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 Post subject: Multi DDS Project for the shack.
PostPosted: Jul Wed 15, 2020 5:06 am 
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Several weeks ago I bought a heath HO-10 station monitor scope, and an HO-13 pan adapter with a 455KC input. I converted them to a VERTICAL CABINET ORIENTATION, so they would fit nicely on my ops bench shelves.

The Project.
Since, I bought the pan adapter to use with several different BA receivers having various first I.F. frequencies, I think the simplest way to accomplish my goal is to make a Block Converter and use one of the E-Bay DDS units to provide the L.O.. My idea is to keep it simple and use an AD-602 Gilbert cell mixer. Of course just making a simple a simple converter quickly became a unit with two DDS modules. One to provide a VFO function with enough drive to run old Crystal controlled transmitters, and the the second unit providing the L.O. for the Block converter.

While doing a little research, I happened across an article which outlined a method for amplitude modulating a AD9850 DDS IC. So I'm going to look at incorporating that function into the mix as well. If that works out I'll have a little A.M. exciter too.

HERE are the schematics. I've built up the Buffer amplifier for the VFO, with somewhat pleasing success.

Click the underlined text link above. Please look things over, and let me know what you think.

First edit; Changed the link.
Second edit; Added this text.

Thanks

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Last edited by Mikeinkcmo on Jul Sun 19, 2020 2:15 pm, edited 3 times in total.

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 Post subject: Re: Multi DDS Project for the shack.
PostPosted: Jul Sat 18, 2020 1:05 pm 
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Your link isn't terribly clear about the DDS module you're using - it appears to be a clone of the the Ham Radio India design? - but, for AM modulation via Rset à la Analog Devices Appnote AN-423, you really do need to take the output from both complimentary outputs, IOUT and IOUTB. The DDS board you show doesn't seem to allow for that without modification (I know the HRI design doesn't).

Otherwise you end up with an asymmetrical 'half-wave rectified' output as shown in Fig 1 of AN-423, for the reasons explained in paragraph 2 of the appnote.


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 Post subject: Re: Multi DDS Project for the shack.
PostPosted: Jul Sat 18, 2020 7:28 pm 
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Hello Rod, Thank you for the reply.

I appreciate the information regarding the APP NOTE. The solution seems simple enough, just another opportunity to help keep the economy going. :wink:

The DDs module I have is this one.
Image

Thus far The plan is to build in a Die cast box, with components arranged something like this. I'll mount the box on a wood wedge, and a piece of 1/2" steel plate cut to size, with rubber feet, such that the panel slopes back from vertical about 15-30 degrees. The plate is, of course, to hopefully make the unit "stay put" while operating the controls.

Image

Mike

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 Post subject: Re: Multi DDS Project for the shack.
PostPosted: Jul Sat 18, 2020 11:09 pm 
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This looks like a very interesting project.

I wonder about that Rset pin’s function on the AD9850/51. The AD data sheet sure seems to imply that your AM modulation scheme with just resistors will work.

Have you found a good reference somewhere that sheds some light? And once you have some hardware working, it would be nice to see what actually happens when you check the AM modulation.

In any case, thanks for sharing!

I don’ t understand why the AD app note implies that a transistor is required. Is it possible that the app note author did not really know how his own company’s IC works?

Quoting the AD9850 data sheet: “[Pin] 12, RSET, DAC’s External RSET Connection.

This resistor value sets the DAC full-scale output current. For normal applications (FS IOUT = 10 mA), the value for RSET is 3.9 kΩ connected to ground.

The RSET/IOUT relationship is IOUT = 32 (1.248 V/RSET).”

IOUT, above, is the current at the output pins of the IC, usually into a low impedance, or a small resistance.

Since the Rset pin’s voltage is fixed at 1.248V, this formula means that the output current is 32 times the current sunk from the Rset pin. This should result in nice AM modulation of the RF output by an audio voltage signal fed through a resistor connected to the pin. As with any AM modulator, you have to set a steady state idle carrier level. Your circuit does this by having a resistance from the pin to ground.

