Sunday, May 12, 2013

Wish I could reveal more, but...

As some of you have been following my blog, I was warned by Jack (AI4SV) that I should not reveal TOO much about my oscillator before I submit my design for publication in QST and QEX.  As a result, I will likely only show photos - no schematics of the work that has been done and no PICAXE programs.  Sorry for the inconvenience this may cause.  

I know there are some on the PICAXE Forum that are clamoring for how I can program a DDS using a PICAXE as there is nothing published.   I can assure you that I have successfully programmed both the DDS-60 oscillator and the AD9834-based FCC-2 kit using my interface(albeit with significant noise margin issues).  This whole effort started because of the sad reality that the FCC-2 PIC-chip program design died with the passing of the FCC-1/FCC-2 designer.  

While I am at Dayton, I am planning to speak with Hendricks Kits about resuscitating the FCC-2 kit as a standalone DDS oscillator, much as the DDS-60 is used.

In any event - I promise that this publication WILL happen - either through QST/QEX, through another QRP group, or this blog.   Preferably ALL of them.

Thursday, May 9, 2013

PCB - Power Amplifier

Figure 4-1:  First Prototype - IRF510 Power Amplifier
As figure 4-1 above shows, the previous design had the FET laid down with the PCB mounted to the heat sink. It was good for a quick prototype, but is not necessarily the most efficient approach.
Figure 4-2:  Updated Power Amplifier Layout
I looked into a different layout where the FET itself was on the heatsink, while the board it mounted on standoffs at right angles as shown in figure 4-2.  At the same time that I sent in the updated DDS interface, I also sent in this Power Amplifier as well. 
Once I get the amplifier design finished, I will be publishing the schematic for the power amplifier - hopefully in QST or QEX - along with the PICAXE interface design.  At very least, VWS will get the design for one of their tech journals when it comes out.  Figures 4-3 through 4-6 show the broadband sweep of the amplifier.



2nd DDS Oscillator Interface Prototype

The second prototype chassis was a more RF-friendly design where the PICAXE is on its own board with very short traces for the CS (Chip Select),  SPI Clock, and SPI Data between the 28x2 and the socket used to hold the DDS-60.
Figure 3-1:  Rev0 PCB with DDS-60 installed.

Figure 3-2:  Front Panel of 2nd Chassis Oscillator

Figure 3-3:  Top View of DDS Oscillator/Interface
The updated version of the interface has four buttons instead of two.  The Blue and Black buttons are the cursor movement buttons like the previous version, but the Green and Red buttons perform band shifts from 80m>40m>...>10m (Green) and 10m>12m>...>80m (Red).

I laid out the circuit and PCB using Express PCB and had it etched by Far Circuits.  The price was right, but there were some alignment issues between top and bottom side that need to be corrected on the next go around.   I will show the board to them when I visit them at Dayton this year.

Wednesday, May 8, 2013

Follow up - Present Status

At present, I have completed two different fixtures for the DDS Oscillators that I purchased.  

One of them uses the PICAXE AXE401 shield with a socket for the DDS oscillator to be inserted.  The shield was placed inside an aluminum chassis and is held up by insulating standoffs.   Though there are no ground planes in the AXE401 shield, I was able to create a controlled impedance channel to surround the RF path with ground up to the transition into the coax which is pigtailed onto a SMA(F) cable mounted to the chassis.

Figure 2-1:  Chassis assembly with display, tuning knob, and buttons.

The LCD is a White Letter with black background and is backlit for ease of viewing. The LCD is hardwired to the AXE401 because I ran out of jumper length. I got it from MicroCenter in Fairfax, but you can get it directly from The two switches are normally-open momentary SPST switches which are tied high with 10k resistors to 5V. 

Figure 2-2:  Front View of Oscillator with Freq Counter next to it.

The dial controls the frequency while the two buttons move the cursor back and forth.  If the cursor is moved onto the MHz Scale, the band will switch 80m>40m>30m>20m>17m>15m>12m>10m>80m, or in reverse depending on which way the dial is rotated.  As figure 2-2 shows, the display and oscillator are in fairly good agreement.  I later recalibrated the MFJ to match the spectrum display as well.

Figure 2-3:  Front View of Oscillator/Freq Counter - tuned to 10.118MHz
Figure 2-4:  Spectrum of Oscillator Tuned to 30m
Fc = 10.118MHz
Span = 100kHz
Resolution BW = 100Hz
My next Blog post will show the other fixture I designed with my own PCB layout.

First Blog Entry - Transceiver Effort


Back in 2008, I joined the Vienna Wireless Society because of a CW Transmitter Challenge that inspired me to try building a multi-band CW transmitter.   Although I didn't complete the transmitter in time, I did manage to build two FCC1/FCC2 DDS oscillator kits along with power amplifiers based on IRF510 Power FETs used in switching power supplies.

FCC1 alone + FCC1/FCC2 DDS Oscillator
3-30MHz Power Amplifier using IRF510

What I learned during that effort was that there was a LOT more to a CW transmitter than an oscillator and a power amplifier.  I also learned that kits are not well maintained as the FCC-1 and FCC-2 programming was not well documented and the kit designer sadly passed away several years ago.

During my presentation at a VWS meeting last year, I presented my design-by-simulation effort for a CW transmitter based on a different DDS oscillator (DDS-60 - programmed using a PICAXE 28x2.    I had hoped to complete the transmitter by QRPTTF 2013, but a combination of illness and other priorities got in the way.   Nevertheless, I am going to start blogging my transmitter efforts in the hope that others will follow this effort and learn from them - and from my mistakes.