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| Voltage comparator |
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| duak:
At one time National made a FET input comparator, the LF311. It looks like it was phased out in the 90's. I built a frequency counter in the 70's using one of these things. Even back then it wasn't particularly fast with a response time of 200 ns. This implies a 5 - 10 MHz frequency limit which was just good enough for the rest of the counter. I think David Hess knows of what he speaks. A frequency counter with a good input circuit is a pleasure to use because it isn't fooled by abberations in the input signat. I preferred Tek scopes over hp's because, to me anyway, they had better and more consistant triggering. Since you can't see the trigger points with a frequency counter, how do you know it's the right frequency? |
| rcbuck:
I used this circuit for the front end of a counter I built last year. It is a slightly modified version of my 1970s Simpson 710 counter that still works fine today. The pot is supposed to be an input sensitivity adjustment. However, I found it has very little effect on the sensitivity. Input sensitivity is about 50 mV from 20 Hz to 60 MHz. It is more sensitive above 1 kHz. You may be better off to just copy the Simpson version. I built an exact copy and it operates the same as my circuit. The Simpson operating manual can be found online and the schematic, although not very clear is at the back of the manual. The sensitivity control in the Simpson also had very little effect on sensitivity so you could just replace it with a fixed 100 ohm resistor. The MC10H116P is available from Mouser. |
| David Hess:
--- Quote from: duak on April 29, 2019, 11:25:47 pm ---At one time National made a FET input comparator, the LF311. It looks like it was phased out in the 90's. I built a frequency counter in the 70's using one of these things. Even back then it wasn't particularly fast with a response time of 200 ns. This implies a 5 - 10 MHz frequency limit which was just good enough for the rest of the counter. --- End quote --- The LF311 is about the only one I know of and I suspect it was discontinued because it was just a dumb idea. Tektronix used custom hybrids for their logic analyzers which included a JFET buffer followed by an ECL line receiver. The major problem is capacitive coupling between the output and inputs and between the inputs. In an operational amplifier, a small amount of feedback capacitance can be added to neutralized the pole formed by the input resistance and capacitance at the inverting input but comparators are not compensated so that will not work for them. It also will not work for a current feedback amplifier without some additional cleverness. Coupling from the output to the inputs and between the inputs when the source impedances are high can cause all kinds of weirdness. And if you use low source impedances to prevent this, then a high impedance input is no longer required. The corner cases left over where a low input bias current is desirable were just not enough. Of course CMOS input comparators have all of the same problems and the same solutions; drive them with low source impedances. --- Quote ---I think David Hess knows of what he speaks. A frequency counter with a good input circuit is a pleasure to use because it isn't fooled by abberations in the input signat. I preferred Tek scopes over hp's because, to me anyway, they had better and more consistant triggering. Since you can't see the trigger points with a frequency counter, how do you know it's the right frequency? --- End quote --- Tektronix's earliest counters used oscilloscope input circuits and they worked great, better than most later counters. The 7D15 shown below is a completely discrete design and includes compensated input attenuators and an HF gain trim. Notice how the trigger level is added to the input signal and only the last stage (1) adds hysteresis and operates with a fixed trigger level; this is effectively a schmitt trigger with one input and one output so no comparator is used even to support a variable trigger level. A tunnel diode based circuit would work this way and support incredibly high speeds. If I was going to duplicate a good but simple design, I would consider the HP 5314/15/16 which uses a JFET buffer to directly drive an AMD (!) AM687 comparator. The comparator then drives a differential pair to create an ECL logic output. (2) These counters support a mode where the level control becomes a sensitivity control and the designer abused the latch enable input of the AM687 to provide this so finding a suitable modern comparator to act this way might be a problem. Replace the JFET with your favorite high bandwidth JFET or CMOS operational amplifier operating as a voltage follower unless you need higher performance. (1) There are two last stages in parallel. One of the two is enabled so triggering on the rising or falling edge is selected instead of trying to invert the analog signal at an earlier point. A modern design could just stick an exclusive-or gate after the comparator to select which edge to use. (2) See the application note I mentioned which discusses comparator level shifters to drive different types of logic. |
| Zero999:
--- Quote from: OM222O on April 29, 2019, 12:33:34 pm ---I'm becoming interested in making a frequency counter and the software part is all done. I just needed a way to convert any incoming signal to a clean square wave so my first idea was to use an op amp / voltage comparator in schmitt trigger mode to do that. my main questions is: what do I need to look out in the op amp spec for high frequency applications? let's say I want about 20MHz bandwidth. should I care about slew rate as it's purely just switching between two states? is gain bandwidth product important in this configuration? would a dedicated voltage comparator be more suitable in this application? And finally if op amps / comparators can't provide that much bandwidth, what's the simplest way I can make a voltage comparator using BJTs? --- End quote --- Yes, use a dedicated comparator IC. Here's some information regarding the pitfalls of interchanging op-amps and comparators. http://encon.fke.utm.my/nikd/latest/sloa067.pdf https://www.analog.com/media/en/training-seminars/tutorials/MT-084.pdf |
| OM222O:
Thanks for the replies, I will buy both the TI and the AD part to see if there are any major differences up to the bandwidth I want (there shouldn't be, but you never know ;)) pretty much every circuit that has been referred to seems to use a JFET input buffer. is this just to create a high impedance input or are there any other reasons? I'm not sure about the input impedance of comparators but op amps have better than good enough input impedance on their own so I'm not sure why the JFET buffer is used. Also I wonder if there is a better way of creating the hysteresis where it doesn't need adjusting the gain based on the input ??? the schematics that have been provided so far don't seem to have that but they're a bit too complex for me to see how they have gotten around the trigger point problem. maybe the hysteresis points are chosen close together at something like 50mv and 100mv? rcbuck mentioned that even the professional ones that include sensitivity adjustment don't seem to do much anyways ... |
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