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Author Topic: Boosting voltage of 200mhz low voltage signal  (Read 634 times)
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I have a harebrained idea on how to make a 200Mhz Frequency Counter using an AVR.  The basic concept is to use a 74VHC4040 counter that is rated for 200Mhz and the AVR just as the UI / counter reader.  The reference is a 1 PPS input signal.  Zero the counter.  Start the counter using the 1 PPS into a T flip-flop.  When the flip-flop goes high, an AND gate feeds the input signal to be counted to the 4040 and the AVR counts the overflows from q12 for the 4096s bit on the INT0 line.  When the next PPS signal comes in the T flip-flop goes low, the signal is disconnected from the 4040, the AVR reads the rest of the bits, and lastly resets the counter for the next go-through.  So you get one count every 2 seconds.  Real basic stuff.  The problem is that the PPS signal is 0.5V and the input signal may not be of a TTL triggering strength either.  Is this a job for an opamp to boost these incoming signals?  What would be the right one to try?  I would assume that 200Mhz is not a speed that that most opamps do not operate at.  Or is this more of a job for a high speed comparator?

P.S.  I haven't sampled a part from Linear in a while.  Suggest Linear parts and I will test them out for you and report the results here.   smiley-cool
« Last Edit: March 20, 2013, 05:06:42 pm by JoeN » Logged

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Why not a bipolar transistor?
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Well the PPM signal would just need a simple voltage translator stage or a comparator op-amp. The input circuitry for the counter is a little more demanding of a circuit as it has to deal with both variable levels at wide bandwidths as you are really dealing with RF circuitry at this stage of the counter, however there are a ton of example input circuits you could study to find one that meets your requirements.

Here is a published project using a similar method that you are considering:

http://www.qrpkits.com/files/ddial_ver1.pdf
Schematic is on page 14 or the document.
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I have a harebrained idea on how to make a 200Mhz Frequency Counter using an AVR.  The basic concept is to use a 74VHC4040 counter that is rated for 200Mhz and the AVR just as the UI / counter reader.  The reference is a 1 PPS input signal.  Zero the counter.  Start the counter using the 1 PPS into a T flip-flop.  When the flip-flop goes high, an AND gate feeds the input signal to be counted to the 4040 and the AVR counts the overflows from q12 for the 4096s bit on the INT0 line.  When the next PPS signal comes in the T flip-flop goes low, the signal is disconnected from the 4040, the AVR reads the rest of the bits, and lastly resets the counter for the next go-through.  So you get one count every 2 seconds.  Real basic stuff.  The problem is that the PPS signal is 0.5V and the input signal may not be of a TTL triggering strength either.  Is this a job for an opamp to boost these incoming signals?  What would be the right one to try?  I would assume that 200Mhz is not a speed that that most opamps do not operate at.  Or is this more of a job for a high speed comparator?

P.S.  I haven't sampled a part from Linear in a while.  Suggest Linear parts and I will test them out for you and report the results here.   smiley-cool

Perhaps a fast LVDS receiver could be persuaded to read the PPS output against a 0.25V reference.  Ditto for your incoming
frequency signal.

There are video opamps that will do this kind of speed but they will all be matched to 50 or 75 ohms and aimed at signals
of about 1V amplitude - a signal transformer/balun could in theory boost this (1:4 ratio perhaps)
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The incoming signal, once it is boosted to TTL level, is going to go through an AND gate which allows the signal to be enabled or disabled to the 4040.  The AND gate that I intend to use has input hysteresis so I think this would square up any slow-rising signal for the counter.  This is the gate:

http://www.ti.com/lit/ds/symlink/sn74lvc1g97.pdf

What do you think of a part like this:

http://www.ti.com/lit/ds/symlink/pga870.pdf

From what I read, you set 6 pins for the voltage gain and it gives you a "gain" of -11.5Db (x0.25V) to 20dB (x10V) with a relatively flat response up to abou 500Mhz.  If I read that correctly, it's pretty ideal.  You could test the signal on an analog pin and just ratchet it up until it's where it needs to be.  Or am I reading this totally wrong?
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This reminds me of a laser control system I built about 35 years ago, when I had to build a counter that would measure the frequency of a very small signal from 0.1 to 500MHz. Getting an ECL-logic divide-by-10 counter that would clock at 500MHz was no problem, but converting the signal to a square wave was hard. Originally I was going to use a tunnel diode, but it took weeks to arrive. So I ended up building a non-saturating Schmitt trigger that worked over the full frequency range.

If you want to cover DC-200MHz, then converting the signal to a square wave (when the frequency is too low for the counter to accept a sine wave) is also something that you need to look at carefully. Using a gate with hyteresis is a good idea, however the one that you linked to has a propagation delay of up to 5.1ns @ 5V. The datasheet doesn't specify a maximum frequency, but with a propagation delay that high, there is a possibility that it may not pass 200MHz very well.
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This reminds me of a laser control system I built about 35 years ago, when I had to build a counter that would measure the frequency of a very small signal from 0.1 to 500MHz. Getting an ECL-logic divide-by-10 counter that would clock at 500MHz was no problem, but converting the signal to a square wave was hard. Originally I was going to use a tunnel diode, but it took weeks to arrive. So I ended up building a non-saturating Schmitt trigger that worked over the full frequency range.

If you want to cover DC-200MHz, then converting the signal to a square wave (when the frequency is too low for the counter to accept a sine wave) is also something that you need to look at carefully. Using a gate with hyteresis is a good idea, however the one that you linked to has a propagation delay of up to 5.1ns @ 5V. The datasheet doesn't specify a maximum frequency, but with a propagation delay that high, there is a possibility that it may not pass 200MHz very well.

Thank you.  I will have to check that out on the scope and see what it looks like.  I have obtained an old Tek 2465B 400 MHz scope so I should be able to check that out at least.  I actually need it to go up to 188Mhz, I am just specifying 200Mhz for headroom.  If you know of a faster AND gate out there that would be more appropriate, I would love the suggestion.  I am getting several of these gates, I can also test them out individually and if there are any that spec out better I can just use the best gate.
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