Easy QRP Low Pass Filters

With an eye toward getting my rudimentary Si5351-based transmitter on the air, I’ve started putting together a number of low pass filters to knock down some of the nasty harmonics that come with using a clock chip as a frequency generator.

The filters are all based on the designs given by W3NQN in the GQRP technical pages from 2015. (This is another point at which I’m taking design inspiration from the QRP-Labs QRSS beacon.) The paper lists both the target values for each of the toroids and capacitors for a 7-pole low pass filter for each band, as well as the required number of turns for each toroid and an appropriate toroid to use. For this project, since my target frequencies range from about 7 MHz to 14 MHz, I’m using all T37-6 toroids from KitsAndParts.com. The basic design of each filter is the same:

All these filters are symetrical, for 50-ohms on either side.

Here’s the values that W3NQN gives for low pass filters for the 20m, 30m, and 40m bands:

Band	C1, C7	C3, C5	L2, L6	L4
20m	180pF	390pF	.773uH	.904uH
30m	270pF	560pF	1.090uH	1.257uH
40m	270pF	680pF	1.380uH	1.698uH

Unfortunately, I don’t have exactly the specified capacitors in my kit, so here’s my approximation to these values. (Note that the number of turns is different at 40m than W3NQN recommends, to more closely match the specified inductance). All inductors are wound on a T37-6:

Band	C1, C7	C3, C5	L2, L6 (turns)	L4 (turns)
20m	1000+220 (series) = 180pF	1000 + 690 (series) = 408 pF	16T = .77uH	17T = 0.87uH
30m	220+47 = 267pF	220+220+100 = 540pF	19T = 1.08uH	20T = 1.20uH
40m	220+47= 267pF	470+220=690pF	21T = 1.32	24T = 1.73uH

Some quick simulations in LT Spice confirms that these little adjustments don’t have an enormous effect on the filter’s behavior. On 40M, for example, it has the effect of moving the cutoff frequency down maybe 100 KHz:

The behavior of the 40m LPF as constructed with the parameters above. Note the nice knee just under 10MHz and that the attenuation is around 40dB at 14Mhz, the second harmonic for this band.

So far, I’ve only constructed the 40m filter. It’s built on a little 2.3cm x 10cm piece of coppper clad. (I’ve got a bulk stock of 7cmx10cm pieces, which cut nicely into threes to make little projects like these).

The 40m filter as built. I love these little screw-on BNC connectors. There’s probably some RF leakage because of them, but they’re so easy to apply and use. And it makes changing male and female BNC connectors as necessary a breeze.

The filter seems to be working great – a little qualitative examination with an oscillloscope shows that the filters are smoothing out the waveform at 7 Mhz quite nicely. Above that, the attention seems to match the slope predicted in the simulation above.

With this filter on my little homebrew transmitter, I finally made my first (official) WSPR contact… but that’s a story for another post.

Hear you on the air!

73