From the data sheet, it looks likely that this pin is a low impedance node driven by some sort of internal feedback to a reference voltage ( +1.248V) It does not appear to have the RF signal on it, as the ap note shows an RF bypass capacitor from the pin to ground.

Probably the idle carrier level must be set to about ½ the level you get with the specified 3.9k resistor, to stay within the limits of linear operation. This would mean a 7.8k resistance from the pin to ground.

If I were doing this, I would probably try a resistor from the audio coupling capacitor directly to the Rset pin. Another 7.8k resistor would give 100% modulation with a 1.248V peak audio-in level. Just a little over 0dBm.

And, I don’t see why both outputs must be summed. I think as long as no attempt is made to exceed the IC’s maximum RF output level, each output will have a usable modulated RF signal. The scope screenshots in the app note include the low frequency modulating AM signal in the “bad” single output pin signal, while stripping the audio for the “good” signal after summing in the transformer.

However, if both output pins really must be utilized, the op amp circuit can be modified to do this.

Ted

The Spare Time Toy Factory schematic:
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The AD App Note modulation circuit:
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The AD App Note output waveforms:
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I believe that you can get A from B by merely passing B through a low pass filter to strip the audio frequency component.

For those following along, the AD AD9850 data sheet is here:

https://www.analog.com/media/en/technic ... AD9850.pdf

The AD product page / overview is here:

https://www.analog.com/en/products/ad98 ... t-overview

Another link to the AD app note touting use of a MOSFET to drive the Rset pin:

https://www.analog.com/media/en/technic ... AN-423.pdf


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 Post subject: Re: Multi DDS Project for the shack.
PostPosted: Jul Sat 18, 2020 11:20 pm 
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I tried using the RSET input to amplitude modulate one of these, and I didn't have very good results. I found that for the amount of circuitry required to get barely acceptable AM, it would have been easier to run the output of the AD9850 through a separate modulator circuit, and that's what I ended up doing. Just to be clear though, my external modulator leaves much to be desired too, but it was good enough for my signal generator application and used no more parts than doing it with the RSET. I'll probably revisit it sometime in the future and come up with something better.


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 Post subject: Re: Multi DDS Project for the shack.
PostPosted: Jul Sun 19, 2020 3:56 am 
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I ordered 4 pcs (8 bucks apiece) of the transformer mentioned in the app note. Its a little 6 pin dip package.

I'll copy the "driver", and see how it goes. I'd be inclined to think the result will be pretty much "as advertised", we'll see.

Possibly an op-amp biased at around 2.25 Volts, with back to back diode strings for a clamp, unity gain, and connected through a 1K or smaller resistor, would work. Simply using an external programming resistor implies the control of a current mirror in the chip, so as with the FET, some resistance is needed in the drive circuitry. The 510Puff bypasses the RF at that node, and will roll off the audio response at some point, but I suspect it'll be far above voice freqs..

Something like this.
Image

Another interesting AD note is AN557.

The buffer/amplifier in my schematic is one proposed by Jim Hagerty, published in the June 2011 issue of CQ Mag. Interestingly the output transistor stage is quite similar to that in Analog Device's App Note.

I've done some preliminary testing using only the output buffer, and the output levels are similar to those with the 1227 IC, so I'm not sure the IC is needed. Results may be better yet with the use of the transformer Rod suggested.
=======
Soldered one of the interconnect cables, going from the Mux to the DDs module, at the Mux end using some computer ribbon cable on 50 mil centers. Using a piece of buss wire as a clamp at one end.

Image

This is my first time personally building with such small devices and it is a bit of a learning curve.
======
I also repaired one of the DDS modules, which early on suffered from over voltage, taking out the 9850. The new chips came today, so got out the solder paste and had at it.

Took a while but I goterdun! I'm beginning to think its time I learned to use the PCB part of Express PCB software.

