Frequency counter front end signal conditioner - Page 1
Forum section: electronics / projects

author: qpit3a
author_rank: Contributor
timestamp: May 20, 2023, 09:06:48 pm
content: Hi,I have made a signal conditioner for my 50 mhz frequency counter from a design that I foundhttps://www.westmountainradio.com/pdf/Article_Fernandes.pdfit works ok but fails a little bit at low amplitidue sine waves. Wondering if there is some single chip solutions (say up to 100mhz) for signal conditioning. I've found another design eghttps://www.siliconchip.com.au/Files/Samples/Silicon%20Chip%20November%201987%20p32-51.pdf, but they use items not available anymore (I think). I think the design is from this old Motorola designhttp://www.bitsavers.org/components/motorola/_appNotes/AN-0581_An_MSI_500_MHz_Frequency_Counter_Using_MECL_And_MTTL.pdf,but MECL seems not available.Thanks for any help.

author: WideBandwidth
author_rank: Contributor
timestamp: May 21, 2023, 04:00:57 am
content: I think the term you're looking for is a limiting amplifier, for which there are plenty available, especially for much higher frequencies. They typically consist of a cascade of amplifiers with well defined limiting conditions (i.e. what happens when you drive the amplifier to its limits). For example, here'sMouser's page. Note that these amplifiers are often used in high-speed optical communications (e.g. > 4 Gbps), and may have a lower frequency cutoff. The reason why they include this cutoff is because they usually have some internal low frequency feedback loop to correct for the DC offset of the initial amplifier stages which would otherwise get amplified until the entire thing railed. To get better results, you may try searching the websites of specific manufacturers (e.g. TI, Analog Devices, Maxim which is now Analog, etc.). For example, the MAX3969 would do the trick, provided its low frequency cutoff (if there is one, you'd have to check) satisfies your requirements.The ECL chips you mention are just high speed differential amplifiers, biased for particular input and output swings. There's nothing particularly special about them that you can't replicate with discretes (except for perhaps the transistor matching and speed advantages of some integrated processes).If you're interested in discrete solutions, I designed a ~300 MHz bandwidth variant using MMBTH10 transistors (the cheapest "RF" transistors you can find on Digikey) that generated a fixed-amplitude output with an input down to around -30 dBm. I was also intending it for use in a frequency counter, but never got around to completing it, as the project queue has since expanded. Check outthis threadfor more info and the attached schematic.

author: mino-fm
author_rank: Regular Contributor
timestamp: May 21, 2023, 08:16:04 am
content: For simple applications I use an 74AUP2GU04 inverter followed by 74AUP1G14 schmitttrigger which can be seen here:https://www.eevblog.com/forum/projects/high-resolution-reciprocal-counter-using-rp2040-pico-board-and-as6501-tdc/msg4807880/#msg4807880An other way would be using TLV3501 comparator for high impedance or MAX961 for high sensitivity.

author: jwet
author_rank: Frequent Contributor
timestamp: May 21, 2023, 09:06:37 am
content: For that range, a really nice, easy solution is an LT1016 or the newer derivatives like the LT1116 and similar devices from Maxim and others. These are very easy to apply 7 nS comparators that just work, ground sensing, no squirelly modes and easy interfaces. I think one of these with a well biased and decoupled threshold pot or a DAC would make a great Freq Counter input.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 21, 2023, 09:47:48 am
content: Quote from: qpit3a on May 20, 2023, 09:06:48 pmIt depends how low amplitude you mean, but a modern amplifier would be a LVDS receiver like TI DS90LV012AThat spec diff in like thisVTH Differential Input High Threshold VCM dependant on VDD (3) IN+, IN− Typ −30mV Max 0 mVVTL Differential Input Low Threshold Min −100mV Typ −30 mVie it has a fixed 30mV offset so a shorted-in is defined, but has little/no? hystersis, so a small pre-bias or trimpot should be sensitive.Or a TLV3603 has a Hyst set pin, which sounds ideal. It costs more than the DS90LV012A, but the resistor-set hysteresis is a nice feature on a counter input.You can adjust both slice level and hyst.

