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Frequency counters & 10MHz standards

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hlavac:
I have two questions:

* Does frequency counter always assume the input is a sine wave / square wave of a single frequency? What happens if it is not the case?
* How does the standard external 10MHz synchronization signal that some of the instruments can use look like? Is there a standard for it?

jpb:
1. my understanding is that the counter has an edge trigger and the level can be set, there is also probably some means of attenuating large signals. So it doesn't assume any shape but if you set the level in a lot of noise it will give erroneous counts and not be very stable.

2. most instruments define the type of signal they expect from an external clock e.g.

for the TTi TF830 counter it just has :

External Standard Socket
An external 10MHz frequency standard (5V/TTL
level) can be applied

while for the Tabor WW5061 AWG it is more specific:
10MHz TTL, 50% ±2% duty cycle

The TTi TG5011 seems quite flexible :
Ref Clock Input
Input for an external 10MHz reference clock
Voltage Range: 1Vpp – 5Vpp
Maximum Voltage: +5V
Minimum Voltage: -1V


so generally they want TTL square waves with 50% duty cycle but I think some accept sine waves.

olsenn:
Counters only measure the number of times a voltage goes from below a value of 'a' to above a value of 'b'. Sometimes these two values are the same, but there is usually hysteresis (schmitt trigger) to avoid any influence of noise.

Try not to think about it in the frequency domain; instead, consider the time domain analysis, as you would see on an oscilloscope. Also, since it counts the total number of low-to-high (sometimes high-to-low) transistions in a set time (say 1 second for example), the reading that will be displayed is the average frequency over that one second interval.

As for external frequency inputs; it also doesn't depend greatly on the waveform; this reference clock merely clocks the digital logic IC's apon low-to-high transistions, just like the counter itself. My 10MHz rubidium frequency standard for instance produces a sinusoid; however, a TTL square wave will also do the trick.

jimmc:
JPB has covered your second question well.
Your first question is covered by Agilent's AN200  http://cp.literature.agilent.com/litweb/pdf/5965-7660E.pdf (p9).

Jim

hlavac:
Excellent appnote, just what i was looking for, thank you!

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