Transceiver Architecture 2.15
News Flash --- The LM373 SSB transceiver is on the air and the 1st contact was with W4DNQ, Ron who is in Florida. The contact took place at 1525 PDST on May 11, 2017 on 14.220 MHz. Here it is "al fresco"
Lets Build the 11.5 MHz Crystal Filter ~ Part III
I re-measured the 9 candidate crystals using the G3URR oscillator only this time using the 1 Hz position for fine tuning on my SDR transceiver. The field is now down to 6 crystals with five of those being exactly the same frequency in the loaded state and the 6th is only off by 5 Hz. As we used to say when I worked in Aerospace --Close Enough for Government Work!
So now I was ready to go back into the Dishal Software and using the new data, I re-plotted the curve for 6 crystals. Again the setting of the Band Pass ripple materially affects the Z in/out and the capacitor values. Less of a ripple translates into a Lower Z in/out. So there has to be some reasonability (trade-offs) between Ripple and Impedance. Below is the new plot and we will now highlight some of the factors.
First and foremost is that it is a 6 pole filter that is characterized by steeper slopes and a 3dB bandwidth of 2.35 KHz. The band pass ripple is 0.13 dB (pretty flat) and the Impedance is 127.4 Ohms. 127.4/50 = 2.55 and a 8 turn to 5 turn broad band transformer is 64/25 = 2.56 ==so really close.
There are 9 capacitors needed for the 6 pole filter and in looking at the values these are easily achieved using high quality (NPO) caps in parallel with very small air trimmers (0-15 PF which were provided to me by a kind ham located on the east coast --Thanks Bob!)
An old time filter measure was to compare the 6 to 60 dB "shape factor". The BW at 60 dB is only 5.43 KHz --so that should do well with those California Kilowatts located just down the street from me. BTW the math shows 1: 2.225.
My next steps will involve acquiring some capacitors and building the actual filter. Stay tuned.