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Filter Table, DDS-VFO Amplifier Pre-Filter
Choices
- 30 MHz Cutoff (this is actually 0.4 dB
down at 30 MHz). It is a Chebyshev design and is the easiest
to build by far:
Omit C22-C24;
L1, L2, and L3 = 330 nH.; C25,C28 = 120 pf; C26-C27 = 180 pf.
Thanks to Mitchell
Lee of Linear Technology Corp. for supplying this design.
- 30 MHz Cutoff (3 dB down at 30 MHz) Elliptic
Design, Supplied By Coilcraft:
C22 = 15 pf., C23 = 68 pf., C24 = 56
pf.; L1 = 330 nH., L2 and L3 = 220 nH.
C25 = 100 pf., C26 = 150 pf., C27 =
120 pf., C28 = 68 pf.
- 40 MHz Cutoff (3 dB down at 40 MHz) Elliptic
Design, Supplied by Dave
Brandon of Analog
Devices (this is quite steep as evidenced by his simulation on
NuHertz design software; can be used if rolloff is less tolerable
at 30 MHz):
C22 = 56 pf., C23 = 27 pf., C24 = 10 pf.,
L1 = 160 nH., L2 = 240 nH., L3 = 270 nH
C25 = 56 pf., C26 = 120 pf., C27 = 130 pf.,
C28 = 82 pf.
- 60 MHz Cutoff (3 dB down at 60 MHz); you
might try this if you want to operate the VFO up at 54 MHz
in a future software upgrade (filter design supplied by Coilcraft):
C22 = 2.7 pf.,
C23 = 24 pf., C24 = 9.1 pf; L1, L2 = 150 nH., L3 = 120 nH.
C25 = 30 pf.,
C26 = 68 pf., C27 = 62 pf., C28 = 27 pf.
5. 160 MHz filter:
Use the values given in the QEX article.
Conclusion: I
recommend using the lowest filter cutoff frequency that is tolerable,
as this will get rid of clock noise and reduce IMD in the output
amplifier.
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