Re: Output Impedance?

Asadullah Mir

Hi Nick!

 I too was wondering how theS22 is +- 50Ohms.

On the other hand I am wondering why was an MMIC used in the first place. I can get equally good results if I directly use the 9851 output. A match with a 4:1 balun or in my case UNUN does the impedance match.

Any idea why an MMIC was used? I just adds to harmonic and other distortion.


I think some more explanation is called for.

I guess you might remember that many moons ago I pasted some pictures of SNA I built from scratch. I was considering posting it to the group but wanted to improve is somewhat and be ready to answer questions regarding the HW/SW.  I could not get around to doing it due to some bit of illness. ( It was all assembly so hard to remember the details)  During this time I decided to learn some embedded C and as an exercise to rewrite the SNA SW in C. I also did not like the slow serial connect to the Graphic LCD ( GLCD!) etc. S0 I decided to use a 3.2 inch GLCD. This one has a 16 bit parallel data interface so the old 32 pin IC would not do and I switched to a more modern C8051F020 from Silicon Labs.

I was looking at the output of the AD9851 and decided that I don't really need the MMIC. I have a few AD9851 boards with the filter built in and decided that a single ended signal take off would do.

I have used a 200 Ohm R on the output of the AD9851 board removed the filter and used a 4:1 Ruthroff UNUN The output is over the top. Enough to do any old testing of networks.

The output level is calculated as follows. ( excluding the attenuation of any built in LP filter. I have removed mine)

The Peak I out is 10mA.  The 200 ohms at the output and the 50 ohms reflected as 200 ohms at the output of the SNA = total 100 Ohms. The 10 mA x 100 ohms gives a peak of 1000 mV and 0.707 V rms output which reduces to .707/2 =353 mV rms at the SNA terminals ( UNUN). This equals +4dBm

Enough for my needs. The out impedance is 50 Ohms. I intend to build a few small external filters for use in different applications. Sweep scan is variable from 480 Hz to 50 MHz. I think the fastest  scan is 300mSec. Slowest is several seconds.


I will try upload the pictures and schematic and SW as soon as I am happy with the details. It is written in C except a few fast routines for the GLCD  in Assembly.  I have used no library routines as  all is home brew. and the SW is as efficient as I could make it. This is an 8051 U-controller so some people might not be familiar but it is the best uC  in the world. ( In my view, that is)

I think the real DIY guys should find it interesting.



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