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Amplitude Modulating a DDS VFO




 
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Author Topic: Amplitude Modulating a DDS VFO  (Read 5417 times)
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Patrick J. / KD5OEI
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« Reply #25 on: March 27, 2017, 07:28:53 PM »

This is a great topic! Thanks to all and please keep advancing it!
It could be useful as a signal generator as well, many uses!
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« Reply #26 on: March 27, 2017, 10:45:43 PM »

THIS!!! and the NPL for the Flex Steve did make a basic, awesome, hi fi building block.

--Shane
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W2PHL
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« Reply #27 on: March 31, 2017, 08:48:33 AM »

You're right Shane. Steve's circuit should be perfect for this gadget.

I've been measuring the IMD incorrectly.  I was looking at the 2nd order level instead of the 3rd! I throw myself on the mercy of the court. In a thread on measuring distortion, WD5JKO posted a very helpful spectrum pic with notes. Thank you Jim!
The pics below are from an Elad FDM-DUO. The FDM-DUO uses Elad's SW2 software which has a built in measurement function (who knew?)
This DDS is much cleaner than originally thought. The top pic is the barefoot DDS. The bottom pic is using the TS450s amplifier deck at 50watts PEP.


* DDS (2).jpg (208.67 KB, 1280x960 - viewed 115 times.)

* DDS TS450s 50PEP.jpg (202.72 KB, 1280x957 - viewed 96 times.)
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W2PHL
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« Reply #28 on: April 22, 2017, 10:26:29 AM »

Here's a link to a neat and super cheap amplifier kit.
http://www.ebay.com/itm/45W-SSB-Linear-Power-Amplifier-DIY-Kits-For-Transceiver-Radio-HF-FM-CW-40db-Q3Q0-/401193735950?hash=item5d6902970e:g:x74AAOSw8gVX44bI

Scroll down the page for the schematic and links to the manual.

It doesn't have an output filter so watch out for the harmonics. I've been using it to drive an Ameritron Al-1500. I imagine any amplifier with tuned input filters will be sufficient to suppress the harmonics. Untuned/ broadband input amplifiers will probably explode if you use this little guy as a driver.

Another issue is the lack of a temperature compensated bias circuit. The gain increases about 3-4dB as it gets warm.
But hey...for $15 bucks....

Phil
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Steve - K4HX
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« Reply #29 on: April 23, 2017, 11:03:55 AM »

Most linear amp input tuned circuits only have a Q of 2. That's not going to reduce the harmonics much.
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W2PHL
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« Reply #30 on: April 23, 2017, 01:08:29 PM »

Thanks for the heads up Steve.
The spectrum view of the unfiltered driver amp is obviously full of harmonics but the spectrum view from the output of the AL-1500 looks the same whether using the unfiltered driver amp or a filtered one. It passes muster without a problem but I've only tried it on 75m. Perhaps the AL-1500 will complain at higher frequencies and/or the output will become dirty.
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Steve - K4HX
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« Reply #31 on: April 23, 2017, 07:14:18 PM »

The output network of your amp should kill any harmonics. The amp may be doing some "extra" work by amplifying the 2nd harmonic. It's probably not a big deal though. How many dB down is the 2nd harmonic coming out of your driver amp?
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W2PHL
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Phil


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« Reply #32 on: April 24, 2017, 02:03:46 PM »

 With the DDS VFO and the ebay amp transmitting on 3.875 MHz, 7.75MHz is 35dB down and 11.625MHz is 8dB down. Running that combo through the AL-1500, 7.75 MHz is 44 dB down and 11.625 MHz is 54 dB down.
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Steve - K4HX
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« Reply #33 on: April 30, 2017, 02:09:42 PM »

Good to go!  Grin
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KA6MOK
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« Reply #34 on: May 09, 2017, 10:22:07 PM »

Gang,
Over the past few years I've been having fun experimenting with cheap DDS signal generator boards from overseas.
>
>
>
I made a video so you can see and hear how Hi the Fi is....

Phil

Wow, that is some VERY Hi Fi...  great results!!    And a nice range of modulation you ran there,  I was liking that too!   Grin

Been meaning to get off the fence and buy one of these,  would be a nice RF sig gen as well as a good core VFO.

So,  I know it's probably been a while,  but could you say which vendor you bought it from?  There seems to be at least 2 or 3 out there now, with varying prices.  I know they're likely a clone design, even the PCB's look..  well...  similar at least if not identical.  Wanting to avoid a 3d or 4th generation knock off of a good working design,  as well as YMMV a lot with vendors on how quickly it shows up once you pay them.

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W2PHL
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« Reply #35 on: May 10, 2017, 08:38:22 AM »

I've purchased several of these VFOs from two different vendors, tomtop_w and kissbuyer. Delivery took about two weeks from either seller. All of the VFO boards I received are identical. My guess is these things are surplus from a radio that was discontinued or never manufactured.

BTW I'm waiting on 4x3 keypad module to try with the VFO. The schematic shows pins for a keypad. Direct frequency entry would be Cool

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