Image

Image

=====
The "microscope" came in yesterday, so this is my first time using it, and it performed quite well. The only issue is that I need to make/buy a very low profile clamp of some sort to hold items securely on the "stage".

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 Post subject: Re: Multi DDS Project for the shack.
PostPosted: Jul Mon 20, 2020 5:55 pm 
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Very nice project! What a handy utilitarian piece of kit to have on hand.

Just an FYI: the LM2402 triple driver can also create signals in the 40Vp-p range allowing modern DDS or PLL synthesizers to emulate a vacuum tube VFO.

Thanks for sharing your excellent project,
Coop, aa1ww


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 Post subject: Re: Multi DDS Project for the shack.
PostPosted: Jul Tue 21, 2020 12:33 pm 
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Hello Coop, and thanks for the suggestion, I'll look into it. First look is VERY interesting indeed.

It certainly would change the power supply requirements, but from a practical point of view, I think cable length/drive issues would quite possibly disappear, or at the very least be somewhat mitigated.

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 Post subject: Re: Multi DDS Project for the shack.
PostPosted: Jul Tue 21, 2020 3:16 pm 
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What microscope is that Mike? Do you like it?

Pete

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 Post subject: Re: Multi DDS Project for the shack.
PostPosted: Jul Tue 21, 2020 11:34 pm 
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Coop,

It seems the 24XX ICs are obsolete and have been for some time. The only place I found them is on my old stand by, E-Bay at two bucks apiece. Got 4.

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 Post subject: Re: Multi DDS Project for the shack.
PostPosted: Jul Wed 22, 2020 12:38 am 
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I never fully completed mine. I just got sidetracked with other projects. The cabinet is from an old spectronics digital display. I removed the three push button switches from the top and have placed the keypad there, I need to make a wedge shaped mount for it so you can easily see the numbers on the pad.

Mike, I’m interested in your buffer amp design. Mine does not have enough output to drive my Heil pine board transmitter.


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 Post subject: Re: Multi DDS Project for the shack.
PostPosted: Jul Wed 22, 2020 7:46 am 
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Pete asked,
Quote:
...What microscope is that Mike? Do you like it?...


Hi Pete, I got it HERE.

I wish the bottom end of the Mag range was a bit lower, but its OK for what I'm doing. The LCD rotates, which they don't show. It has a 7" display which was bigger than most.

Need to make some sort of low profile vise/clamp to hold things steady when soldering.

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 Post subject: Re: Multi DDS Project for the shack.
PostPosted: Jul Wed 22, 2020 2:21 pm 
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Hi Mike,

I just wanted to chime in on the low profile vise for the microscope stage.
I purchased this one several years ago and quite like it.

https://stickvise.com/

It sits very low and close to the working surface which is great for hand soldering. Its easy to flip boards over (springs let the jaw open) or just pick up and reference the the other side for through hole etc... Its easy to position in the field of view. Its got just enough weight and surface area to keep everything stable while working.

From a maker/DIY perspective its got a myriad of jaws you can print if you have access to a 3D printer and many people have built the entire vise themselves. If you follow this link and scroll through the images you will see a version where they have added the stick vise directly to the microscope stage.

https://hackaday.io/page/5464-running-list-of-diy-stickvise-builds

Here are some photos of mine on the bench.
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good luck with the rest of your project,
Eric


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 Post subject: Re: Multi DDS Project for the shack.
PostPosted: Jul Wed 22, 2020 10:35 pm 
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Hi Eric,

THanks for the pics and links. Got one on the way.

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 Post subject: Re: Multi DDS Project for the shack.
PostPosted: Jul Wed 29, 2020 9:17 pm 
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Well, I got the "stick vise", Cool little tool. Thanks Eric!!

Ordered and got the transformers from Mini-circuits that were called out in the App. note Ted (Usually Lurking)mentioned in an earlier post, and am in the process of installing it.

