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Input stage for VHF frequency counter in an FPGA?
The other day I found myself needing a short gate time ~200 mhz
frequency counter for an automated test, and since I had an FPGA board on hand I whipped one up quickly. Getting it reading and reporting to my computer was the easy part. Ah, the input stage.... I've got about 4dBm of RF into 50 ohms to play with - about a volt p-p or a little more if it's high-Z. The output of the device under test has a transformer and then a series cap to create an unbalanced output. I did something ugly with a 3.3v cmos 7406 varient and a feedback resistor, which works well enough to get an accurate reading on one version of the device under test, but not on the other (both have been verified with real test equipment) It also tends to self-oscillate with no input... What would be the right way to do this using on hand parts, such as abused logic, little 1:1 or 2:1 RF transformers, etc? One idea is to use another gate with a feedback resistor and cap to ground in the hope of establishing the threshold level, and then using a transformer to swing another input above and below this. Most parts on hand are SMD - which means dead bug construction in SOIC scale under the maginifier - discourages extensive experimentation. Why do most abuse-of-logic RF applications seem to use NAND gates rather than inverters? From a digital perspective NAND gates are a universal element, but once you tie their inputs together, is there something to be gained from having two inputs in parallel? Is there a way to use a differential input configuration on an FPGA to input a balanced RF signal directly? Theoretically this should be an FPGA clock input... The device in use currently is an Altera Stratix II, but a Xilinx S3 kit is available. If ordering things, what would be a good default low supply voltage HF/VHF gain component to have on hand? I seem to recall lots of last-millenium ham designs using the MC1350P video IF amp, but what would make sense today? |
Input stage for VHF frequency counter in an FPGA?
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Input stage for VHF frequency counter in an FPGA?
For gain, have a look at MMICs. Minicircuits sell them in kits that
give you a good range of performance. They're easy to use, and run on a single supply. I think the idea of using the LVDS inputs on the FPGA is a good one, if you can get that into the clock lines you need to use. Otherwise, you only need a little gain to get to levels you need for logic input. Do your clock input lines have a bit of hysterisis? Do they have high enough input impedance to be used with a step-up transformer? Can you reliably bias the transformer DC return to a point between the hysterisis trip-points. I suspect you can do a better job with your logic-gate amplifier, too. Or you could make a fancier input using one of the very fast comparators. Cheers, Tom |
Input stage for VHF frequency counter in an FPGA?
As Jim has said LVDS is a good way. Just watch the common mode input range.
You can use a RF transformer to rebias the DC level to get arround any issues. Another way is to use a single ended standard like SSTL and DC bias the input to the reference voltage used for the SSTL. You do need to have access to Vref pins on the Spartan-3 to use this technique. We have support for LVDS and SSTL in our boards but I am not sure about the Spartan-3 starter kit. John Adair Enterpoint Ltd. - Home of Raggedstone1. The low Cost Spartan-3 Development Board. http://www.enterpoint.co.uk wrote in message oups.com... The other day I found myself needing a short gate time ~200 mhz frequency counter for an automated test, and since I had an FPGA board on hand I whipped one up quickly. Getting it reading and reporting to my computer was the easy part. Ah, the input stage.... I've got about 4dBm of RF into 50 ohms to play with - about a volt p-p or a little more if it's high-Z. The output of the device under test has a transformer and then a series cap to create an unbalanced output. I did something ugly with a 3.3v cmos 7406 varient and a feedback resistor, which works well enough to get an accurate reading on one version of the device under test, but not on the other (both have been verified with real test equipment) It also tends to self-oscillate with no input... What would be the right way to do this using on hand parts, such as abused logic, little 1:1 or 2:1 RF transformers, etc? One idea is to use another gate with a feedback resistor and cap to ground in the hope of establishing the threshold level, and then using a transformer to swing another input above and below this. Most parts on hand are SMD - which means dead bug construction in SOIC scale under the maginifier - discourages extensive experimentation. Why do most abuse-of-logic RF applications seem to use NAND gates rather than inverters? From a digital perspective NAND gates are a universal element, but once you tie their inputs together, is there something to be gained from having two inputs in parallel? Is there a way to use a differential input configuration on an FPGA to input a balanced RF signal directly? Theoretically this should be an FPGA clock input... The device in use currently is an Altera Stratix II, but a Xilinx S3 kit is available. If ordering things, what would be a good default low supply voltage HF/VHF gain component to have on hand? I seem to recall lots of last-millenium ham designs using the MC1350P video IF amp, but what would make sense today? |
Input stage for VHF frequency counter in an FPGA?
I did something ugly with a 3.3v cmos 7406 varient and a feedback resistor, which works well enough to get an accurate reading on one version of the device under test, but not on the other (both have been verified with real test equipment) It also tends to self-oscillate with no input... 7406 is open collector. Did you mean 7404? What size feedback resistor? What sort of oscillations? I've had reasonable luck with that sort of hacking. Not great. What's the output of your gate look like? Is it cleanly switching or struggling to switch at that speed? You might want to skip the external gate and use an inverter in the FPGA out to a feedback pin. That gets the feedback covering the input pin that you are really interested in. -- The suespammers.org mail server is located in California. So are all my other mailboxes. Please do not send unsolicited bulk e-mail or unsolicited commercial e-mail to my suespammers.org address or any of my other addresses. These are my opinions, not necessarily my employer's. I hate spam. |
Input stage for VHF frequency counter in an FPGA?
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Input stage for VHF frequency counter in an FPGA?
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Input stage for VHF frequency counter in an FPGA?
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Input stage for VHF frequency counter in an FPGA?
Jan Panteltje wrote:
Just a partial reply... I think 7400 series should stop way below 200mHz, perhaps 50MHz? It's either a 74AC04 or possibly a 74HC04 (it's upside down so I can't tell) and it's self oscillating at 294 mhz - (it's stable enough for the counter to read... a fast scope shows it approximately as a sinewave. It seems to be oscillating at about 1/tpd... can't even really pull it much with finger capacitance - only about 10 mhz. Interestingly, if I short a the floating input-output pair of an unused inverter with the scope probe, that runs a bit slower around 260 mhz... wheras the gate in use has about 20k of resistance in the feedback path. I would make a small diff amplifier, did something 40 years ago (yes 40!) with I think it was BFY90 transistors, then invert with 2 more and drive the LVDS input. I may give your transistor circuit a try, either with components or simulation, thanks. |
Input stage for VHF frequency counter in an FPGA?
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