A Snow Day Well-Spent: Chris Modulates an AD9850

Feb 10 at 7:40 AM
Hi Bill and Pete,
My work is closed today due to the snow so I wanted to show you what I've been up to.
I built sort of a test fixture a while back for those cheapie EBay AD9850 boards based on AD7C's DDS circuit using an Arduino Uno as a controller.
Last summer someone posted a link to Analog Devices App note AN-423 on QRP-Tech I think.
It was always on my list of things to try.
Armed with a schematic of the eBay DDS gleaned from Doug Pongrance's website, I got to work. It involved cutting a trace or two on the DDS board using an X-Acto knife.
Two outputs of the DDS chip are fed into a wideband transformer as in the app note's second page and I hung a scope probe off the secondary of the transformer.
Basically you are removing the Rset resistor on the DDS board (marked R6 on mine) and using a 2N7000 MOSFET as an electronic version variable resistor and modulating the MOSFET.
This lead to much frustration over the afternoon. No output on the scope!! Did I make an error or connect something up wrong? I was using the 600 ohm output of my HP652A audio generator. I thought about it during lunch and decided to try the 50 ohm output. Bingo!! At the 3V RMS range setting on the generator I now have some kind of signal. Not a nice sine wave shaped AM output but at least something resembling a clipped sine wave. The circuit is really touchy as far as needing a hefty audio signal in. The output of my iPhone at max volume doesn't turn on the DDS chip.
I set the DDS for 1200 kHz and can listen to it on a Radio Shack portable radio. Varying the audio generator varies the received tone on the radio just like I was expecting.
I just need to figure out what's up with the audio levels.
Chris
KD4PBJ

Feb 11 at 7:53 AM

I did manage to reduce the audio a little and get clipping to go away. I'll send another set of oscilloscope shots tonight. 

The only thing I don't get is why the audio level needs to be so high. The app note said 1 V peak to peak should be enough but I am having to drive it with 7 V p-p audio to get a decent waveform with 0.8 V p-p RF. 

I did cut the traces going to pins 12,20 and 21 to the DDS chip rather than pick up the signals elsewhere in the board. This was because once the traces go into vias down in the board, I'm not sure what else they connect to. 

Chris

Feb 11 at 1243 PM

Found the original email from May of last year about the app note. It was on the EMRFD list. After hearing you guys lately talking about AM it has me excited about an AM transmitter. 

I just got a thought over lunch that I'm going to try tonight and that is biasing the 2N7000 gate with a couple of volts to see if I can get the transistor on. Then maybe I don't need as much audio. I'll definitely put a series cap in though to my audio source so I don't fry my iPhone!!!

More to come this evening!

Chris

Feb 11 11:23 PM

I'm so excited!!

That's exactly what I needed to do-bias the gate of that 2N7000 so I'm not depending on the audio to turn it on, only to modulate it. 

I hooked up a 9v battery across a 10 k trim pot and connected the wiper to the gate of the fet. A 10 uF cap is in series with my audio line as not to feed DC back into my source. 

I have audio and now the output of a small radio or iPhone modulates it fine. 

So this is a great proof of concept! Those cheapie DDS modules from China can be AM modulated. 

I need a mic amp now, filter (surgery to the board disabled the on board LPF, but yes Steve I'll install one right away), buffer amp and some kind of linear RF amp to get a few watts. 

Chris 

Here's a video of Chris's project:

 

 

Gravitational Waves, A GREAT VIDEO, Phasing, and Joe Taylor K1JT

Wow, you really have to spend 20 minutes and watch the video (above).  It is really well done.  I loved it.  I give it FIVE SOLDERING IRONS!

And big news today!  They did it!  Gravitational waves finally detected.  Here is a good New York Times article that includes a recording of the signal, a nice NYT video that has a good explanation (with phasing!) of how lasers are used in the massive detectors, and mention of Joe Taylor, K1JT, whose Nobel Prize winning work contributed to this great discovery.

http://www.nytimes.com/2016/02/12/science/ligo-gravitational-waves-black-holes-einstein.html?partner=rss&emc=rss&_r=0

Life in the Fast Lane: Potato Semiconductor Chips

Even I, with my luddite tendencies and analog preferences, have recently bumped up against the speed limit of 74 series logic chips.  The Si5351 chip in the I and Q VFO for my phasing receiver will run up to 160 MHz.   But the 74 series inverters and flip flops that I have attached to the output don't seem to want to go beyond about 120 MHz.   Our old friend Thomas LA3PNA tells us how to break this speed limit:

http://www.potatosemi.com/  

Be sure to go their "Milestones of 74 Series Logic" Page.

