Low Pass Filter Board: Part 3

So its been a busy day here in the workshop. Caps arrived today and we set about to making low pass filters for the transmitter. I started by sticking in the caps and then in turn winding the inductors for each of the 3 filters, 80, 40 and 20m bands and giving them a test by feeding in a square wave from my it makes nice distortion signal generator.

80m Low Pass Filter Under Test

The output was fed into the spectrum analyzer on the Red Potato. The spurs either side of the fundamental are crud from my signal gen, it does that when you start to turn up the output. This was a 10 volt peak 3.6mhz square wave being fed into filter and I through up a few cursors to get some levels, first harmonic is -60dBm down on the fundamental. That should be more than good enough.

I did the same for the 40m and 20m filter, again -60db down on the first harmonic, the above image being for the 40m band.

Something I was worried about was isolation on the relays. These are no uber doober premium rf relays, no no no, they are 10 for $1 Chinesium Grade floor sweepings and seconds off Ebay. So we stick the signal in the in port and connected the 100meg Hantek scope up to the other end and turned the volts per division down to the noise floor and looked for signal leakage. Nothing, maybe a couple of microvolts but that might have been an aberration. Either way, good enough for the kinds of girls i go out with.

Final board all together and soldered up.


A bit of an idea of how its all going to go together as a stack. I think things might actually work ok after all. Touch wood.  Tomorrows job is to program an ATMEGA IC to work as an iambic keyer, add in some switching and buffering to get the transmitter up and working. Not sure if i am going to have an external PA yet, or just use a couple of BD139’s and get 3 or 4 watts out. Will see how we go.


2 thoughts on “Low Pass Filter Board: Part 3

  1. Hi Rob,

    I’m not sure if I follow your logic regarding the 2nd harmonic being 60dB down. If the signal generator was a true square wave then there would be no second harmonic at all. Or any even harmonics for that matter.

    And since you can’t get 60dB attenuation from a 7th order Butterworth filter at the second harmonic I’m confused as to what you actually measured. Can you elaborate a bit please?


    1. The measurement is of the First harmonic, and its 60dBm down on the fundamental. dBm, being the ratio of dB per Milli-watt. And my signal gen is not very clean when making square waves, adding a lot of other junk other than just the desired frequency and its harmonics.

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