Establishing how an electronic circuit works

Hello

In 1978 I designed a frequency counter for use with a FM radio I built. The objective was to display the actual frequency of the tuned station - cutting edge technology in those days! This will sound utterly stupid but on revisiting the circuit diagram 36 years later I can not work out how some parts of it work. Is there any software on the market that will analyse a circuit and say how it works? Circuit mainly uses CMOS counters and nand/nor gates, RF frequency divider and crystal oscillator. CMOS deviices feed LED display via resistor network.

Thank you

If that software is made available, that certainly will be a big help. But sadly, I know nothing of a kind. Research is all you can do on that device. Mostlikely, the concept of frequency counters still apply until today.

Since you have a schematic you could put it into a logic simulator, there are a number of them around that are free to download, just do a search.
http://www.softpedia.com/get/Others/Home-Education/CEDAR-Logic-Simulator.shtml
http://smartsim.org.uk/
are typical of what is available.
Some a rather simplistic, so you will need to sort through them to find one that will do what you want. You may have to break the circuit down into smaller blocks, and as Kevin said, nothing much has changed for a lot of the circuits, they may just have become more integrated since you did the design that many years ago.
If it is still working, an 8 or 16 channel logic analyser may do the trick, but you still have to figure out where the critical nodes are in the circuit to probe,
cheers,
Richard

Hello Richard
Thank you for your help. I will try out your suggestion. I have analysed my circuit and now know how individual elements work. I seem to have generated a 1k oscillator controlled signal for use with the time period for which counting takes place. It is in this area where I fall down, not knowing how to initiate the start of a count and how to finish it and then interpreting the final state of the counters. No doubt with perserverance it will come back and all will fall into place. It seems like I have forgotten the principles of a counter, I do remember that F=1/T !

When the input frequency is high, you count the input cycles using a longer internal gating signal.
Where the frequency is low, you do it the opposite way, you use a high frequency internal clock and gate it withe the external signal.
Either way, knowing the count and gating time,you can compute the frequency, as you say F=1/T.
Cheers,
Richard

Thank you for your input. I have a basic problem understanding my original design.
I have a series of divide by 10 counters representing !00M,10M,1M,100K and10Khz.
The RF signal entering the first counter, i.e the 10KHz counter is gated using a 1KHz timing signal. I can not work out how the counter stops at the correct frequency if the gating signal is not 1 second. How does the counter know where to stop when the correct frequency is reached.
Best Wishes
Norman

I have now worked out how my circuit works. I had forgotten one fundamental fact. The gating signal has to be a sub multiple of 1 second which mine was. Your reply to me triggered me off. Thank you. I can't believe my radio is still working after 39 years. I must admit it did have quite a lot of military standard components in it and those that weren't were BS screened.
Best Wishes
Norman