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About 2nd harmonic measurements

Question asked by wangyiliu99 on Jul 18, 2013
Latest reply on Sep 3, 2013 by wangyiliu99
Hi, all,

sorry for asking question again.

This time, it is about the second harmonics measurement.

Please refer to the attached jpg file for the measurement setup.

Some fundamental explanations.

SG generates the signal at frequency f0, then use low pass filter (LPF)to filter out the SG 2nd harmonics, isolator to reduce the reflected signal, hence to stabilize the SG output. 2dB ATT to improve impedance matching. SA measure the 2nd harmonics at frequency 2*f0. Further, SA and SG 10MHz source is locked together.

In the attached file. (a) is for the setup; (b) is for the DUT measurement. 
in (a), SA is carefully tuned, RBW=1Hz, SPAN=100Hz, input attenuator is optimized to reduce the SA mixer distortion. (input attenuator increased in its 2dB step, until the 2f0 harmonic signal level varies within 1dB).
in (b), SA setting is the same with (a), DUT inserted and do the measurement. 

DUT is passive, with its insertion loss being 0.5dB at f0, attenuation at 2*f0 is 15dB. 
LPF(LC type) insertion loss at f0 is 1dB, attenuation at 2*f0 is 55dB.

Here is our measurement result:
In setup (a), harmonic level at 2*f0 is -65dBm
In setup (b), harmonic level at 2*f0 is -80dBm

My questions:
(1) Normally, as for harmonic measurement, the setup should have greater performance in order to do the DUT measurement correctly. In my measurement, I found the DUT result better than the setup. Does this mean that the setup is not well configured/optimized?

(2) From the measurement results, it seems that the DUT attenuation at 2*f0 (15dB) exactly matches the setup (a) and (b) difference. Therefore, in setup(b), I replaced the LPF with two identical LPFs and expected to have improved results, however, I only see the 2dB improvements, which is due to the LPF loss (1dB x 2 = 2dB)

Your help would be highly appreciated!