3
\$\begingroup\$

I am building a circuit that uses a 4 MHz reference carrier. The used ICs are basically XOR 7486 and NOT 7404, CD4013 flip-flop, etc.

I have a function generator which is capable of providing up to 5 MHz. I tried to use it but I have a hard time getting the circuit to work.

When adjusting the amplitude to be a ±5 V square wave the ICs don’t seem to sense the input. I tried using the CMOS TTL output, but for some reason it’s stuck at 15 V peak (or is it designed to be? I don’t know) and the ICs work, but I am afraid that’s too high an input voltage.

Another issue I faced is that when I drive several ICs (say two) using the same clock from the function generator the ICs do not output as intended; it’s like there’s some loading effect which diminishes the function generator signal.

I tried searching data sheets for the used ICs but honestly I am at loss.

Below are pictures of the function generator, circuit, and schematics:

enter image description here

enter image description here

enter image description here

enter image description here

\$\endgroup\$
6
  • 1
    \$\begingroup\$ Are you trying to feed a negative input to the logic? The inputs to the logic want a uni-polar input, i.e. 0 to +5 volts. Use an oscilloscope to set the function generator levels. \$\endgroup\$
    – qrk
    Feb 12 at 23:55
  • 1
    \$\begingroup\$ 4 MHz on solderless breadboards with such a complex circuit is not something I'd consider. It was hard enough for me doing about that on wire wrap sockets and multiple boards. \$\endgroup\$ Feb 13 at 10:48
  • \$\begingroup\$ No, my original plan was to get an oscillator like the indicated on the diagram but decide to use available function generator instead \$\endgroup\$ Feb 14 at 0:25
  • \$\begingroup\$ @periblepsis You are right, I am having a big trouble connecting the circuit and getting it to work.. the thing is I don’t have the capability to design/order PCB. Do you have any recommendations/suggestions? I am currently using 3 breadboards \$\endgroup\$ Feb 14 at 0:27
  • \$\begingroup\$ @Mohamed If this were me, I'd either make an attempt at wire-wrap or else manhattan style construction. The wire-wrap pins can be found in 1x40 female 17mm long pin SIPs, but they cost about 5 cents per pin. So that's expensive these days. (Used to be more affordable and more available.) Manhattan is really easy and readily available. Just takes some practice. (Well, wire wrap also requires some practice.) Dead-bug wiring would also be cheap. But I'd get a roll of wire-wrap 30 gauge and a wire wrap tool for stripping the wire. 30 gauge is easy to work with. I'm staring at a roll of 1000' now. \$\endgroup\$ Feb 14 at 0:48

1 Answer 1

6
\$\begingroup\$

I tried to use it but I have a hard time getting the circuit to work, firstly when adjusting the amplitude to be 5V peak (+5-5) square wave

It should be 0V..5V, not -5V..+5V!

the ICs don’t seem to sense the input

They may latch up occsionally when you feed them with negative voltages. Make sure you're using a regulated power supply and watch the output current. The output current limit should be set low, say 100mA. That circuit won't need nearly that much. If it "pegs" the power supply, you've got bad chips, miswiring, etc.

I tried using the CMOS TTL output but for some reason it’s stuck at 15Vpeak (or it’s designed to? I don’t know) and the ICs work but I am afraid that’s too high input voltage?

The circuit can only accept a square wave with 0V low leve and 5V high level. Period.

  1. Sort out the function generator output first, unplugged from any loads, using an oscilloscope and ensure you're getting 0V..5V square wave. Use DC input coupling on the scope.

  2. Connect the clock input to the circuit to GND, and power up the circuit from 5V, 100mA current limit. Ensure that nothing is hot, and the circuit should be consuming way less than 1mA of current, typically some microamps (μA).

  3. Unplug the GND from the powered circuit and connect the function generator, ground first, then the signal lead.

Do not leave the generator turned on with the circuit unpowered. Always power the circuit first, then connect the generator BNC connector.

\$\endgroup\$
1
  • \$\begingroup\$ Do I use the (offset) knob to adjust the levels?? Because when I do that it seems there’s no effect, although I only tried that with AC coupling on oscilloscope! Secondly, the schematic is using an XTAL oscillator ( which I think have positive and negative swings “I am not sure”) how does that differ? As for the steps, step 3 is a bit unclear in order can you elaborate? And thank you for taking the time to explain! \$\endgroup\$ Feb 14 at 0:30

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.