This post is part of a series: Building the VK3YE Beach 40 DSB Transceiver.
Next on the bench is the low-pass filter, which is mostly responsible for curtailing the harmonics generated by the VFO and mixer. Since limiting out-of-band signals and noise is also useful on receive, the LPF is connected directly to the antenna socket to be useful in both modes.
This LPF is particularly simple, using just three RF chokes and six capacitors. VK3YE’s original low pass filter used a couple components I didn’t have (namely the 820pF caps), so I played around a little in LTSpice to work out a viable filter with the components I had. Here’s what I ended up with:
SPICE simulations show that this filter maintains a similar low-pass characteristic to the original VK3YE design. Moreover, the insertion loss is approximately equal. The match to 50-ohms isn’t great, but is acceptable. And perhaps most importantly, attenuation at the second harmonic is strong at about 42dB:
Constructing the LPF was simple – just six capacitors and three molded chokes that I picked up on a recent trip to California. I laid the filter out on a small piece of scrap copper clad. I enjoy when the layout of a board can highlight the symmetry of circuit itself.
Before attaching the filter to the rest of the project modules, I did a quick manual sweep of it with my SI5351 board. It’s somehow very satisfying to roll the frequency up past the filter knee and see the higher frequencies just drop off. One word of caution – the SI5351 doesn’t have perfectly uniform output power across all frequencies, so I put one channel of the scope on the input of the filter and one on the output, to observe relative attenuation. Of course, terminating the filter is a must.
Hear you on the air!