Unable to read frequency of a tone signal with a frequency meter.

[Alby]

Can anyone tell me why I'm unable to read the frequency of a tone signal with my frequency meter? Equipment used in this question is a Victor VC3165 Frequency counter and a Instek GOS6112 oscilloscope.

Circuit being tested is an oscillator that is generating a 67 Hz tone, Note: The signal is a triangle sine wave and has been set to a very close 67 Hz using the oscilloscope.The output of that oscillator is going through a resistor to the base of a NPN buffer. The output of the NPN (emitter) then goes through a split resistor bias to ground, The signal at the junction of the split resistor is then outputted to an audio preamp circuit through a .01uf npo capacitor.

I can read the output from the oscillator, at the base of the NPN, at the emitter of the NPN and at the junction of the split resistor with the oscilloscope but can't read the frequency of the signal with my frequency counter anywhere in the circuit. I've tried to couple the freq meter with various values of resistors, various values of capacitors and even direct coupling, The display on my meter either freezes or radically displays random numbers. Note: I checked the freq meters function with a known Audio signal generator and is working ok.

Most likely left out some details, just ain't perfect these days. Got questions...please ask. Any help will be greatly appreciated.

 

[Arouse1973]

Hello
What is the amplitude of the waveform?
Thanks
Adam

 

[Arouse1973]

What setting and channel are you using. Looks like you have to use DC coupling for that frequency and you are limited to 80 mV. That seems quite a low voltage to me, is that right?
Thanks
Adam

 

[Alec_t]

Are you using your meter on Channel A with a x1 attenuator setting?

 

[Alby]

What setting and channel are you using. Looks like you have to use DC coupling for that frequency and you are limited to 80 mV. That seems quite a low voltage to me, is that right?
Thanks
Adam

Wave form is 2.2V p-p @ emitter.
On the freq counter I'm using channel 3 Input "a" and using DC coupling.

 

[Alby]

Are you using your meter on Channel A with a x1 attenuator setting?

yes

 

[Arouse1973]

Have you tried changing the attenuator setting to x20? For DC coupling the manual say less than or equal to 80 mV. Does that mean that is the maximum voltage you can apply to the counter with x1 set?
Thanks
Adam

 

[Alby]

Have you tried changing the attenuator setting to x20? For DC coupling the manual say less than or equal to 80 mV. Does that mean that is the maximum voltage you can apply to the counter with x1 set?
Thanks
Adam

I've tried so many things Adam, I can't say for sure that I tried that. I will though. It seems that when one gets frustrated, the simple things are never thought of. I'll post the results in a short while. I'll make sure the input to the meter is less than 80mV.

 

[Arouse1973]

Ok cool. Strange one this is

 

[Alby]

Ok cool. Strange one this is

OK Adam. I knocked the the voltage down to 43 mV going to the freq counter. Made every attempt to read the frequency and it wouldn't display anything but zero's when I connected the freq leads. I verified that the signal was there and the voltage was correct with the oscilloscope. Switched the freq meter to my Audio Generator with an output of 20 mV and the freq meter read it just fine. I've tried every possible combination I could think of. I don't understand why it won't read that signal.

BTW...the 67 Hz has to be right on frequency is why I need to have the freq counter work, Just setting it with the scope doesn't work most of the time, unless you get lucky..... scratching my head...mumble mumble...scratching my head some more.........

 

[Alby]

Here is the circuit I'm working on.

 

[AnalogKid]

Where is this schematic from?

Do you have a schematic that shows the individual gates as components so the drawing conveys some kind of information about the circuit? Also, why are pins 12 and 13 brought out of the device as a single pin?

Pin 11 is a NOR gate output. The signal there probably is a square wave of some sort. The C10-R16 node is where a pseudo-triangle wave will be.

ak

 

[Alby]

Where is this schematic from?

Do you have a schematic that shows the individual gates as components so the drawing conveys some kind of information about the circuit? Also, why are pins 12 and 13 brought out of the device as a single pin?

Pin 11 is a NOR gate output. The signal there probably is a square wave of some sort. The C10-R16 node is where a pseudo-triangle wave will be.

ak

Sorry AnalogKid, but this is the only schematic I have to work with. This is a schematic for a 2 metre transceiver tone oscillation for PL repeater access. To be specific, a KDK FM-2015R.

Yes, Pin 11 has a square wave of some sort, and yes, C10 is where the pseudo-triangle wave is and can be measured. I believe R-18 is the VR that controls the frequency refinement. R-17 along with R-18, C10 and C9 that control the upper and lower frequency spectrum.

No, I don't have a schematic that shows the individual gates as components. The only thing I have is a data sheet for that chip. It being a Quad 2 input NOR Gate.

No, I don't know why they tied the inputs,12 and 13 together for that gate.

The VDC for this circuit is a regulated 10 VDC.

Al

 

[Alby]

Here is the data sheet for the MCI4001B, which is the actual IC in the radio. The above schematic has some errors in it.

 

[Arouse1973]

Are you able to get some pictures of the scope trace of the signal on the output of your circuit?
Thanks
Adam

 

[Alec_t]

I'm using channel 3 Input "a" and using DC coupling

Is that a 50Ω input? If so, it would overload the circuit under test.

 

[Alby]

Are you able to get some pictures of the scope trace of the signal on the output of your circuit?
Thanks
Adam

Hope this is good enough Arouse1973. Never took a pic of my scope trace before.

 

[Alby]

Is that a 50Ω input? If so, it would overload the circuit under test.

It's the lead that come with the counter. I suppose it is a 50Ω. I didn't think it would make any difference when doing this test. The manual says it is a 50Ω input impedance when used on CH B 50 MHz - 2.4 GHz but for CH A it says it's a 100 meg input impedance from 1Hz - 100Hz.

 

[Alec_t]

The manual says it is a 50Ω input impedance when used on CH 2 50 MHz - 2.4 GHz but for CH 1 it says it's a 100 meg input impedance from 1Hz - 100Hz.

You said you were using Channel 3.

 

[Alby]

You said you were using Channel 3.

Sorry for the typo.....yes CH Function 3. I meant to say CH a and CH B in previous post. I edited it to read the right thing.