Saturday, October 31, 2015

SPECAN 2nd IF filter UPDATED 11/4/15

Got back to work on the spectrum analyzer, with the 2nd IF filter board.     Revised 11/4/15

 This is two filters that are relay switched.  The narrow filter about 1Khz uses 9 crystals and has two in line amplifiers between each set of 3.  The wide filter should have about a 400Khz bandwidth and is comprised of 5 LC networks.

I did a board layout using mostly SMD components. During the layout of the narrow (1KHz.) filter, I found it would be easier to use leaded 2n3904s instead of the smd components.  Just too lazy to redo everything I had already finished to use the smd part.


I had ordered a batch of 30 12MHz. crystals, and using my SNA  to match them, I got 9 that were within about 100 Hz. of each other. 
After building this half of the board I decided to test and see what kind of a frequency response I had.

 Using the SNA jr. to check the bandwidth,I found the signal at just a few KHz. below 12MHz.  Narrowing the sweep range I found a nice peak and a bandwidth  of about the 1KHz. required.  From everything I have read, for a Spectrum Analyzer filter you want more of a peaked response than that for a SSB filter.  So this response looks like it will work.  After I finish the board I will check the responses with the Chinese SNA  that has  options for measuring bandwidth.  But, the SNA jr. is so easy to use on the bench it is what I usually go to when building.

Next to do the wide bandwidth filter.  I have not decided If I want to wind some toroids, or use some molded RF chokes.  If I use the chokes I had had added places for paralleling a couple of smd capacitors to tune to the correct frequency.  With the toroids I can try  to adjust spacing of the turns to help tune the filter elements.

I wound some toroids and tried to tune the segments by adjusting the coil spacing on the toroids.  This turned out to be a large pain, and I could not get them adjusted to the bandpass I wanted.
I had some 1.8uH molded RF chokes and some small 60pF trimmer capacitors that would fit on the board.  I  had placed pads for additional capacitors if needed, but found that I could tune the segments without adding any additional capacitors.

After finishing this filter I added the relay switching circuitry.  The relays I had were through hole, but I bent the leads and trimmed the length so I could solder directly to the elongated pads I had on the circuit board.

After completing I checked the bandpass filters.  The narrow crystal filter was about 1.1 kHz at 6dB. and the wide band filter was 456 kHz. at 6dB.

Just need to solder some .020" circuit board material as a shield around the assembly and on to the mixer stage.

1 comment:

  1. Nice build. Have you finished?

    I wonder what insertion losses you got on the two filters?