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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!?

?

I did some further measurements comparing a IAM81008 with a ADE-11X passive mixer in a typical SA setup
Configuration was:

IAM81008 -> 434MHZ filter-> AD831->10.7MHz filter->log detector
and
ADE-11X->20dB amplifier->434MHZ filter-> AD831->10.7MHz filter->log detector

The LO leakage is comparable
The IAM81008 setup produces 10dB more noise and has an 10dB lower input IIP3 level so in total 20 dB less spur free dynamic range

I also tested the AD831 in the same configuration

AD831?-> 434MHZ filter-> AD831->10.7MHz filter->log detector

But the AD831 looses 20dB with the 1st IF at 434MHz versus the 1st IF at 110MHz
So the AD831 is a nice mixer but do not use it with an output substantially above the indicated IF limit of 200MHz

Christian,

Excellent? overview "H mode" mixers, thanks!

I am considering the ADL5801, seems to be a good active mixer, a bit more noise compared to a passive mixer.
The main reason I am looking to an active mixer is not having to design a diplexer between the first mixer and the 2.5GHz cavity filter.
In the current setup without a diplexer the performance of the passive mixer I tried (SIM-83+) is not as expected.
I tried with 10dB attenuators between LO and LO port or between mixer output and cavity filter and with a buffer amplifier between mixer and cavity filter but that did not make a big difference, maybe something else is wrong?

Eric,?

I prefer building my equipment with dead bug style, even when working with SMT components. The schematic, photos and some notes are enough.?

Christian W.?
HK4QWC

On Mon, Feb 4, 2019 at 11:14 AM, <erik@...> wrote:

The PCB should not be difficult to make but I will/can not do it.
Is this group interested in building dead bug style from a schematic and some photo's using SMT components?

YES

Hi all,

anything that can inspire others to do builds is great in my opinion. Even though I have a boat anchor VNA I might build something similar but simplified to get a portable antenna analyzer.

/Marcus, SA5PMG

I'm making progress with a 400kHz till 1.2GHz VNA design using a SI5351 breakout board and less then 10$ of other components but building will required the usage of SMT components otherwise the upper frequency is below 100MHz. The PCB should not be difficult to make but I will/can not do it.
Is this group interested in building dead bug style from a schematic and some photo's using SMT components?
Or only if a PCB is available?

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
- 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?

The IAM-81008 is a bit better than the good old SA612 and it also goes much higher in frequency

After some measurements a simple comparison

AD831:
Usable dynamic range (from noise output level till output 1dB compression) about 100dB
Output iP3 at output 1dBC about -40dB

IAM-81008:
Usable dynamic range (from noise output level till output 1dB compression) about 80dB
Output IP3 at output 1dBC about -30dB

A good level 7 balanced diode mixer has about 10 dB more usable dynamic range compared to the AD831 with somewhat lower IP3? at output 1dBC

Of course active mixers need much less LO power and are easier to impedance match so its easier to build something but for high performance there are better options so I guess its back to the drawing board for my SA and to all passive mixers

I found this wonderful spreadsheet to do the calculation (See attachment)
It really helps to understand alternatives for certain stages (like mixers) and where to put your amplifiers
Example output


Anyone suggestions for even better tools? Like a true (free) System Level modelling and simulation tool?

The pc software is visual c++ using .net objects. The graphing is using the standard .net chart object.
Much faster then drawing the chart with lines yourselves and much more funtionality. Of course some things are not possible and you have to accept

It may be worthwhile to use a 6dB attenuator between mixer LO port and low pass filter. That way, the LO port always sees 50 ohms. It requires 6dB more LO signal, but gives 12dB more isolation.?
All the ports of a mixer should be considered as both input and a output port. Even tho you use a gilbert cell mixer, using attenuators to improve the isolation should help.
Remember that shielding is important when doing a project like this, feedthrough capacitors and coax connectors help keeping the unintended signals out of the spectrum analyser.?

What graphing library are you using? it looks great!? This is a interesting project and I will be following along.?


73 de Thomas LA3PNA.?

On paper this seems to be usable as 2nd mixer in high IF system, IP3 only 3dB but 8dB gain, usable from DC to beyond 2GHz, NF=17dB
Guess not good enough for 1st mixer

Any experience?

The PJRC stuff is pretty good:
? ??

The ATMega328P normally runs on 5v in Arduinoland, but is also rated for 12mhz max
when run from 3.3v, this thread says does fine at 16mhz operating from 3.3v.
? ?
So if you take a cheap Nano clone and short across the LM1117-5 regulator, it should
just work when you feed it 3.3v at VIN.? ?
? ???? ??
Could still re-program using the USB port (powering from the host, so will then operate
the processor at 5v, have nothing attached to VIN), or could program it at 3.3v using
an external USB-to-UART-3.3v dongle into PD0,PD1 instead of the CH340.
If not planning to use the CH340, might remove the 1k resistors from TxD, RxD
to be sure it won't interfere.
?
Jerry

Due and Zero are clearly dead as a Dodo, any other 3.3Volt arduino compatibles?