I like their explanation of the brand name: 

Single Sideband + Carrier -- The Collins KWS-1

Back on 2 February I was on 160 AM with the DX-100, talking to WA4PGI.  At the end of our QSO we got a call from a station.  I was at first confused -- was this station calling on SSB?  Or was he calling on AM?   Turns out that he was -- in a way -- on BOTH.  K4DBK was -- I think -- running an old Collins KWS-1, the transmit side of the famous Gold Dust Twins.  Aptly named:  It was built in 1955. 1000 watts output.  $2095.00 in 1955.  Gold dust indeed.  

The really interesting thing about this rig was that it put out CW, SSB and SSB plus the carrier.  I think that was what we were hearing from K4DBK.  FB.   

Does anyone have an e-mail address for K4DBK?  I'd like to drop him a line.

http://www.collinsradio.org/cca-collins-historical-archives/the-equipment-of-collins-radio/the-black-boxes/kws-1/

Back to Divide by 4 -- Big Improvement in Receiver Performance

Thanks for all the comments and advice.  I have come to understand the wisdom of divide by 4 IQ circuits.  

Fortunately it was very easy to convert the divide by two 74AC74 circuit described earlier to a version of the divide by 4 scheme seen above.  (From the SDR Ensemble II Receiver:  http://www.wb5rvz.com/sdr/ensemble_rx_ii_vhf/04_div.htm)

This change provided a great way to observe 1) the improvement in the output signals from the VFO and 2) the resulting improvement in receiver performance, especially opposite sideband rejection.

Here are some numbers. I was very pleased to discover that my Rigol scope will measure duty cycle and phase difference. Thanks Rigol!

AD9850 Divide by 4 :  7.212 MHz  Duty cycle: 48.3  Phase Difference:  87-90 degrees

Si5351 Divide by 2:      7.212 MHz  Duty Cycle 49.6  Phase Difference:   83 degrees

Si5351 Divide by 4       7.212 MHz   Duty cycle 49     Phase Difference:  85-90 degrees

Additional improvement came when I switched the power supply to the IQ inverters and Flip Flops.  I switched from 3.3 to 5 volts:

Si5351 Divide by 4       7.105 MHz   Duty Cycle 49.7    Phase Difference:    90 degrees

When I took the VFO box and put it back in the receiver with the divide by 4 scheme and the 5 volt supply I immediately noticed a big difference in performance.  It was obvious that opposite sideband rejection was back to what I had had with the AD9850, perhaps better. 

I have a quick and dirty method of measuring opposite sideband rejection: I put an RF signal into the antenna connector.  I put the 'scope on the audio output.  I tune (on the desired sideband) for 1kHz audio and I measure the output voltage.  Then, with the audio gain and RF sig gen output in the same positions, I tune to the opposite sideband, again tuning for 1 kHz, again measuring audio output.  With the divide by 4 scheme and the 5 volt supply, the opposite sideband was so weak I had trouble measuring it.  I estimate the rejection to be at least 32 db -- this is back in the range of what I had with the AD9850, and significantly better than I had with the divide by 2 scheme. 

Now I just need to figure out how to get the Si5351 VFO sketch to tune above 42.94 MHz.  For some reason it quits at this point, switching down to 2 kHz output, and keeping me on 30 meters and below.

Thanks again to Todd VE7BPO for a lot of help with the hardware and to Tom AK2B for help with the Arduino code.  
   

Ambition, Greed, and Experiments with a Divide by 2 IQ VFO

Fresh from a great success with the use of the M0XPD divide by 4 I and Q VFO in my Frankenstein Phasing Receiver, I got ambitious.  And greedy.  I wanted more.  More frequency coverage.  More bands.  Divide by 4 can really limit your frequency range.  The AD9850 only goes up to 40 MHz.  Divide by 4 and you can't even get the 30 meter band. 

