Saturday, April 15, 2017

The SA becomes the SNA jr.V.3 for now UPDATE 4-17

I have been working on the Spectrum Analyzer, and had hoped to get it done in time for the FDIM  'homebrew' contest. Between wrong parts shipped , or not arriving in time, along with software problems it did not look like I could get it finished in time. So on to plan B.  I have gotten great response for the SNA Jr version 2, and even quite a few from people who have built their own version.  The most common requests for changes, are a larger screen, and higher frequency coverage.  I have the nice 2.8" screen and basic display routines from the SA, and I still have a couple Adafruit si5351 modules left from several other projects. The si5351 would give me frequency coverage to over the 2 meter band, but it has a square wave output.  From early experience in a simple K6BEZ style antenna analyzer I built a couple years ago, I had found problems with harmonics from the square waves. 

Looking around the web, I saw some information on an antenna analyzer by IW2NDH.  It used two clocks on the si5351, one at the test frequency, and the second offset by the frequency of a simple crystal filter.  This second clock is fed to a mixer, and the output goes through a crystal filter before being measured by an AD8307 LOG detector. This should remove the harmonic problems I had found with the earlier circuit.  It also had a built in directional coupler, instead of using an external RLB like I used with the SNA Jr II.  I made a few changes and came up with this block diagram.

Looking at the block diagram, I realized by just using the second clock, the mixer and crystal filter, it could also be used as a basic measurement receiver.  This might be adequate for measuring the harmonic output of a home brew transmitter.  

Basic functions to include
SNA  1 to 150 Mhz.
Antenna analyzer (SWR only)  1 to 150 Mhz.
Measurement Receiver  1 to 150 Mhz. there will probably be some unusable area around the IF frequency I select.

Since I still have some problems with some of the libraries for the stm32 board, I will go back to the old reliable Nano.  This will mean adding some level translation between the Nano and the display, and having a slower update on the screen.  But, I will live with that.  I added a couple of relays to change functions and to add a switched input attenuator.  I went with a 3 crystal filter, and added pads for several attenuators that I might need to properly terminate the filter and mixer stages.  I laid out and after a couple of tries I etched a single board that should do the job.  

The software is coming along nicely, I could use most of the display routines I had for the stm32 board, and also pulled in some of the code from the SNA Jr II.  Still going with the joystick instead of a rotary encoder for the input device,  It is much faster and easier to use than the encoder.


Here is a picture of the partially populated board being tested with just the display and si5351 connected.  Things looks fine so far, so I will probably have something for FDIM

UPDATE  4-17-17 

I had a little trouble getting the si5351 library to work after I copied some of my earlier code into this sketch.  The version I had been using is almost 2 years old, and the newer version has some major changes.  After making them most things looked OK except the frequency was off.  My frequency selecting code uses a resolution of 1 Hz., and after re-reading the documentation for the latest version of thesi5351.h file I saw that it has a resolution of  0.01 Hz. Just a quick multiply by 100 from the computed value to the value used to set the 5351 frequency took care of that. 

I needed to make a directional coupler, and used the instructions for making a VNA directional coupler at   www.oh6hgn.net/Miten_kaamia_VNA_muuntaja.pdf
Except for the number of turns through the core, this is basically the same as I used in my SWR/Power meter.  After building it, and installing on the PCB, I made some measurements to check how it worked.  
The small AD8307 power meter probe I built last year worked great for the job.   The difference function I added to the software made it very easy to check the coupling loss. I measured a  unloaded output level of around 12 dBm. in the center of the frequency range. Depending on frequency I measured around 15 - 18 dB. coupling loss from the through signal with no load,and around  40 - 44 with a 50 ohm load.  These values look usable, and probably are affected by the harmonics in the square wave, and possibly an impedance miss match on the input to the coupler.  I built in pads on the board for a small attenuator before the directional coupler.  I will solder in resistors for about 6 dB. and see if that makes a difference.
I checked the AD8307 log amp output with a voltmeter, and the values look very similar to what I found with other power meter circuits I have built.  Next to copy some of the code from the SN A Jr 2 software to get that working and do a simple sweep of the output of  the si5351.  Then to build the mixer/filter circuitry and test them





2 comments:

  1. Very interesting project Duwayne. Are you planning a switched bank of LFPs after the oscillator to eliminate harmonics?
    / Gerry

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  2. No, not planning on using LPFs. When doing a sweep the signal sent to the DUT and LO frequencies are both changed to maintain the IF frequency of the crystal filter. This way, only the fundamental frequency is measured.

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