Analog Device's AN-423 App Note.

It's A LOT bigger than the other components on the DDS board, but I found how I can mount it on top of the "band select" output transistors using some clear, Hi-Dollar caulk. I attached wires to the two complimentary output pins and ground, and tomorrow, after the caulk has had time to sufficiently cure, I'll clip the transformer leads, and attach the wires from the I.C..

Those wires are a single strand from a piece of 26 Ga stranded wire. Pretty small stuff. I used some solder paste and my trusty ole Weller soldering Iron to connect them to the I.C. pins. Delicate work for sure.

Image

Well, the caulk didn't adhere to the transformer so I used some JB Weld.

Soldered the wires to the transformer and added a 50 Ohm load per the APP NOTE and now the output looks pretty nice. I'll connect the transformer's output to the board's final amplifier and see what the output looks like.

Image

Image

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 Post subject: Re: Multi DDS Project for the shack.
PostPosted: Sep Tue 15, 2020 2:01 pm 
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Haven't posted here for a while so I thought I'd do a little update. I finally got all the switcher supplies in, wired them up, finished the Keyboard assembly, and did some work on the heat sink.

Finished wiring and testing the keyboard/Mux assembly and all is O.K..

Image

I got my SMB connectors today, so I'll be working on getting the LM2402 Amplifiers working in the next few days. Don't know if they will eventually work out, but they certainly have the bandwidth to do the job, and if the output is pretty flat across frequency that'll be a big plus. I did get the three amplifier outputs DC biased at half VCC and they draw a couple Watts each.

Image

The little open board switchers I got are pretty slick, I strung the output of a pair of 48V 300mA supplies in series and built up simple regulator for the +80. The 12V supply is a tiny little thing about one cubic inch in total, and it squeezes out 400mA according to MPJA. Its able to run one of the synthesizers and it looks like it may be able handle both synthesizers, plus the keyboard.

Image

The output of the DDS is pretty clean and the output level is about 0dBm. According to the FFT on the scope almost everything is 50dB down. The grid is 10dB or 200mV depending on which trace you're looking at, the center frequency is 5MC, and the displayed signal is 3.9MC.

Image

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 Post subject: Re: Multi DDS Project for the shack.
PostPosted: Sep Wed 23, 2020 7:35 pm 
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Quote:
Mike, I’m interested in your buffer amp design. Mine does not have enough output to drive my Heil pine board transmitter.


Yes, thanks for posting...this is lighting a fire to finish my own 9850 VFO project...

The microscope and clamp information was very helpful.

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 Post subject: Re: Multi DDS Project for the shack.
PostPosted: Oct Sun 04, 2020 9:07 pm 
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Hi Jeff.

A little update since my last post.

I've been doing some CAD drawing, lay out, and metal work. Got the front panel, heat sink, and box more or less done with the exception of paint and some labeling. This is as far as I can go till I get the TX DDS modulation and "VFO" drive figured out.

Image

Had to cut some Heat Sink material to fit the box, and used my table saw to do the task. I've cut aluminum a number of times on the saw and never had any problems with the cut or blades. I use a Carbide blade and feed the material fairly slow. Needless to say you need to keep a firm hand on the stock.

Image

Image

Image

Image

The DDS module comes with tact switches that have long actuating stems, and I want to keep and use them, so I had to re-configure the hardware stack and eliminate the inter-board connector. This meant I needed another tool, darn, so I could precisely cut some new spacers from Brass tube. The idea worked out pretty well, and there is still room to incorporate the mods needed to modulate the unit.

Image

I spent a couple days trying to calm the 2402s down without much success, so I may well stay with the original op-amp transistor configuration for the VFO. If so, the 80V supply is gone.

For the transmitter option, I'm looking at one of the little 70W amp kits on the bay, but I don't yet know how much drive they take. This switch means I need about 10+ amps of 12-14 volts, so I added a connector for an external Switcher, as there's no room to put one in the box.

Image

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