What is the use of LC circuit in frequency measurement setup?

Dear All,

I am working on a test system that measures the frequency of 16.367 MHz TCXO oscillator and verifies within the range of +/- 33 Hz.

When I put the DUT Module onto the test fixture, the test pin probes the output of TCXO and measures the frequency with reference to ground.

Immediately after the test pin to TCXO, on its way to frequency counter E1420B (agilent), one adjustable inductor coil (max: 0.12 uH) is added in parallel with capacitor 266 pF in between.

I wonder what is the use of LC circuit in frequency measurement.

From practical experiments, I found that adjusting the inductance affected the stability of the measured frequency value in E1420B.

Could anybody please explain to me the reason why such kind of LC circuits are added in frequency measurement?

Why can't I just directly connect to the Frequency Counter?

They call such circuit "Sniffers"

Regards

Reply to
Myauk
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I would suggest that the circuit is a band-stop preventing the

2nd harmonic from misleading the frequency counter.
Reply to
Geek

A capacitor in parallel with an inductor forms a resonant circuit but also could be used a band pass filter

Check here

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Put in Frequency: 16.367 (MHz) and Capacitance: 266(pF)

The result is 355 Inductance: (nH) = which equals .355uH

There may be other stray inductance an capacitance on the board not accounted for in the values yo mentioned

Joe

Reply to
Joe G (Home)

The TCXO is probably generating a square wave and therefore full of harmonics. A frequency counter has difficulty measuring the fundamental frequency in that case. The network is providing filtering of the fundamental frequency you are trying to measure.

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©
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Reply to
RFI-EMI-GUY

Thank you all for your kind explanations.

Now I am able to figure out the use of LC circuit in the test system.

Regards

Reply to
Myauk

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