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Verifying the IP3 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!?

?
 

The root cause for the mismatch in ip3 turned out to be a wrong distribution of IF gain over the total chain. This reduced the 1dB compression point so a input of -10dBm resulted in -13dBM.
After correcrion the IP3 measured at the spec level of 10dBm.
 

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.
 

Thomas.

Thanks for the excellent feedback. I am still learning.
The mixer i am investigating is the first mixer of the SA.
I have a step attenuator before that mixer to indeed check if the IMD is generated by that or later mixers or before the attenuator.
I have used ADIsym to calalculate where to have gain (if any) in the SA as any unbalance either makes later mixers the cause of IMD or noise just as you stated.
All three (level 7) mixers in the SA have about equal perfomance so I hope it can work out by just compensating for conversion and filter loss between them, max 9 dB, and have the rest after the last mixer. The log detector with 120dB range for sure is not the limiting factor. Would you not lose SNR without any amplification between the mixers?
For the compression point I use the step attenuator and step up till the step becomes less than exectly 1dB. Your AM modulation trick is something to try.
There is still so much to learn and unfortunatily I learn best by doing things wrong and then try to understand why somthing works different from theory
 

Erik,
If you are investigating the first mixer, it would need variable
attenuator after that (tack it in, its temporary) to be sure that you
are testing the IMD of that mixer, and not the chain behind it.
Level 7 mixers may not be strong enough for use in a Spectrum
analyzer, W7ZOI used 17dBm mixers in his SA, and those are a bit too
weak. I buildt a copy once, that now goes to 1.8GHz. I guess mixers
are a part that is fairly easy to change once you are sure that the
rest is working.

You should be optimizing the gain in-between your stages like you
explained. 9dB sounds reasonable, accounting for some 6dB mixer loss
and 3dB filter loss. A additional 3dB or 6dB pad on the output of the
mixers may reduce several of the unwanted products you may see, by
terminating it into 50 ohms.

Having no gain between the mixers will reduce the SNR, or more
correctly stated, it will increase the noise figure of the spectrum
analyzer. As you may understand, this is a series of trade-offs, and
knowing what to trade is the difficult part.

Keep experimenting, and understanding what the changes you do means,
that is how science is advanced.

73 de Thomas.

s?n. 10. feb. 2019 kl. 21:17 skrev <erik@...>:


Thomas.

Thanks for the excellent feedback. I am still learning.
The mixer i am investigating is the first mixer of the SA.
I have a step attenuator before that mixer to indeed check if the IMD is generated by that or later mixers or before the attenuator.
I have used ADIsym to calalculate where to have gain (if any) in the SA as any unbalance either makes later mixers the cause of IMD or noise just as you stated.
All three (level 7) mixers in the SA have about equal perfomance so I hope it can work out by just compensating for conversion and filter loss between them, max 9 dB, and have the rest after the last mixer. The log detector with 120dB range for sure is not the limiting factor. Would you not lose SNR without any amplification between the mixers?
For the compression point I use the step attenuator and step up till the step becomes less than exectly 1dB. Your AM modulation trick is something to try.
There is still so much to learn and unfortunatily I learn best by doing things wrong and then try to understand why somthing works different from theory
--
With Best regards, Thomas S. Knutsen.

Please avoid sending me Word or PowerPoint attachments.
 

Thomas,

Thanks again! To test I removed the amplifier after the first mixer to get 9dB attenuation. Results are the same so I am happy.
W.r.t the level 7 mixers. I did not know the importance of IP3 when I started with my build and second, they where comparable cheap. Did some investigation and found some cheap ADE-42MH mixer that match the required frequencies and are expected give a IP3 of 17dBm. Going above that will require some serious cash and I will wait with that till I have ironed out all the other unknown's, like how to put the SA in a box? and shield the mixers against 25MHz leakage from the LO XCO reference, for the rest the SA is clean with noise floor at -105dBm with resolution bandwith of 300khz/30kHz from 100kHz till 1.8GHz. I also need to build a narrower resolution filters (if the ADF4351 phase noise will allow) but I'm contemplating why not do all resolution filtering from 30kHz and below using a final IF at 80kHz into the audio in of a PC and the rest in DSP. My home build VNA is already using a I/Q amplitude/phase detector in audio DSP so it should be doable. But my current log detector has 120dB dynamic range and even with a 24 bit audio input I'm not sure if I can reach 120dB. Still so much to learn.
 

Thomas:

This technique looks interesting. Would you mind sharing a source with details?

Thanks.

Christian W.


On Sun, Feb 10, 2019 at 07:03 AM, Thomas S. Knutsen wrote:
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.??
 

The basic procedure is outlined here:

There is no need to use that given modulation meter, the important
part is to look at the change of the demodulated output. This can be
obtained with a diode detector and read with a oscilloscope. The
important part is that you calibrate to find the detector constant at
low levels, so it does not compress.

C# code for the above meter is here:


73 de Thomas LA3PNA.

Den ons. 13. feb. 2019 kl. 07:40 skrev Christian W. Correa <hk4qwc@...>:


Thomas:

This technique looks interesting. Would you mind sharing a source with details?

Thanks.

Christian W.
HK4QWC


On Sun, Feb 10, 2019 at 07:03 AM, Thomas S. Knutsen wrote:

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.

--
With Best regards, Thomas S. Knutsen.

Please avoid sending me Word or PowerPoint attachments.
 

Thomas:

Thanks for sharing the procedure and your source code. I added both to my reference library.

Christian W.
?
On Wed, Feb 13, 2019 at 10:05 AM, Thomas S. Knutsen wrote:

http://hpmemoryproject.org/an/pdf/an_286-2.pdf

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