Saturday, March 31, 2018

The DR. Validates Cascading Filters

IC-7000 + TS-450 + 752-C + NIR-12 + R.S. Equalizer
This blog entry is unique in that I am writing it "around" an excerpt from the Sept. 2014 QST Magazine's "The Dr. is in" column wherein Joel Hallas answered a long-standing question for me.

As I have shared in many Contest BLOG entries, the Autek QF-1a has become my favorite external audio filter. I liked it SO much that I bought a 2nd unit.

Because the FT-1000mp is a dual-receive unit, both receivers now have a QF-1a on the output; and of course the main receiver has its built-in eDSP.

Unfortunately, the Sub-RX has no built-in DSP.   I am strongly considering putting the JPS-NIR 12 in the Sub-RX audio line, giving a DSP-equivalent to the right ear as well.

With the NIR-12 in place, both RX's will have shapeable bandwidth and an auto notch, to vanquish broadcast carriers
and unwanted RTTY QRM that often
shows up in the phone bands.

Now, there is an old saying that if one
is good, two will be better. Then, if done carefully, THREE may provide the best combination; although not necessarily. 
Over the years, I have brought various audio filters and DSP devices into my operating environments. 

Of course, radios w/built-in DSP (like
my previous FT-920 and the current FT-1000mp), even if only audio-based can transform a frustrating contest weekend
into one that is largely under control.

After writing the QST-Dr, I spent a weekend running a portable contest operation from N6GEO's cabin QTH in Twain Harte. 
 During this operation I experimented with switching the cascade order of an MFJ 752-C, a JPS NIR-12 and a Radio Shaft Stereo Equalizer (15 settings per channel).
When I settled on the above order as the most effective approach, I was not surprised that Dr. Joel's detailed analysis confirmed my discovery; or did my discovery confirm his analysis?
Either way, that combination of audio devices gave me a DSP notch filter (in the NIR-12) along with the ability to shift/narrow the audio bandpass; all before being finally processed by the amplified equalizer.

While there might be an advantage to equalizing certain narrow frequency bands BEFORE the 752-C and the NIR-12, the active audio-amplification of the equalizer seems to work BEST at the end of the audio chain; not in the middle of it.

Taking things one step further, my biggest 
disappointment with the MFJ 752 is that the so-called NL (Noise Limiter) circuits are all but worthless. (The difference between the B, C & D models is cosmetic - the circuits are all the same,)

W6A in 2012 A-A Contest

The NL switch positions could be better used for other things. In place of the NL diodes, I wired in the circuit board from an old MFJ CWF-2 audio filter. The "SSB" switch position now invokes the 180hz Cw filter setting, while the "CW" position invokes the 80hz filter setting.  Doing this allows yet another form of cascading filter elements, with the amplifying stage at the END of that audio line.

Adding filters inside of filters also furthers one of my design goals, which is to repurpose old electronics rather than sending it to the trash heap.

 What all of this has also taught me is that the newest technology is not always the BEST technology for eliminating certain combinations of QRM and QRN. 

For me, having many knobs to twiddle
in a variety of combinations, gives me
a greater likelihood of copying that marginally weak signal.  In radiosport, every point or 10-points can eventually add up to a winning score.

Do YOU make use of external audio filters?

What discoveries have YOU made?


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