Friday, March 23, 2018

Q-Filtering for Fun and Profit

WQ6X/6 @ W7AYT's Concord QTH

Ever since the beginning days of radio operators have been plagued my QRM (interference) & QRN (Static and Noise). Each time a new receiver or transceiver is introduced they promise us the latest gimmick circuit advance to help us deal with the interference problem. What they DON'T often disclose is that the circuit design to reduce/rid us of QRM & QRN often introduces noises or interference of its own; something we generically call "artifact" (unwanted effects added or
subtracted from the desirable signal).

As a radio operator, I am a "knob twiddler" (switches too) - I LOVE fiddling with receive signal adjustments to obtain that LAST db of signal-to-noise ratio (SNR). Lucky for me, during the last
60+ years equipment manufacturers have devised various receiver add-ons; some for the front
end, some for the IF-line and some to process the audio end of things. Also during this time frame 100's of construction articles have appeared in magazines like QST/QEX, CQ, 73-Magazine and
even Popular Communications.

As detailed on this WQ6X Contest BLOG website, I have run dozens of different style portable operations. To help overcome the inadequacies of portable operation, I always bring along a
bunch of outboard filters, plugging them together in whatever order will yield the highest efficiency. That I chose the correct equipment order was confirmed by none other than the QST Dr. Joel Hallas - W1ZR. He gave me a personal reply which QST chose to print in their "The Doctor is in" column a couple of years back. In a follow up installment of this series I will publish his reply to my question about the proper order of audio filters.

This BLOG entry is the 1st of a series discussing the pros/cons of Q and Audio filtering. Using examples from the many WQ6X portable operations, I will explain how these devices have worked
for me, while detailing the drawbacks or artifact problems. As I am currently in the process of adapting some old construction article circuits into a useful combination, as each circuit comes together I will spend time with it in this BLOG section.


For all intents and purposes while regeneration was the first form of Q-filtering, it was the IF Q-Multiplier that gained strategic use by traffic handlers and contesters (a form of traffic handling). Heathkit alone produced 3 different (altho near-identical) models; the QF-1, GD-125 and the HD-11; all utilizing the high-gain 12AX7 (dual-triode) vacuum tube. Making use of an IF-level Q-Multiplier requires opening the receiver and tapping the 455khz IF output; something not everyone is comfortable with doing.

If you want to know more about Q-multiplication do an internet search for "O. G. Villard" - inventor
of the Q-multiplier - it was originally awarded a U.S. patent.  [CLICK HERE] to read a Wikipedia
write up about his life.


The 1970's & 1980's brought many QRM circuits & devices to the amateur world; some internal to the evolving transceiver technology and some in the form of audio-based external units. Even before [so-called] DSP units came into being, companies like Autek Research (with their QF-1 and QF-1a) and MFJ (with their 752 series of signal enhancers) devised ways to apply Q-Filtering for "shaping" audio. A pair of Autek QF-1A units are my current favorite for the dual-receive FT-1000mp
(one for each ear)  - they WAY out-filter the pair of MFJ 752's (a "B" and "C" model).
Currently, the 752 pair have been relegated to augmenting laptop audio during
remote running of NX6T.

MFJ 752-C

All the above technologies attempt to provide Low Pass (LP) filtering, High Pass (HP) filtering,
High-Q Peak (PF) Filtering and some sort of High-Q notch filtering (MNF or ANF). Additionally, devices like the QF-1A and 752-C provide a secondary Q-filter usually designed best for notching (i.e.. eliminating) troublesome carriers and noises.

When actual DSP filtering first emerged (approx. 25 years ago), what was processed was the audio stream. While such filtering can process the audio and remove annoying interference in a remarkably intelligible way, if that interference is PUMPING the AGC, altho now you can't hear it, it's deleterious effects are still present. As technology has improved over the years, DSP filtering is now
commonly found at the I-F level INSIDE the AGC loop.


The first REAL external DSP unit I had access to was the JPS NIR-12. While it can be run well with diverse modes like RTTY & SSTV, the NIR-12 does best in an SSB environment (altho CW works rather well, as well). Nighttime 40-meter SSB operation above 7.200 is helped DRAMATICALLY by the Auto-Notch filter in this unit. The NIR-12 CPU chips must work VERY HARD as the unit is often nearly HoT to the touch.

My current radio of choice is the 20 year old Yaesu FT-1000mp, with it's [audio-based] eDSP circuits, for both transmit, as well as receive. Because each receiver in the MP sports a final IF frequency of 455khz, I am proposing a side project this year of tapping both receiver's 455khz signal to run in my classic Heathkit QF-1 sitting on the shelf waiting for something to do.

In preparation for this BLOG entry I took a walk down memory lane to examine the number of different radio and filter combinations used by WQ6X in various portable venues. This is what it looks like:

NIR-12 & MFJ 752-C
NIR-12 & MFJ 752-C
NIR-12 & MFJ 752-C
------ " ------
------ " ------

Now that I have settled in on the Yaesu FT-1000mp as the main radio, I am working on a number

of cool audio filter ideas; some a combination of circuit design adaptations along with the QF-1A
and the NIR-12. In the next installment to this BLOG series I will share many of the "accidental" discoveries I've happened onto while playing with the different radio combinations.

Do you use outboard DSP and/or audio shaping devices.

Please write me and share YOUR findings:

Ron @ WQ6X.Info

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