Universal Counter is not stable with a load why?

[Davewalker5]

I use these Universal counters
HP5315B Universal Counter
HP5334B Universal Counter

I'm using a Ling Function generator for a waveform generator
Sine waveform at low frequencies

I use the Universal counter to display a digital readout of what the frequency is of the sine waveform

When I plug in the function generators output to the Universal counters input and it displays the frequency

The problem I'm having is when I plug a load or to a circuits input ( board under test ) it makes the universal counter not stable, the display is jumping around and won't get stable

Any reason why?

I tried adjusting the Universal counters input trigger and sensitivity

The only thing that makes the universal counter stable, is when I unplug it from t the circuit board under test when is loading the universal counters input?

 

[Davewalker5]

Why doesn't this circuit work in square waveform? only works in sine waveform

The frequency is 10 hz input

Is Capacitor C13 blocking the low frequency Square waveform?

Is that why the circuit works only with a sine waveform

What does U1 , Q7, Q8 do? is this a draining off the voltage circuit, any reason to use a Logic Gate?

 

[Harald Kapp]

What do you mean by "it doesn't work"? What do you expect to happen/see? What does happen, what do you see instead? What frequency do you use?

U1, Q6-Q8 could be a peak detector (for the output of U4, pin 14) which turns on or off (Q6) a current source (Q7, Q8). Since the rightmost part of the schematix is missing, I can't say more.
A logic gate may have been used to control the current source from the logical AND of the peak signal and a separate control signal. This is not good practice, but can be seen sometimes, especially with CD4xxx components.

 

[Harald Kapp]

Measure the output voltage of the function generator without and with load. A function generator typically has an output impedance of 50 Ohm. When loaded, the output voltage will drop. Do you have acccess to an oscilloscope to observe the waveform of the function generator?

Another possibility is hum (50 Hz or 60 Hz depending on your mains frequency) introduced e.g. by a ground loop (protective earth connection of the different devices used in the setup). DO you have access to an insulating mains/mains transformer? using this on one of the instruments can break a ground loop.

 

[davenn]

the 2 threads were directly related ... I have therefore merged them

Dave

 

[Harald Kapp]

I've seen both but thought the ycould stand each for himself since they deal with different issues. But it's o.k.

Harald

 

[Davewalker5]

Measure the output voltage of the function generator without and with load. A function generator typically has an output impedance of 50 Ohm. When loaded, the output voltage will drop. Do you have acccess to an oscilloscope to observe the waveform of the function generator?

Yes on the O scope , the frequency and voltage is good

The problem is that the Universal counter display is not stable, displaying the frequency is jumping around random only when it's loaded or hooked up to the circuit , under test