GROUP 7155 ARTICLES


 

1. Roofing Filters by Keith Kumm, AI7SI

2. A Letter to a Friend, Where Ham Radio Can Lead You,by Phil Hogan, W8PNZ

3. Ham Radio Tuners by Gil Gray, N2GG

 

 

Roofing Filters by Keith Kumm, AI7SI

A “roofing” filter...

Is a term in ham radio receivers probably coined around the first appearance of DSP rigs. On the diagram at right, a roofing filter lies where a “1st IF filter” was in analog superhets back in the day. That was a good enough name. Why change it?

The short of it is this: The new name suggests providing “shelter in a storm” (of signals and noise power) to a vulnerable function downstream of the filter. A “rain” of converted signals, making it through the broad frontend “band” filter, plus the wideband (WB) noise there showers down from a receiver’s first mixer toward the demodulator. Best to have roofing in place for flood control!

A “roofing” filter is a term in ham radio receivers probably coined around the first appearance of DSP rigs. On the diagram at right, a roofing filter lies where a “1st IF filter” was in analog superhets back in the day. That was a good enough name. Why change it?

The short of it is this: The new name suggests providing “shelter in a storm” (of signals and noise power) to a vulnerable function downstream of the filter. A “rain” of converted signals, making it through the broad frontend “band” filter, plus the wideband (WB) noise there showers down from a receiver’s first mixer toward the demodulator. Best to have roofing in place for flood control!

Now just a bit more detail, the long of it. But no serious math!


SHELTER-FROM-THE-STORM

Back in the day, what followed a 1st IF filter was a second mixer forming the 2nd IF of an analog double conversion receiver. That 2nd mixer would have a pretty high IP3 (for high dynamic range plus high power handling to conserve a softer analog-filtered converted spectrum). Yes, there were crystal filters back then, but they were less common and likely viewed as very premium.

Fast forward to DSP ham rigs, other than “direct sampling” types, the 1st IF filter often drives an analog-to-digital converter (ADC). Generally, an ADC can feature either high dynamic range and power handling, or a larger effective number of bits (ENOB) for better digital SNR, particularly useful with floating point DSP. Alas, for a reasonable part cost and power dissipation, an ADC can’t support both of these disparate high goals at once. On the signal path, the reduced power just adjacent to a desired signal plus power excised by wide and deep filter stopbands of a very sharp filter passband nicely accommodates high ENOB ADC.
So this is the reason “roofing,” pluralized, is the word. One roof is sharp edged yet broad on one side. Two roofs, above and below fIF, suppresses two neighborhoods that are still at RF frequency.


A superhet first mixer output can pack a lot of power, today or in yesteryear. At right, Ptotal = PSB– + PPB + PSB+ . From VLF-UHF, other than AM and FM, hams are restricted to 3kHz transmit BW, and so the most receive Hz needed for a QSO. Stopband power (PSB– + PSB+) holds merely irrelevant information. At IF, passband (PB) power is what’s relevant! But those SBs likely hold almost all of Ptotal. Consider 300kHz /3kHz SSB in 40m; that’s potentially 100x the PB width, or a 20dB “load” of unnecessary power. Call that the “rain” from the storm we’d like to protect the ADC from, not just for the ENOB, but also for “conversion headroom.” You never want an ADC to “hit the rails.” A continuing role for AGC!

Some other things are worth mention. Older rigs had adjustable “Preselector” RF filters, a compromise tailoring the total power making it to the 1st mixer, thus adding onto the 1st IF filter shape, lightening the load downstream. That’s a bygone thing. Hams today expect to command a VFO and everything falls into place! Encountering a low noise floor with few big signals, it might even be useful to add RF gain up front, and so legacy and modern rigs both do that, many moderns up to 20dB. All the more reason that roofing filter needs to be steep and deep, its signature attributes! Roofing filters are often high order, low ripple crystal lattice types with that might well have been considered a costly bit of overkill back in the day. Not today! The diagram illustrates an idealized 300 Hz roofing filter in a notional “bandscape” of various CW signals. Just scale it up for digimodes, RTTY, SSB or AM, even FM! The idea is general purpose. The goal is the same.

roofing

A Few Comments in Closing
The receive PB of interest, “open to the sky,” is what the DSP deals with. It has to take whatever rain drops fall there, near a desired signal, and render them harmless enough. But the DSP needs a linear representation of the PB to do its magic, and it’s the role of a roofing filter to keep it that way. Otherwise, if the ADC going got too rough, it could succumb to that old adage, garbage in, garbage out! But relax! We ultimately can fall back on AGC to save a spectral garden from a mudslide. The DSP “long loop” to AGC will tap us out of SNR perdition! Better some than none. As for those fancy rig specan waterfalls, that’s going to bypass the roofing filter onward to a second ADC tailored for power. You almost never get something for nothing!

