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Old 13th September 2010, 12:31 AM   #11
TheGimp is online now TheGimp  United States
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Looking good.
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Old 13th September 2010, 10:18 AM   #12
Merlinb is offline Merlinb  United Kingdom
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That seems like an awful lot of parts for the job? Since the frequency of the anode pulses and the signal to the Schmitt trigger is always the same (mains frequency), why do you need a PLL, or indeed, anything other than the opamp staircase generator?

My tracer currently uses only a comparator to generate a square wave from the AC, followed by a 4520 counter feeding a simple DAC using resistors and an opamp. Less than 20 components, not including the power supply.
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Old 13th September 2010, 04:10 PM   #13
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Quote:
Originally Posted by Merlinb View Post
That seems like an awful lot of parts for the job? Since the frequency of the anode pulses and the signal to the Schmitt trigger is always the same (mains frequency), why do you need a PLL, or indeed, anything other than the opamp staircase generator?

My tracer currently uses only a comparator to generate a square wave from the AC, followed by a 4520 counter feeding a simple DAC using resistors and an opamp. Less than 20 components, not including the power supply.
Hi Merlinb,

I used what I had in by parts bin. Since I've never done this before I studied the circuit of a Tek 570 that creates the stairs at 240hz rate. Since I am using a Miller integrator to charge up the staris, the PLL gives me a very even charge from step to step.If I want fixed steps per family it is easier. But I wanted to be able to adjust the number of steps per family so I can have enough traces equally spaced at whole step values for different tubes. With the comparator at the end i can adjust the number of stairs up to more than 20.

The less parts the better. Can you post your design? I am not sure how it works. Aren't the steps at 60 hz frequency? If this is the case the oscilloscope may have starting refresh problems past a family of 4 steps. Is it possible to change the number of steps per family?
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Old 13th September 2010, 05:35 PM   #14
Merlinb is offline Merlinb  United Kingdom
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Originally Posted by avincenty View Post
The less parts the better. Can you post your design? I am not sure how it works. Aren't the steps at 60 hz frequency? If this is the case the oscilloscope may have starting refresh problems past a family of 4 steps. Is it possible to change the number of steps per family?
I'll try and post it later (the schem is drawn in Proteus and annoyingly takes up the whole screen since the symbols are so large)

My circuit produces eight steps at 3V per step. I added a potential divider so the volts-per-step can be reduced to 2V, 1V or 0.5V, but it always draws eight curves; you just select whichever option fits on the screen most conveniently.

I use full-wave rectified AC, so the steps are at 100Hz (UK), and I found that at least eight curves can be displayed without serious refresh problems, at this frequency.
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Old 13th September 2010, 08:25 PM   #15
Merlinb is offline Merlinb  United Kingdom
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Here's the schem.

(On my actual PCB the values for the volt-per-step divider were changed when I realised I had to use 3V per step as a maximum, but the rest is as drawn.)
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Old 14th September 2010, 06:14 AM   #16
gootee is offline gootee  United States
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I haven't fired one of mine up for quite a while. But I used 16 steps (4-bit counter) and don't remember there being any significant problem with the usability of the display at 60 Hz. It didn't all appear to be continuously on, at that frequency, of course. But it was usable. (I did definitely prefer the solid higher-frequencies' displays, though!)

I guess maybe you could just have the counter reset at whatever number you want, too, to vary the number of steps.

Best bet is probably to rig up more steps and see what it looks like.

Are you sure you wouldn't enjoy designing a high-power ramp-generator circuit, with a higher frequency? There are some very easy low-power ones with two opamps and then all you need is an audio-range power amplifier.

The classic two-opamp ramp generators usually also make a square wave as a byproduct, too, making it easy to then generate a short pulse to clock the digital counter. I, too, then used a simple resistor-ladder-and-opamp DAC, but followed it with a high-precision switched attenuator that had quite a few steps, and then a small power amplifier that could be switched to produce either current or voltage steps, both under voltage control by the output of the switched attenuator that the stairsteps had gone through.

And then you could add a rotary switch with capacitors to set the ramp generator to multiple frequencies.

(See how it starts? <grin!>)

Last edited by gootee; 14th September 2010 at 06:16 AM.
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Old 16th January 2012, 04:56 AM   #17
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Default LME49810 as step amplifier and grid current source

I am going to try to add a LME49810 as a step amplifier and grid current source so I can plot curves for tubes with large grid swing and also plot some A2 region curves.

I ordered 4 of the drivers but when I received them I realized that the pins are very close and do not fit on the breadboard. I searched all over and could not find an adapter for purchase. So I went to Radio Shack and purchase some parts.

My plan is that if the driver works I will also use it to make a A2 SE with some Jan1619 I have laying around.

I had to slightly bend some of the pins to make it fit the hole spacing on the perforated board.
Click the image to open in full size.


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Below is the first one I made, after it worked I made the 2nd one in the photos above.

Click the image to open in full size.

I already ran some tests with it and it does what it is supposed to do. I followed the sample circuit depicted in the datasheet.

alfredo
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Old 16th January 2012, 05:49 AM   #18
gootee is offline gootee  United States
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Cool!

I did mine for low-voltage semiconductor stuff. So I wanted both voltage and current steps of either polarity, for gates and bases of transistors. After the staircase generator (4-bit counter, resistor-ladder DAC, opamp buffer amp, switched-in-or-out opamp inverting amp for negative polarity, and stepped attenuator), I used a Howland Current Pump (see AN-1515 at national.com), which is a voltage-controlled current source. But after its opamp, inside of its feedback loops, I added a small current-boosting power amplifier. I used push-pull transistors for that but today I would probably use an LM1875 chipamp. So then the staircase could go to 15V max (1 V per step max) and 1.5 Amps max (100 mA per step max). To change from current steps to voltage steps I basically just had a 100 Ohm power film resistor to ground, on the output, that could be switched in from the front panel. Worked great.
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Old 17th January 2012, 12:30 AM   #19
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Today I hooked the chip to +-100v to check the maximun swing. Almost 190v pp. Next i am wiring up a 1619 se and se how well it cna drive the tube into A2. The swing for the 1619 will be around 70 vpp.

Love my iphone and the app Web albums. I can take a picure and upload it directly into picasaweb, then copy the link in my ipad to place in the post.

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Old 17th January 2012, 01:24 AM   #20
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It will be interesting to see how much current that thing can source and how much distortion results.

I have been considering a curve tracer but would make it microcontroller based(I want to record curves to import to excel) so I am interested in how this high-voltage chipamp performs.

Are you AC coupled at the input like on the datasheet schematic?
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