author: qpit3a
author_rank: Contributor
timestamp: May 21, 2023, 11:52:09 am
content: Thanks. I did try at one point a fast op-amp and comparator LT1720 (and was adjusting hysteresis) but seemed very un-reliable. (Althought this is not as fast a comparator as the TI part). My board generated a lot of extra signal at low input amplitudes although that was an early board (in my pcb design experience) that didn't have correct ground planes so... Problem is if you have to adjust to get the correct reading how do you know what the correct reading is unless there's another counter which kind of defeats the purpose. My amplitude goal for sine wave is ~20mv.

author: qpit3a
author_rank: Contributor
timestamp: May 21, 2023, 11:56:42 am
content: Thanks for all the replies and info. Sounds like a comparator should work - perhaps if I do a better pcb...

author: Kleinstein
author_rank: Super Contributor
timestamp: May 21, 2023, 04:22:04 pm
content: If the signal is not clean, it may just need some adjustment to get the right setting. Chances are there is no fixed setting that works in all cases. Ideally one would have some monitoring output to send an amplified / bufferd signal to a scope to check and or some kind of crude amplitude measurement (e.g. pos and neg peak voltage with some simple diode circuit).

author: Georgy.Moshkin
author_rank: Frequent Contributor
timestamp: May 22, 2023, 02:19:12 am
content: I had good results with battery powered comparator (dual supply). So there was a positive voltage, negative voltage and gnd between batteries in series. This way input signal is easily biased to gnd, can set thresholds without adding noise. Also I like this idea to eliminate negative supply:https://www.ti.com/lit/pdf/snoa989but haven't tried it yet

author: mino-fm
author_rank: Regular Contributor
timestamp: May 22, 2023, 08:15:35 am
content: Using 2 x N-channel FET you don't need negative voltage as shown below.Nowadays it is a little bit difficult to get these transistors (PBMF J620).Pot1 50 Ohm is needed to trimm offset of J620 and pot 10 k lin to adjust DC input offset. BSS138 can switch on low pass filtering.

author: David Hess
author_rank: Super Contributor
timestamp: May 22, 2023, 08:45:31 am
content: Quote from: jwet on May 21, 2023, 09:06:37 amUp to 50 MHz that is my preferred solution. An LT1016 or LT1116 is effectively a limiting amplifier followed by a comparator anyway, and if extra sensitivity is required, then an external differential limiting amplifier can be added with just a pair of transistors or an IC.It is not required for sine waves, but Jim Williams published an adaptive trigger LT1116 design based on Tektronix trigger circuits which works better than AC coupling when duty cycle varies. See page 15 of Linear Technology application note 61.Update: Fixed Spelling

author: jwet
author_rank: Frequent Contributor
timestamp: May 22, 2023, 02:59:52 pm
content: That Jim Williams circuit is elegant (as most of his circuits are,,,). I think what you meant was "Adaptive" but the spell checker must have eaten that.To the OP, this overcomes your objection nicely- adaptive in this case meaning it will track the input and set the threshold- very nice.Comparator circuits require good layout- on one side you're trying to resolve small overdrives- a few mV. On the other you have very high slew rate signals- just a pF of inadvertent feedback gives CdV/dt all it needs to cause instability, double triggering or other ills. I think the LT1016 DS has some layout guidelines. The LT1720 you mention is essentially a more modern LT1016, I suspect your problems might have been layout related.

author: qpit3a
author_rank: Contributor
timestamp: May 27, 2023, 12:17:22 pm
content: Thanks for the replies, I am interested in doing another design with a better pcb layout using comparators. I also have to admit that I contacted designer of the circuit I am using and was told that I had put JFET with source and drain swapped... Works a bit better now .