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Extending frequency range question
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vincent coppola
Hi, I am using the Midnight Design solutions DDS-60 with a Rev 8.40 PHSNA board. Besides the DDS 60 and software modification, are there any other hardware restrictions for extending the SNA up to around 115 MHz or maybe even 2 meters? I believe when I first got my system working, it appeared to be operating well around 50 MHz once I got the compensation formulas working correctly. However it has been awhile now and my memory is hazy. I am building the QST W7ZOI SA and of course a 110 MHz bandbass filter with a 2mhz BW is required. It is still under construction but the testing tools are an antique HP-8704A VNA and an HP8601 sweeper. Tantalums caps are old and can self destruct at any time and paper caps are not reliable either. I also have access to a 7L12 SA but without a tracking generator. I am going to experiment with different approaches to testing it accurately. I was just wondering what it would take to get the PHSNA to
cover that range. In addition I believe that the power meter is capable of
operating much higher in frequency, so that should not be a restriction. Please advise or correct me if I am wrong on the above. Thanks, Vince N1VC |
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John Lawson
Vince, good morning....you don't need to modify your DDS60 stuff.....here is what I did back in the day when I faced the same problem at Wes's suggestion. I'm hoping you have a freq. counter that you can use to monitor the frequency up around 110MHz and also have the Power Meter working also.
Reference Figure 7 that shows the 100 MHz Osc. Build it ugly on a slab of PC Board. Important: make sure the 470 pf (C202) cap's leads are extremely short. Build the Osc per the instructions in the article including the 6dB pad. You may have to adjust the turns slightly on L201 slightly to 4 turns to get it to Osc properly 110 MHz. Once built and working with the 50 ohm resistor in place of the crystal, you can now peak your filter for maximum output. There will be "some" frequency drift but you can work around it. Now, once you have the filter tuned at 110 MHz for max output, then you can vary C205 cap to change the Osc frequency and sweep above and below 110 MHz while observing the power meter's needle to see if the slope is correct, etc. In my case I used a panel mounted variable cap rather than installing the trimmer cap for C205 as the one I used gave me more range......the Osc should be able to be adjusted upwards to around 140 MHz or so depending on the cap used for C205 as well as downward to aroun 80 MHz if I recall correctly. If you need additional information let me know off line at jmlcs2000@.........best of luck, John K5IRK |
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You can get to >150 Mhz if you use one of the si5351 modules and software library from NT7S. The output is a square wave but for filter testing at those frequencies harmonics should not be a problem. You can get a module from several places. The cheapest is from Adafruit, around $8.00 for an assembled board. You can also get a kit from Jason NT7S for a little more, and QRP Labs also has one.
Software change over would be very easy. Include the library and replace the send frequency function with the corresponding one from the 5351 library. I changed from a ad9850 to a si5351 in a simple VFO with about 10 lines of code to initialize the si5351 module. If you want to go higher than that you could go to a SI570 based system. Farhan has a SNA on his blog at VU2ESE's radio experiments: Home He also has his updated version of the W7ZOI SA on the same blog. DuWayne KV4QB
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John Lawson
Hi again Vince, Opps, I forgot to mention that I also included U201 the MAV 11 circuitry following the 6 dB pad to get more output for more downward slope readings on each side of the peak.....John
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vincent coppola
Hi DuWayne, and thanks for the heads up on the NT7S library etc. Actually, a few months back I did purchase an si5351 module from Adafruit for another application. It is still in the box. I was thinking of building a stand alone DDS but ended up using an AD9850 based one for various reasons. At the time I was only considering operation up to 40MHz max, or in other words operation thru 10 meters. I wanted to select any frequency on the fly and run on a UNO. I made some slight modifications to AD7C's software he had posted online and it now works well. Credit however goes to Toby for the initial code.
The other question is how do you handle the power meter side of things? Are you just relying on the analog meter or does your idea plot a graph on a PC screen? Please elaborate. Finally yes, I did see Farhan's blog fly by and I agree that might be another place to investigate and/or a route to consider. Thanks Vince N1VC ---In PHSNA@..., <duwayne@...> wrote : You can get to >150 Mhz if you use one of the si5351 modules and software library from NT7S. The output is a square wave but for filter testing at those frequencies harmonics should not be a problem. You can get a module from several places. The cheapest is from Adafruit, around $8.00 for an assembled board. You can also get a kit from Jason NT7S for a little more, and QRP Labs also has one. Software change over would be very easy. Include the library and replace the send frequency function with the corresponding one from the 5351 library. I changed from a ad9850 to a si5351 in a simple VFO with about 10 lines of code to initialize the si5351 module. If you want to go higher than that you could go to a SI570 based system. Farhan has a SNA on his blog at VU2ESE's radio experiments: Home He also has his updated version of the W7ZOI SA on the same blog. DuWayne KV4QB
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vincent coppola
John,
Thanks for the reply. This is worth looking into. Vince |
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You could use the existing PHSNA and bring out A4 A5 5v and ground to an
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external 5351 board. Then change the existing PHSNA code to control the 5351 and use the same graphing function. I have an extra board done up for my SNA Jr. and am going to replace the ad9850 with a si5351 board. Plan to make a converter board for the Adafruit board that will plug into the existing DDS socket. Shouldn't take much to update the code for the 5351 board. Will post info on my blog kv4qb.blogspot.com DuWayne On 9/22/2015 2:18 PM, n1vc@... [PHSNA] wrote:
Hi DuWayne, and thanks for the heads up on the NT7S library etc. |
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vincent coppola
DuWayne,
You have my interest here. I checked out your blog and will also look at the driver source code. If you want to contact me at my email address, it is N1VC (at) yahoo (dot) com. Thanks, Vince |
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alan_r_cam@...
If you want higher frequencies, you need a different generator - or redesign the current one. The AD9851 typically runs at 180MHz, and the "cleanest" sine waves are under 60MHz. Due to the wonders of aliasing, there IS an image signal: this is what the low pass filter is supposed to eliminate. For a higher frequency, go to the Analog Devices website and read up on the AD9958: run with a clock of about 500MHz, produces signals up to about 200MHz. There's currently even a board available on Ebay for about $85 USA. There are even higher frequencies possible, but the chips get rather expensive - and I suspect the design would be more sensitive to noise / layout issues. |
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