80/40m band pass receiving filter

25.07.2008

So here we are, this is a picture of the "test filter" :

As you may see, I added 68 pF capacitors in parallel of the 680pF on the 40m filter, this sharpens the bandwidth, while increasing slightly the insertion losses.

Corresponding schematics :(measured capacitors on 40m were around 620 pF;..Adding 68pF brings close results than when using 680 pF capacitors directly )

This is the curve I should obtain:

And this is the curve I obtain :

OK, the image is not that good,
- Vertical scale is 7 dB / cm
- Horizontal scale is 800 KHz / cm.
- Calibrated reference level is -20 dBm

But here is a summary of the results

80m band filter real performances:

Minimum insertion losses @ 3.741 MHz : -1.8 dB
IL @ 3.5 MHz : -14.8 dB (I'm not CW op...)
IL @ 3.6MHz : -7 dB
IL @ 3.7 MHz : -2.3 dB

-3 dB BW : [-145..+250 KHz] referring to 3.741 MHz
-6 dB BW : [-185 +300 KHz]
-12 dB BW [ -250 +365 KHz]
-20 dB BW [ -375 + 465 KHz]
-40 dB BW [[ -920 +780 KHz]

40m band filter real performances:

Minimum insertion losses @ 7.073 MHz : -2.2 dB
IL @ 7.0 MHz : -2.5 dB
IL @ 7.1MHz : -2.3 dB
IL @ 7.2 MHz : -2.9 dB
--> the response is quite flat over the 40m band actually.

-3 dB BW : [-250..+290 KHz] referring to 7.073 MHz
- 6 dB BW : [-370 +385 KHz]
-12 dB BW [ -560 +560 KHz]
-20 dB BW [ -830 + 805 KHz]
-40 dB BW [ -1.88 +1.8 MHz]

With only the "620 pF" (theoretically 680...), the rejection above the central frequency was less good :
-3 dB BW : [-250..+345 KHz] referring to central frequency.
- 6 dB BW : [-357 +473 KHz]
-12 dB BW [ -560 + 675 KHz]
As a trade-off, IL increased from 1.8 up to 2.2dB...But I thought the slope was not sharp enough on high frequencies for the 40m filter...

Next steps:

- Properly place the component on a adequate board (this was just a demo board for "tuning" !!!!!!!!)
- Start the fabrication of the preamplifier !!!!!!!!!!!!!!!!!!!
With a calculated 21-22 dB gain, this will compensate for the my insertion losses, HI !!!!!

73
Ronan