Do You Need That Filter?

The Yaesu FT-DX10 comes standard with a 500 Hz crystal (xtal) roofing filter, but offers an optional 300 Hz roofing filter.  Should you purchase the optional filter?

The 300 Hz roofing filter is twice the size of the 500 Hz filter so it must be twice as good right?  

If you casually switch back and forth between the two filters on a noisy band, it sounds like the 300 Hz filter markedly improves selectivity and quiets the noise.  But try this: Select the 500 Hz filter and narrow the bandwidth (using the bandwidth control) to 300 Hz, then switch to the 300 Hz filter. 

When you digitally narrow the bandwidth of the 500 Hz filter to 300 Hz you will "hear" the same reduction in noise as you have cut out 200 Hz of higher frequency sound.  Engaging the 300 Hz filter lowers the volume a bit (3-6 dB) due to insertion loss.  

So what you are actually "hearing" when you switch back and forth between the filters without changing the digital bandwidth is the reduction of the higher frequency noise that can be accomplished using the bandwidth control alone with the 500 Hz filter.

So, from a selectivity standpointthe 300Hz filter doesn't gain you anything over using the digital filtering with the 500 Hz filter.  The real benefit should come in the form of adjacent signal rejection.  So let's look at that.

In the video below I demonstrate the signal rejection of a 40 dB over S9 adjacent signal to a weaker S3 - S5 signal.

From the video you can hear that there is a very small demonstrable difference in strong signal rejection when using the 300 Hz optional filter, but the difference is so small that I doubt many of us would find practical benefit over simply narrowing the DSP bandwidth while using the 500 Hz filter. Even when contesting.  The digital filtering built into the FT-DX10 is really, really good when using the included 500 Hz roofing filter alone.

Yes, I spent the $200 for the optional filter thinking it would help, but I wished I had known what I do now.  I would have $200 for some other nifty radio gadget to spend instead. 

That's all for now.

Lower your power and raise your expectations

Richard AA4OO

https://www.hamradioqrp.com