Friday, November 6, 2015

S9 signal generator Updated 12/1/15

While waiting for some parts for the Spectrum Analyzer, I was looking around for a quick project.

There was a post on the QRP-tech yahoo group about the NorCal S9 Signal generator
http://norcalqrp.org/files/NorCal_S9_Assy_V1.pdf

 








This is very close to the circuit for the Elecraft  XG2 Receiver Test Oscillator

http://www.elecraft.com/manual/E740084%20XG2%20Manual%20Rev%20F.pdf


This looked like something that might be useful for testing the Spectrum Analyzer after I get it finished.  I decided to build a similar unit, using parts from both design.  Looking at the NorCal circuit, I saw that they used  20 and 14 db pads instead of the single 34 db pad used in the Elecraft model.  I went with the NorCal circuit and added switching around both pads to have options of 50, 10, 5, and 1 uv. outputs. 


I want to put the signal generator in an Altoids tin, so I used the same template as my AD8307 power meter as the basis for the board layout.  I have found that I get more consistent etching of the ground pour if I do it as a hatch instead of solid.  I plan on using  2 row .1" header strips with jumpers instead of switches, so I placed extended solder pads on the board.  This way I can just bend the bottom part of the header pins out to the side and solder directly to the top of the board.

Now to etch the board after I get back from a hamfest tomorrow.

 11/8/15


Well it was a little damp and chilly at the Hamfest, so I didn't stick around as long as I usually would.  This gave me plenty of time to etch the circuit board.  For SMD boards I have been using the blue press-n-peel transfer paper to eliminate etch through, and I get a cleaner pad for the SMD components.  I also let the boards sit in some "Liquid Tin" solution for about a minute to give a nice tin coating to the traces.  This seems to solder a little better than the bare copper.


I built the oscillator portion of the board to test how much output I was getting.  From the Elecraft document it looks like the oscillator should put out about -50 dBm before the attenuators.  Hooked mine to my home-brew AD8307 power meter and got a reading of -48dBm.  Checked the voltage to the oscillator and had about 1.6 volts, I might try adding another diode in series to drop the voltage  and see what that does to the output. 
Now on to the attenuators,  I am missing a one of the values for the 20 db pad but will try with the  closest value I have for now.  Will replace with correct value when the assortment of low value SMD resistors I have comes in. 

Since it is raining out I decided to finish the board and add the attenuator  components.  With the attenuator pads out I measured -68.2 dBm on my power meter.  I added another diode in series with the existing diode, this dropped the voltage to the oscillator to 1.07 volts.  Measuring the output with the additional diode I measured a value of -72.8 dBm.  This is more than close enough to the desired -73dBm output level of the Norcal S9 generator.



Only have two crystals in the circuit for now (3.561 and 7.030) will add others as needed.  Listening to the output on a receiver. The 50uv gives about a S7 reading, readings for other levels are below S1 but I can hear definite changes in the signal as I change attenuator settings.
Unfortunately I do not have any calibrated test equipment to verify the values I found.  But I believe the accuracy should be enough for testing the home-brew equipment I am working on. 



Link to dropbox for Eagle files and toner transfer image.

https://www.dropbox.com/sh/p6jdf2z4cg31ut1/AAD0zL4tcr9kYS_v0HMFjzCza?dl=0


I finished drilling some holes in an Altoids tin and and mounted the circuit board.  I also printed a simple instruction card showing the jumper settings for the different signal frequencies and attenuator settings.  I sized this to fit inside the top cover of the Altoids tin.