rather than a shared cathode resistor.

I don't need simultaneously green and red.

As the PIC and the 74AC14 are running at different Vdd voltage

Yeah it slipped off, thanks for bringing it up: I'll fix it.

Thanks Ian.M for the feedback: it's great to have another pair of eyes. At work (I'm software engineer) we do mandatorily code reviews and this practice is alone improving a lot the quality of our code and consequently the quality of the product.

[cosenmarco]

New version of the schematic with most of the problems addressed.

[StillTrying]

Here's my about £0.02's worth.

The regulator's OP and FB connections to the voltage setting resistors are wrong.

If I'm reading the data sheet right, the current into the SD pin isn't good at about 30uA while SD.

1.4uA though the sense resistors might not be enough with changes in the PIC's comparator's input current, - can't find it on the PIC's data sheet.

You could just use the regulator's Error output as a dead battery warning.

Simulating the output resistor network, I get the VCC to the 74AC14 to be 3.47V, I'd be tempted to use a proper on-off SW and power the PIC from the 3.47V as well because of the number of problems it solves. Or use one ON-OFF-ON SW.

If the 10K between the PIC and 74AC14, stays I think 10pF across it would be enough to keep the switching edges fast without putting spikes on the 74's VCC.

[cosenmarco]

StillTrying: thanks a lot.

The regulator's OP and FB connections to the voltage setting resistors are wrong.

Thanks for spotting. Should be fixed now. 

If I'm reading the data sheet right, the current into the SD pin isn't good at about 30uA while SD.

30µA minimum at 2.4V. We're feeding in 5V so it might be more... too much. Do you have any suggestion? I'm thinking about adding a MOSFET based switch like suggested in https://electronics.stackexchange.com/a/208942 to switch the output circuitry (regulator and 74ac14). This has also the positive effect that I can increase the current going through the SENSE divider addressing your concerns around the comparator's input. The datasheet says "A maximum source impedance of 10 kOhm is recommended for the analog sources" so I will have a divider with 33K and 12K giving a Thevenin equivalent Rs of 8.8K which should be within spec. This sucks 144µA but then only when the system is ON.

You could just use the regulator's Error output as a dead battery warning.

Nope... quoting datasheet: "The Compactor produces a logic low output whenever the TS2951/A output falls of regulation by more than around 5%" which is for me too much. I still want to detect that fact in the PIC to maybe make the red led flash.

I'd be tempted to use a proper on-off SW and power the PIC from the 3.47V as well because of the number of problems it solves

I would loose an important feature: auto shut-down which I think is a very nice feature to have. And the switch is there anyway in case one wants to completely isolate the battery.

If the 10K between the PIC and 74AC14, stays I think 10pF across it would be enough to keep the switching edges fast without putting spikes on the 74's VCC.

Yeah it might be a bit ugly to let current flowing through PIC and 74's Vcc. I think a proper resistor divider on the PIC's output should solve this.

[StillTrying]

The power switching looks OK to me.
  Are you sure you can't now get the PIC onto the 3.47V VCC line. 

Some decoupling is need on the PICs power, 100n very near the PIC plus 10 to 100uF.
A couple of uF is probably needed between the power switch FET and the regulator.

The 20k and 33k divider doesn't work very well when the PIC's voltage gets down to about 3.5V, the divider's output berely reaches the 74's high switching point. If you want it to still work with the PIC's voltage down to about 3V (which will be below the 74's VCC) you'll probably have to replace the 2 resistor divider with something like this.

Edit: I was thinking of something like this for the On/Off, don't spend any time on it if you're happy with your powering methods.

[Ian.M]

Personally, I'd stick to the 10K series resistor with parallel pF speedup cap for the level shifting.   If carefully controlled protection diode current is really an issue, add a fast small signal Schottky clamp to the adjustable Vdd rail.  However, as when the input is high, the 74AC14 output is low, you'll also need a load on the adjustable Vdd rail to prevent it rising.  The regulator's divider chain may be sufficient, but check that you meet the regulator's specs for minimum load current with the max possible clamping current flowing into its output.

[StillTrying]

Personally, I'd stick to the 10K series resistor with parallel pF speedup cap for the level shifting.

Yes, just a straight 10k plus a 10 to 20pF seem like the simplest solution that works down to 3V at the PIC, and when the PIC is at 5V the 10 to 20pF doesn't cause spikes above the 74's 3.47V VCC.
I'm assuming the PIC output is about 25R and 15ns and the 74 input about 4pF.

[cosenmarco]

Hey StillTrying,

Edit: I was thinking of something like this for the On/Off, don't spend any time on it if you're happy with your powering methods.

Yes, I was thinking myself about something like that and I like it.
I'll come up with something this evening or during the day if I get the time.
Thanks a million.

[cosenmarco]

Here we go!. I hope it has some sense.
The SHUTDOWN PIC output should be set to high during normal operation. When it goes low it makes the FET switch turn everything off resulting in the PIC committing suicide.
The button, when everything is ON, if pressed would put some higher-than-vdd voltage on the PIC SW input but current is limited to, I think, ~500µA so it should be fine.

EDIT: I added some de-bouncing caps to the button.

[cosenmarco]

Hi there,
I breadboaded the power supply stage. Here a couple of pictures about how the TS2951 is doing.
The regulated voltage stability is quite OK. I put a load here of 330Ohm.
There was some noise on the VDD (if I remember correctly around the 2mVpp) but I think it was coming from the power supply and various RF sources.
I'll see with the actual batteries whether the output cleans up.
Next step PIC programming and testing. I'll keep posted as soon as I make some progress.