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I'm going kind of crazy trying to solve what I thought would be a very simple problem. I'm not massively experienced but will try my best to spell out what I'm trying to do and what's happening.

I'm trying to build a frequency counter using an Arduino Mega to hook in to a homebrew HF radio receiver I'm working on. The library I'm using on the Arduino (http://interface.khm.de/index.php/lab/experiments/arduino-frequency-counter-library/) requires logic (5V) level signals to sample to calculate the frequency of the input signal. The signal I'm trying to measure comes from an LC tank attached to a SA602AN and in the current configuration runs from ~3 MHz to ~5 MHz, and on my 'scope it measures 180mV peak to peak. I taking signal via a 10pF ceramic cap to avoid effecting the tank.

No problem I thought, I'll rattle a little 2N3904 common emitter amplifier, nice and simple and we've all built a few I'm sure, but I didn't seem to be able to squeeze any gain out of the circuit - output was 180mV peak to peak. Next idea, use a LM385, signal to (+) via a 100nF cap, a 25k pot between output and - to adjust gain, and a 1k resistor between - and ground, which I've seen many times online for similar applications. I had exactly the same problem - in fact wherever I set the pot, the output signal of the circuit was < the input.

Now I'm perfectly happy to accept the idea that I don't know a) what I'm doing or b) what I'm talking about, since this should really be an easy problem to solve but I'm just going round and round in circles. I have other similar 358-based op-amp circuits in my radio (audio preamp and to drive an RSSI indicator LED), the main receiver circuit has 5 2N3904s slopping around (and a couple of 2N2222s) so I kind of thought I knew what I was doing, but obviously not! Would anyone be able to point me in a useful direction, toward either what I could be doing wrong or a circuit which would serve the purpose? Thanks for your time!

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  • \$\begingroup\$ If you look at the frequency response graph on the LM358 datasheet, you'll see that it drops to unity gain at 1 MHz, so there's no chance of it being useful at 3 MHz. \$\endgroup\$ – Peter Bennett Jan 5 '14 at 23:57
  • \$\begingroup\$ Ah well bless my socks! Should have checked that, thanks. Still very puzzled why my bog-standard NPN circuit won't play though :( \$\endgroup\$ – DrMistry Jan 5 '14 at 23:59
  • \$\begingroup\$ ~3 mhz to ~5 mhz Please use a capital M when referring to MHz \$\endgroup\$ – alexan_e Jan 6 '14 at 0:07
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Try an op-amp in comparator configuration - nothing in the feedback loop, ground the non-inverting input (or use an offset as needed) and your signal to the inverting input. No feedback. If you use a chip designed as a comparator, like LM319, it won't have internal compensation and is much faster than an equivalent op-amp.

LM319 has full scale response of 80nS (can stretch out to 180 with over-voltage input) which fits your frequency needs. Use capacitor coupled input for zero-crossing detector, or add about half your full scale input to the + pin. LM319 works with single +5 supply but so will a lot of others. Note the old standby LM311 you will see in a lot of examples is too slow.

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  • \$\begingroup\$ That's excellent, I have a few stray comparators kicking around so I'll have a play around! Thanks very much! \$\endgroup\$ – DrMistry Jan 6 '14 at 1:12

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