KitsAndParts.Com Forum

Bseres · 2012-02-19 21:35:15

Hi
I'm new to this forum, and Kits and Parts, but have ordered the 5 watt amp and low pass filters from Diz. I have also ordered the N3ZI dds Vfo and want to mate the two for a nice little qrp tx. Has anyone tried this or played with this Vfo? Do I need a buffer between the Vfo and the amp? Has anyone had experience keying the Vfo? I plan to start by building the tx and using my ten tec 1254 as the rx, and then move on to some kind of dc receiver or simple superheterodyne in the same box as the tx using the same Vfo. This will be the rig for our summer cabin. Looks like Diz has some nice building blocks, and any advice or info forum members could give would be most welcome.

Tnx,
Brent
VE3CUS

ak2b · 2012-02-20 11:18:30

Bseres wrote:
Do I need a buffer between the Vfo and the amp?

H Brent,
From the pongrance website, my guess it that you would need a buffer. Diz's 5 watt amp needs about 0dBm. According to the specs on the N3ZI vfo it puts out 300mv into 1K load. With the buffer shown you should get between 2 and 3 volts p-p output which is about +10 dBm. You can drop this down with a 10 db pad.
By the way, I use DL5SWB's handy dB calculator for all things db.
Good luck,
Tom, ak2b

ae4gm · 2012-03-08 20:38:48

The DDS VFO I just received from N3ZI says it puts out 250 millivolts. I'm using that voltage to drive the 5 watt cw amp from a buffer I built for my Heathkit HG-10 VFO. 0 dBm is 225 millivolts. I've been working on a toroid transformer to go from the 200 ohm impedence in the N3ZI manual out of the DDS VFO to the 50 ohms needed to run the 5 Watt CW amp. I've never done this before, so don't just make up what I'm putting here and hook it up before you know it's right. I'm using the formulas on the link
www.dxing.info/equipment/impedance_tran … bryant.doc
plus information on toroid constants, etc. in the 1986 ARRL Handbook. I'm coming up with 70 turns primary to 35 turns secondary on a T50-2 toroid. Like I said, I've never done this before, so be aware of that before you use my information. I'm having an Elmer look over my findings before I start winding it. If someone sees that I am wrong please let me know so I can take off this post and get a better understanding of what is needed.

Bseres · 2012-03-08 21:57:39

225mv is only 0 db if driving 50 ohms. I found I could not drive the amp directly from the dds, but it seems to work ok using buffer circuit on the N3ZI website. I'm still testing though... I didn't look at actual number of turns on your transformer for your core material, but your 2:1 ratio is right for a 4:1 impedance step down. Remember, though, that the secondary voltage will only be 1/2 or around 125 mV. Across 50 ohms, that would be around .3 mW or around -4dbm. Hope my math is more or less right...don't have a calculator. Good luck!

Brent
VE3CUS

ak2b · 2012-03-09 08:12:07

bseres wrote:
225mv is only 0 db if driving 50 ohms.

225mv 'RMS' is 0dbm into a 50 ohm load. 225mv p-p is -9dbm into the same load. On the Pongrance url that I gave above, Doug states that
"The voltage is rather low (~300mV P-P) but it drives active mixers (e.g. SA612) with no problem." The input impedance to an SA612 is about 1.5K. One of the important things about the simple buffer described at N3ZI's site is not only the gain but the output match to a 50 ohm load.
One of the reasons that I recommended DL5SWB's db calculator is that you can easily switch from peak to peak, peak or rms voltage calculations for dbm. On the bottom of the first tab (U --> dBuV, W, dbm) you can also change the load. Of course, some may not need this type of tool. For me though, I use it all the time

Tom, ak2b

ae4gm · 2012-03-09 16:55:15

I don't see the SA612 buffer on the Pongrance website. Can you give me a link to that page?

Bseres · 2012-03-09 20:57:01

Thanks for the info on th db calculator. Looks great!

ae4gm · 2012-03-20 09:45:11

I have the N3ZI DDS VFO BUFFER AMP finished and working. I'm running the KITSANDPARTS 5 WATT CW AMP and it's doing FB. I ordered some opto-couplers, but for now I'm using a 12 volt relay to key the buffer output to the 5 watt amp and switch from main frequency to rit frequency on the DDS VFO. Thanks for all the help from here on the forum. That's for 40 meters right now. I used more toroids from Diz to make a 30 meter 5 element low pass filter for the buffer output to go to the KITSANDPARTS AMP. I still need to wind 2 more toroids for the KITSANDPARTS AMP low pass filter output for 30 meters.

ae4gm · 2012-03-20 10:26:57

I forgot to mention this. My N3ZI DDS VFO Buffer amp puts out 700 millivolts RMS. The Kitsandparts amp is looking for 250 millivolts. I have the buffer going into another buffer I built to run off the Heathkit HG10 VFO and adjusted it from there. I recommend padding the output of the N3ZI buffer down to the correct 250 millivolts.

Bseres · 2012-03-24 17:07:39

Could you just use a trim pot on the output of the buffer?? Im still figuring out how to 'box' everything..

ak2b · 2012-03-27 20:03:47

bseres wrote:
Could you just use a trim pot on the output of the buffer??

Funny thing. Diz just made a suggestion to Ian to check out the MiniPig 10 meter transceiver as a first project.
I went there myself and while I was reading Diz's notes I saw that he made mention of being able to adjust the transmit power. So I looked at the schematic and saw the same/similar buffer as the URL I gave (above) for the Pongrance website.
Diz shows his solution in the schematic (look at Q7 and Q8).

Tom, ak2b

Bseres · 2012-03-30 05:18:54

Looks good. Exactly what I had in mind!

Bseres · 2012-04-01 19:34:39

Well, everything seems to work! I will post some photos of the transceiver when I have it propery boxed up. Using junk box relays for semi qsk right now, so I must build a proper t/r switch. Has anyone successfully used opto isolators on the rit line of the n3zi dds? I tried a 4n25 and it seemed to pull the line low even with the opto input off. Leakage current? Do I Need a pull up on the rit line?

KitsAndParts.Com Forum

#1 2012-02-19 21:35:15

n3zi dds

#2 2012-02-20 11:18:30

Re: n3zi dds

Bseres wrote:

#3 2012-03-08 20:38:48

Re: n3zi dds

#4 2012-03-08 21:57:39

Re: n3zi dds

#5 2012-03-09 08:12:07

Re: n3zi dds

bseres wrote:

#6 2012-03-09 16:55:15

Re: n3zi dds

#7 2012-03-09 20:57:01

Re: n3zi dds

#8 2012-03-20 09:45:11

Re: n3zi dds

#9 2012-03-20 10:26:57

Re: n3zi dds

#10 2012-03-24 17:07:39

Re: n3zi dds

#11 2012-03-27 20:03:47

Re: n3zi dds

bseres wrote:

#12 2012-03-30 05:18:54

Re: n3zi dds

#13 2012-04-01 19:34:39

Re: n3zi dds

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