Today I'll discuss a filter that is imperative to have so I can use the Pixie on the air. I'll cover two types - a low-pass and a band-pass, and explain why you may want them on your project.
The Low Pass Filter
A low pass filter is a kind of filter that allows frequencies below a certain cutoff to pass then attenuating the frequencies above the cut off.
The low pass filter on the Pixie is rather appalling to say the least. The second harmonic is only suppressed by -6dB. FCC regulations require suppression to be at least 50dB - obviously this doesn't pass! Despite the Pixie being a QRP kit, the second harmonic can and will transmit just as well as the fundamental on a good propagation day.
The stock, barebones Pixie kits comes with a low pass filter with a cutoff of around 7.3MHz and uses standard E12 (10% tolerance) components. 2 470pF capacitors and a 1uH axial inductor make up the filter, which, as I mentioned earlier, only attenuates the second harmonic down to 6dB.
In doing some experiments as well as online research, I have found that adding a capacitor in parallel with the inductor on a 3-pole LPF will significantly attenuate the 2nd harmonic as well as the other harmonics that follow. In order to do this, the LC circuit comprising of the inductor and parallel capacitor should be resonant at or near the second harmonic's frequency.
Let's take the stock 7MHz filter. We need a capacitor that will form a resonant circuit on the second harmonic, or 14MHz. A 120pF capacitor seems to do the trick, as the circuit with a 1uH and a 120pF is resonant on 14.5MHz - close enough to the second harmonic.
For 60 meters or 5.3MHz, the values will be a bit different. You will need 2 1000pF (or 1nF) capacitors, a 1uH inductor, and a 220pF capacitor placed in parallel to the inductor. This will be more than sufficient for 60 meters and will reduce the 2nd harmonic down to at least 50dB. Other harmonics will also be attenuated down to around 29 or 30dB.
Here is what the 60m LPF looks like:
Band Pass Filter
The Pixie is VERY prone to receiver overload, as a reminder. I don't have very many issues with it since there aren't any strong shortwave radio stations near NW Missouri, but if you live by a powerful AM station or WTWW, you might have an issue with overload.
A band pass filter is just like a low pass filter but it allows a set bandwidth of frequencies through. In this case, we want the 5MHz band to pass through on both transmit and receive, but cut off frequencies that are lower in the HF spectrum as well to avoid overload.
To create a BPF, all we need to do is place a capacitor in parallel with an inductor, a capacitor in series with an inductor, and another capacitor in parallel with an inductor. This, like the LPF, will create a resonant circuit that will filter out the unwanted frequencies.
Our 60m BPF looks like this:
Note that the LC circuits created are resonant around the 60 meter band. There will be a small amount of loss due to insertion losses, but it shouldn't affect the overall performance of both the transmitter and receiver. In fact, losses under 1dB should be negligible. On 60 meters, we're looking at a loss at most of -0.3dB.
Band pass filters are generally not used in transceiver circuits since they can affect transmission losses. However, due to the limitations and shortcomings of the Pixie's direct conversion receiver, constructing a BPF is almost imperative to prevent breakthrough.
The Decision
The final decision rests in your hands. You must take things into consideration, such as where you live, where you plan to operate, what kind, type, and value of components you have, etc.
For me, since I live in a rural city with zero to no AM breakthrough, I will opt to install a low pass filter. However, if I lived somewhere like Chicago, New York, etc., I'd opt to go with a band pass filter. The low pass is also 4 parts big whereas the band pass would be 6 parts big. While this isn't too much of a constraint for me, QRP is all about minimalism, and it only involves adding on one part instead of 3 more.
That's this section modified. Stay tuned for the final build!
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