DDS VFO voltage adjustment
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Aaron: Once again, I'm the software guy, not the one who designed the board; that was done by Farrukh and he is better able to answer these types of questions. That said, I do know that the pots can be used to change the drive on J3. The fact that you are getting 5W out suggests that maybe it's being over-driven, since it is a 3W transceiver. I'd back that down a little, as I do not know the power rating on the final transistor.? Farrukh: any ideas on this? 73, jack, W8TEE From: "aaronhev@... [SoftwareControlledHamRadio]" To: SoftwareControlledHamRadio@... Sent: Monday, April 11, 2016 6:19 PM Subject: [SoftwareControlledHamRadio] DDS VFO voltage adjustment Jack, and all... As you know, I'm in the process of putting together a very detailed construction manual for the radio which will be handed out to everyone who takes on the project as part of the class in our club. I wanted to ask you about the voltage adjustment of R1.? I've been putting my prototype of the radio through some serious testing the past few days. My goal is to also use the DDS VFO to drive some of the older transmitters I have in my collection. But for the 49-er, I see that you indicate that J3 should be used to connect the 49-er to the DDS board at J6. If I'm understanding your instructions correctly, J3 is supposed to have a 4v peak-to-peak voltage on it via the buffer amp on the DDS board. This voltage can be adjusted with R1.? As it stands now, I'm not seeing any difference in the performance of the 49-er both in receive and transmit by using either J2 or J3 on the DDS board. According to the schematic, J2 is coming directly from pin 20 on the DDS through C2, while J3 goes through the buffer amp and supplies the 4 volts voltage. I attempted to view the output voltage at J3 but saw absolutely no difference when I adjusted R1. In fact, with either J2 or J3 feeding the 49-er, I still had 5 watts output power on the radio. Can you please clarify exactly where this voltage measurement would be taken? I simply took it directly from J3 and ground but saw nothing but a few milliwatts of DC voltage regardless of the rotation of R1. Thank you and best 73s. Aaron - N2HTL
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Got this today: Hi Jack, There are two pots on the DDS VFO board. R1 adjusts the GAIN of the RF buffer amplifier and hence the AMPLITUDE of the sine wave output on J3. R8 is for calibration of battery/power supply voltage sample into Nano analog input A7. If we add the code for A7 analog input, we can display the power supply voltage as part of the initial splash screen. If the J3 output sine wave amplitude on a DDS VFO board does not change from 0 to 4V pk-pk when R1 is adjusted something must be wrong with the buffer amplifier circuit. It is also possible the person is not aware that R1 is multi-turn (>10 turns) pot, and they are only turning it one rotation. There are no hard stops at the end of the multi-turn travel range of R1. Only very soft clicks. So the person might keep turning it towards 0V position without realizing it has reached the 0V position, unless they are looking at J3 output on a scope or DVM. 73, Farrukh From: "Aaron Heverin aaronhev@... [SoftwareControlledHamRadio]" To: SoftwareControlledHamRadio@... Sent: Monday, April 11, 2016 10:17 PM Subject: Re: [SoftwareControlledHamRadio] DDS VFO voltage adjustment Thanks for the relay, Jack. And therein lies the rub. Rotating R1 has no effect whatsoever on the output... but I'm not thoroughly convinced that I tested the output voltage at the right spot on the DDS board. Having Farrukh's input on this would be a huge help. I'm actually experimenting with this now... documenting everything as I go.? And did I tell you I LOVE this thing? :-) Aaron - N2HTL On Mon, Apr 11, 2016 at 9:52 PM, Jack Purdum jjpurdum@... [SoftwareControlledHamRadio] <SoftwareControlledHamRadio@...> wrote:
Aaron
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Good to hear! That was one of the design goals that the three of us shot for when we started the project. As I mentioned in the article, we wanted a VFO that would be capable of powering older rigs, which tend to need more drive than modern rigs. Dennis did the original design for the Arduino Projects book and Farrukh expanded it with a second buffer. The result is a pretty functional VFO for a variety of rigs. Glad you got it working. Jack, W8TEE From: "Aaron Heverin aaronhev@... [SoftwareControlledHamRadio]" To: SoftwareControlledHamRadio@... Sent: Tuesday, April 12, 2016 10:58 AM Subject: Re: [SoftwareControlledHamRadio] DDS VFO voltage adjustment Thanks for the reply, Jack. I successfully made the adjustment to 4 vDC using an RF probe and my VTVM. And Farrukh is absolutely correct... you need to give R1 quite a few turns in order to see any sort of variance on the meter.? Once I had R1 adjusted for 4V, I decided to take this experiment up a notch. I took the output of J3 and fed it into my Globe Chief Model 90 transmitter. I dialed up the DDS VFO to 7.106 and keyed the transmitter. Adjusting the oscillator tuning on the transmitter was a bit different that if there was a crystal in rig, but once I had a peak, I switched to transmit and keyed the rig. Wow! It loaded up perfectly to its full 30 watts out. I tried various frequencies up and down the CW portion of the 40m band and it worked flawlessly. If it wasn't for the fact that I need to get ready for our radio club's hamfest this evening, I'd be making contacts all afternoon! So not only do we have a really sweet QRP rig, but we also have a digital VFO for use on crystal-controlled transmitters. It doesn't get any better than this. Aaron - N2HTL On Tue, Apr 12, 2016 at 8:54 AM, Jack Purdum jjpurdum@... [SoftwareControlledHamRadio] <SoftwareControlledHamRadio@...> wrote:
Aaron
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