Verifying the IIP3 of a mixer

Measuring harmonic products with a Spectrum Analyzer (SA) requires understanding of the artifacts created by the SA. One of the well known artifacts is intermodulation distortion create by the non-lineair mixing of two signals in the first mixing of the SA. The dominant products are IMD2 (second order) en IMD3 (third order). IMD2 increases with 2dB for every 1 dB signal increase and IMD3 with 3dB for every 1 dB signal increase.

The key number for a mixer is the third order Intercept Point (IP3) where (theoretical) the signal and the harmonic products become equal in strength. The first mixer of my home build SA is supposed to have a input IP3 level of about 10dBm.

To measure this we need two independent signal generators generating equal streng signal with somewhat different frequencies, then combine the two signals and use a switchable attenuator to set this as input signal level for the SA (see above picture for " Key 3rd order terms" )

You need two measurements with different input levels that both show the wanted signals and the  unwanted harmonic products

First two signals at -30dBm, resolution filter is set to 15kHz (still not computer controlled)

The input signals are at -30dBm and the IMD3 products are at -77dBm, so -47dB below.

Now increase the input level to -13dBm

The signals are at -13dBm and the IMD3 at -36dBm, so -23dB below. So 17dB signal increase gives about 41dB IMD3 increase. 

Theoretical an increase of input signal of again 17dB would bring the input at +4dBM and IMD3 at +5dBm but the compression point is around 0dBm so this can not be directly measured

This is 5 dB below the spec of the mixer but not too bad. The current LO drive level of the first mixer may be a bit at the low side.
More measurements to do! 

Tuning a 2.5GHz cavity filter with a home build VNA

While improving the SA I'm building I learned something about tuning a cavity filter.
The VNA I'm using is home build and consists of only 
- 2 x ADF4351 (or one SI5351, range 1MHz till 1.2GHz)
- a resistive bridge
- 2 x IAM-81008 mixers with some passive components (10 dB attenuator at the RF input and some capacitors)
- Arduino
- PC with some software



The black brick is the 10 pole cavity filter supposed to be very good, but when I measured the input impedance I got this.


Then I learned that for good input impedance measurement you have to terminate the output of a cavity filter with 50 ohm.
This improved the measurement.


When tuning the cavities the first cavity determined the reactance of the input impedance so I was able to center the circles better around Z0. 
The other cavities could be tuned to decrease the size of the loops around Z0 and finally resulted in


The first cavity still needs a bit of tuning but the S21 of the filter is now nice and flat
Passband ripple is less then 2dB and loss is also very good. 



This was the result of only one round of tuning so with some iterations it probably can be further improved.
Due to complete absence of any shielding the dynamic range of S21 measurement is limited to about 40dB-50dB. 

The IAM-81008 turns out to be the perfect component for building a quick and dirty GHz VNA