3
\$\begingroup\$

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!

\$\endgroup\$
3
  • \$\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
    Commented Jan 5, 2014 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
    Commented Jan 5, 2014 at 23:59
  • \$\begingroup\$ ~3 mhz to ~5 mhz Please use a capital M when referring to MHz \$\endgroup\$
    – alexan_e
    Commented Jan 6, 2014 at 0:07

1 Answer 1

Reset to default
3
\$\begingroup\$

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.

\$\endgroup\$
1
  • \$\begingroup\$ That's excellent, I have a few stray comparators kicking around so I'll have a play around! Thanks very much! \$\endgroup\$
    – DrMistry
    Commented Jan 6, 2014 at 1:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.