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Re: Verifying the IP3 of a mixer

 

When using a spectrum analyzer to measure intercept point, it is important to use a step attenuator between the DUT and the SA. This way, you can isolate if the Intermodulation products are generated inside the SA or in the DUT.? By adding 1dB to this step attenuator, all the products on the screen should change 1dB. If any of the intermodulation products changes more than that, they are generated inside your SA, and your measurement are invalid.
I spent a lot of time on this when measuring some H-mode mixers some time back. It turns out that when measuring a mixer with Intercept point near 60dBm, most spectrum analyzers don't have the needed dynamic range needed.?

Making sure your generators are a clean sine should be fairly easy, and some 7 order lowpass filters are easy enough to build to clean them up. Clean, low phase noise generators are important as you try to measure higher IMD powers. A couple crystal oscillators with isolation amplifiers are a great way to measure, if you can get the crystals.?

When measuring compression point, it is quite often difficult to find the exact 1dB compression point. Both due to inaccuracies in the analyzer, in attenuators, and variation in power levels from generators. I have started to use a new approach to 1dB compression point in mixers and amplifiers where you take a 30% AM modulated signal and feed through the DUT, then increase the level until that AM becomes unsymmetrical.? Then the 1dB compression point is a simple calculation.??

Spectrum analyzers, like any receiver, I have come to the realisation (Wes W7ZOI may have tried to pound it in) that you should avoid gain before the bandwith determining filter. In a classical spectrum analyser, this is often in the 2. IF. This makes it quite deaf, and you will probably have a hard time seeing anything interesting.? There are some high IP MMIC amplifiers for CATV around that may be worth a look, AP560 from Teledyne comes to mind. The approach taken here:?? Should be a viable one for increasing the?Intercept points of a amplifier, at the cost of a couple 90 degree hybrids.?
Remember that in a typical receiver chain (SA) your 2. mixer needs to have better IMD products than the first mixer, by the gain between them. So if you have a mixer with?+10dBm intercept point in the front end, and a amplifier with gain of 10dB, your 2. mixer needs a IP better than?+20dB. That should be the main reason to keep the gain as low as practical possible in the front-end.? 3. order intercept point in a mixer is what you are fighting, the 2. order products in a mixer is the mixer products.?

73 de Thomas LA3PNA.


l?r. 9. feb. 2019 kl. 18:08 skrev <erik@...>:

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!?

?



--
With Best regards, Thomas S. Knutsen.

?Please? avoid sending? me? Word? or? PowerPoint? attachments.
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