Tuesday, July 28, 2015

"Canned Frog" part three UPDATED

Now that I had the VFO and controller completed, it was time to start putting the transceiver circuit board together.  This is a very nice double sided board with plated through holes, and good silkscreen on the top for parts placement. I started with the receiver portion, and checked that I had good audio to the headphone jack.

Since I am making this cover the whole 7Mhz band, I had to replace the single crystal input filter with a double tuned band pass filter. I wired this up on a small piece of perf-board, and tack soldered SMA connectors so I could easily connect to the SNA Jr.

video

    I could have used the Chinese SNA for this, but I decided to go with the SNA Jr.  I like to use the SNA Jr. for quick alignments because it is small and self contained.   Do not have to connect to my computer and run the software on there,
I set the frequency range from 5.5 to 8.5 Mhz.to put 7 Mhz on the center graticule., and set the SWEEP mode to FAST.  This mode is a reoccurring display instead of a one shot like the other SWEEP modes.  Took just a few minutes to adjust both tuned circuits, and the coupling capacitor.  Much faster than other methods I have tried in the past.


Now to put it in the circuit, connect up the VFO signal and see what I get.  After I connected everything I found that the receive level seemed to be a little low, and there was a constant tone on the signal.  I quickly rewired the input filter as a single tuned circuit, and that brought up the signal level.  Tuning across the band, I decided it would be adequate selectivity for normal operation so will leave it as is.  Looking at the values for the input filter, low-pass filter on the transceiver, and low-pass I put on the VFO board it would be no problem converting this to 10 Mhz. operation.  Just different tuning of the input filter and changing frequency settings in the Arduino code.  I had included a couple of extra Arduino pins when I did the circuit board, so could even add a relay and with a little code change make it a 2 band rig.

The other problem I found was the constant tone. Since I was using a separate transmit and receiver oscillator, I found that I was picking up the transmit signal all the time.  A little change to the code to toggle  the transmit  oscillator off  when receiving took care of that.  With the selectabel tuning step size it is easy to cover the whole band, and switch to the smaller step size for smooth fine tuning.  There are virtually no PLL lockup clicks when tuning in the smaller step size. The only time I heard any was when I made large frequency changes, then a quck click or two when the software switched to a different PLL divider setting. The RIT function works well, and since there is no limit to how far you can tune off the transmit frequency, it acts like true split mode.

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