Tuesday, May 31, 2016

DDS Buffer

In order to see if I could get better readings on the antenna analyzer, I built this DDS buffer circuit from here.

First go of this I forgot the output cap, and when I hooked it up to my 50 ohm dummy load, I was getting 6 volts peak-to-peak.  I blinked a few times, confused, then checked the transistors.  HOT!

After the cap was added, it gave a more reasonable 3 volts p-p.  Not the prettiest output in the world - the first harmonic was pretty strong - but amplified.

I tried hooking this up to my antenna analyzer circuit and measured the result on the dummy load.

Hard to really see what is going on here, but it is DDS->Amp->Analyzer->Dummy load.  I'm still not seeing what I expect, though - the readings actually look worse.  I guess it could be that strong harmonic, not sure.

Antenna Analyzer

Doing my version of k6bez's antenna analyzer here (warning, pdf).

First, please notice that there are far more resistors on this board than there should be.  I could not find 50 ohm resistors anywhere on my workbench.  I ended up putting four 200 ohm in parallel.  (Also used two 10k in parallel for the 5k resistors, and used a 680 ohm instead of the 648(?!?) ohm he calls for.)

As a test, I just put the output of my VFO into this, measured the voltages out of the opamp with my DVM and calculated the VSWR in a spreadsheet against a couple of different resistor values.  Immediately noticed that where I should be getting 1:1 on a 50 ohm load, I was getting like 3.4:1.  Started poking around and found some cold joints in my bridge circuit.

Testing again, I get happier results.  However, they still aren't what I'm expecting.  I would expect a 50 ohm load to have a 1:1 reading, but is either 1.3 or 1.1 depending on what method I use to calculate (it should be (fwd + rev)/(fwd - rev), but I've seen people use the square of the individual values as well).

I saw some discussion on the Yahoo Group where someone was asking about the accuracy of the numbers.  They talk about the nonlinearity of the diode in there, and I also noticed in his slides that he talks about needing more power for accurate swr readings.  Maybe I'll make an amplifier next.

As a side note, something in my setup seems to be sucking down power.  Nothing is getting hot, but my setup browned out while I was testing.  Found the 9v battery I was powering this all off of was down to 7-ish volts.  I put in a new one, and in a few minutes it was down a volt, too.

Tuesday, May 17, 2016


You know when you have a project where you thought "I'll just throw some things together and see how it goes" and it works amazingly well on the first try?

I guess there is a first time for everything...

In any case, I got an AD9850 in the mail the other day, and I wanted to have a go at playing with it.  I found a bunch of write-ups of people doing this, but this one looked really well thought out.

I started out trying parts of his program, just to make sure I (a) understood it and (b) wired things right.  I started with the lcd and got hello world working.  Then I took his code and commented out most things to get the rotary encoder part working.  Then I wired up the whole thing and loaded up his code.  The only issue I had was that the for me, the include for the rotary code needed it spelled "Rotary.h" instead of lower case r.

After playing around with the step settings, etc, as he suggests in his writeup - I immediately got treated with a beautiful sine wave!

Encouraged, I started pulling the VFO components from my DC receiver I put together the other day.  I put the output of the DDS into pin 6 of the NE602 via a 10pF cap.  You can see the messy result here:

I excitedly turned on everything and attached a long wire to the antenna...  and I immediately heard code!!  The receiver is definitely crunchy (not enclosed, no filters, questionable antenna, etc), but it is super easy to tune with the digital VFO.  So excited!!

I'll probably play with this it in this form for a few days, but already trying to decide what to do next.  Thoughts include a bpf for the front end, some sort of narrow cw filter, and a simple agc circuit.  Oh, and maybe getting it off of a breadboard and into some sort of actual enclosure.

I want to thank Jan Ciger for sharing his awesome project!

Monday, May 16, 2016

Direct Conversion Reciever

I've been in a holding pattern on some other radio projects, so I decided to start another one - the "IC BASED DIRECT CONVERSION RECEIVER" from section 1.5 of EMRFD.  (Chapter 1 that has this project is online here.)  This is your basic ne602/lm386 combination you find lots of places.  I built mine for 40m.

The variable capacitors are little polyvaricons.  I got these cute little dials for them, but still need to find some screws to hold them in.

I threw some wire out the door and was able to hear some stations!  The thrill of hearing anything on a radio you built yourself is *awesome*.

Unfortunately, it is quickly replaced by the annoyance that it isn't doing *quite* what you want.  For one, it isn't quite on the frequency I want.  I made a rookie mistake and mounted the variable capacitors on veroboard (it was late and I was excited to be close to done...), which seems to have added a bit of capacitance.  I also found I had a wrong cap in somewhere.  So, I haven't quite got it to the 7-7.125 MHz range I was going for.

Also, it is *really* hard to tune.  I think a third padded down cap would help a lot, but getting it right on a station took a lot of practice.  Even as I got the hang of it, it was kind of harsh to listen to, and the whining as you got close to a station got old quick.

All in all, I'm counting this as a huge success.  But I don't think I'm going to keep it as is for long.  I got an AD9850 off of Ebay the other day that I think I'm going to try in place of the tuned circuit here.  Partially because I want to play around with the DDS, partially because the little variable caps here are fiddly.

This is my first Manhattan-style build, and I looked at a *lot* of examples before starting.  I need to give huge props to KK6GXG - his build was a huge inspiration.