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Lowpass filter for 30m

Lowpass filter for 30m

I just finished a 30m lowpass filter for the Softrock Ensemble RXTX tranceiver.

I used the schematic provided by WB5RVZ

L200 and L201 is wound to about 0.9uH, whereas C200 and C202 is 100pF and C201 is 330pF. Everything is soldered to a PCB (ugly construction) and I added two switches and a coax to enable on and off switching of the filter.

Ugly construction…

I tested the filter using a 25 MHz function generator and my Rigol 1052 scope. The filter was connected to a dummy load. The first two pictures below show 1 Vptp at 10 MHz with the filter off and on respectively. The two pictures thereafter show 1 Vptp at 20 MHz (on and off).

10MHz lowpass filter switched off

10MHz lowpass filter switched on

20MHz lowpass filter switched off
20MHz lowpass filter switched on
As a three pole filter it should have a roll off at 18db/octave. This seems about right when I compared 12 MHz and 24 MHz (9.3V and 0.9V ptp respectively). I measured the -3dB point to be around 14.7 MHz. The purpose of the filter is to reduce the the 2nd harmonic when transmitting in the 30m band (10.1 MHz). The second harmonic is down about 13dB.
Function generator on burst mode, Oscilloscope on FFT, center frequency is 10.5MHz and filter is off

Same as above, but lowpass filter is switched on

I also looked at the FFT on the Rigol scope while using burst mode on the function generator. In the pictures above, the center frequency is 10.5MHz and the grid is 12.5MHz in the X axis and 10dB in the Y axis. Here it seems like the filter contributes to 20dB attenuation at 23MHz which is about right.  I have not measured the second harmonic while using the Softrock, since I have no real spectrum analyzer. However, as the filter is working, I guess I can legally transmit on 30m. 73!

 

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