So I started looking at other options.  Si570 looked nice, but here the lower limit was the problem: 10 MHz.   Even with divide by 4,  that knocks out 160 meters, a band I am very interested in lately, and that seems to sound especially good in a direct conversion receiver.

Once again, the controversial Si5351 was calling my name.  It would go down to 8 kHz and up to 160 Mhz.  Woo Hoo!  If I could build a divide by 2 IQ VFO, I could cover 160-6 meters.   

Here is the basic idea.  From:
http://www.markimicrowave.com/blog/2015/04/top-7-ways-to-create-a-quadrature-90-phase-shift/

 The Flip Flops are set up to change state when the input signal is going up.   By putting an inverter at the input of the bottom FF input, in effect you have that one changing state when the input signal is going down.  Look at this for a minute or so.  Look at the square waves at the bottom.  See it?  See how it takes an ordinary signal and spits out two signals, one 90 degrees off the other?  Pretty cool, don't you think?

With lots of hardware help from Todd VE7BPO, and software help from Tom AK2B (wizards both), I got my Si5351 divide by 2 circuit working today.  You can see the resulting I and Q in the picture at the top.   But I am discovering that there may have been wisdom  behind those divide by 4 circuits.  My opposite sideband suppression isn't as good with this /2 scheme as it was with the AD9850 divide by 4.  I'm still trying to figure out why. I may have to go back to divide by 4.  Stay tuned.    

Michael's Log: AA1TJ Has FIVE Contacts with the Unijunction Transistor at 1-2 milliwatts

Michael writes:

Dear Friends, The UJT transmitter circuit was improved considerably today. The power output has increased to 1.48mW and the start-up "whoosh" is now far less objectionable. It's currently running in beacon-mode at 3687.8kHz. I'll resume "CQing" as soon as I've returned from an hour's walk in the woods. I hoping to work K1QO among others. 73, Mike

Added five QSOs today. Seabury/AA1MY is in Maine...exactly 100 miles from my doorstep. It's wild to think that we made a one-hundred mile radio contact on a unijunction.

Beautiful BITX17 Presentation by Chris PA3CRX

This is a really amazing presentation on our beloved BITX rigs.  This presentation takes the viewer from block diagram to schematic to photos of the actual circuits and throws in great graphics showing spectra and filter curves etc. 

There is no sound.

Here is the link in case the embed above doesn't work:

https://prezi.com/yn2loy4mi0wo/bitx20/

Thanks Chris!

A Probable First: First Ever Radio Contact Using Unijunction Transistor as the Transmitter

AA1TJ writes:

I spent most of a week working to raise the RF output power from my unijunction transmitter to nearly 1mW. I was rewarded this evening with two contacts.

Jim/W1PID exchanged (599/449) signal reports with me from Sanbornton, NH (112km) at 2210z!

Dave/K1SWL did the same (589/229) from Newport, NH (95km) some four minutes later!

I should think these were the first-ever radio contacts made using a unijunction transistor as the transmitter.

FYI: my receiver was comprised of a single 1N34a germanium diode mixer followed by a single 2N35 germanium transistor audio amplifier. Great signals on this end.

Wikipedia on Unijunction Transistors: https://en.wikipedia.org/wiki/Unijunction_transistor

New VFO for the Frankenstein R2 Phasing Receiver

I've wanted to change the VFO in my R2 phasing receiver.  The AD9850 DDS VFO with a divide by 4  I-Q generator limited me to 160, 80 and 40 meters.  But an Si5351 chip will go up to 160 MHz.  With a divide by 2 I-Q generator, this should allow me to cover 160 through 6 meters. I got the Arduino, LCD and rotary encoder all working tonight.  Thanks to Thomas LA3PNA for the code, and to Tom AK2B for help with the Arduino. The Si5351 board that you see sitting atop the Arduino is the work of Dean AC9JQ.  Thanks Dean.  

The flip-flop and inverter IC's should arrive this week.  That will allow me to finish up this VFO conversion project.