 

Keith Kumm, AI7SI, is a member and of Group 7155 living in Tucsan, AZ who says that he enjoys "a bit of SSB and the Zen of CW.

 

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Where Ham Radio Can Lead You - A Letter to a Friend

A Letter to a Friend

Toledo, Ohio
I was a student and friend of your grandfather Louis Galambos. He was a great man and he inspired me, as well as so many others into a lifetime in the communications industry, including telecommunications and the space program at Vandenberg AFB, CA. I met Lou first in January 1953 at Macomber High School, Toledo(photo, left), in the Radio and Communications class. I was 14 about 5'0" and 85 pounds, the smallest and youngest of the class. The days in his class started with 5 minutes of Morse code and then into the basics of electronics. In the summer of 1953, we were living on Lewis Avenue, across from Whittier School (Where Danny Thomas went to grade school) and he would pick me up in the mornings and then on to school. Under his coaching, I built and tested a simple 25-Watt radio transmitter.

MACOMBER HIGH
Macomber High School

 

greyhound bus to Detroit for CW text in 1953
1953 & Off to the FCC

 

I also learned basics of sheet metal work, components and how to solder correctly etc. What a feeling of ecstasy when it worked and did. I learned a lot!

In the fall of 1953, I practiced Morse code more at home, using makeshift equipment as I could not even afford a telegraph key. I also studied for the Novice written exam. The big day came, and I traveled to Detroit by Greyhound, took and passed the test. In those days traveling from Toledo to Detroit for a 15-year-old was a big deal and I had to be at the FCC office by 8:00 am. Lou was quite proud of me.

     

Unfortunately, my family moved to the Cincinnati area in April of 1954, so I could not continue at Macomber High. I did not let this stop me and I finally came on $25.00 to buy a used receiver and I was soon up and running. In September 1954, I passed the General class license at Cincinnati and began working part time in a television shop. Ham radio operations included lots of television interference those days.

By fall 1955 we moved back to Toledo and in November I was interviewed and hired by AT&T Long Lines, starting to work in June 1956 for $49.50 a week. Multiplex single sideband long distance transmission systems, microwave relay and early transcontinental network television transmission were right down my alley. Thank God for the running start and inspiration that your grandfather gave me. I am certain there are many more of his students who could tell similar stories.

Titan VII finished my career, heavily involved in the space program, on the technical side of the ending of the Cold War. The Soviets could not compete with the US Air Force, Star Wars, The Rail Garrison and the satellite surveillance programs emanating from Vandenberg as well as other locations. Vandenberg is known as the Western Missal Test Range and many persons Civilian and Military participated in the effort. I was lead engineer on many of the communications programs, especially Titan IV, working for a government contractor. There were also government program directors which interfaced closely.

I am aware that Lou served in the Navy in WWII, on the USS Denver, a Light Cruiser. Lou never talked of his battle experience and he probably joined the ships company a bit after Pearl Harbor. The Denver won eleven battle stars and participated in nearly every major battle in the pacific. There were several instances of major battle damage, including kamikaze attack. Being the great man that he was, Lou never spoke of his battle experiences.

I gave up ham radio for many years, as it was too like my work and even let my ham license expire while I was working on communications in Saudi Arabia. I retired quite young from Vandenberg and lived in Baja California Sur Mexico for a time, returning to the live in the US in Southern California.

After about ten years of retirement, I realized that I missed the technical side of life and went back into ham radio in a big way. I obtained the Extra class license in a short time and proceeded with not just operating but teaching and helping others into the Hobby. I moved to Florida several years ago and live in the beautiful area of Rainbow Springs, near Ocala. I have a super high-tech Soft Ware Defined station and communicate around the world on a regular basis. For young people, ham radio is a wonderful gateway into a life of engineering and science.

Today, I contacted another of Lou's students and a friend of mine since the late 1940's. He lives in the general area and we will have lunch next week. We had not spoken in about sixty-five years. I met him in the 4th grade, Wayne Heter is his name, W8PNO. How I located him is a long and very neat story, which I will save for another day.

All of this is a tribute to your Grandad and the great man that he was. He touched our lives and we all miss him. High School teacher, college professor, businessman, husband and family man. They simply don't make that kind anymore!

Phil Hogan,
W8PNZ

 

More Photo Images from Phil

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Ham Radio Tuners

Cloudy with a Chance of . . . . . . .

I think all used equipment should come with a big shiny sticker pasted on it that says, "A Surprise Inside Every Box." You know, that sort of thing. But while not all used equipment have surprises in the downhill direction, we need to be aware of the things they might not be immune to, stuff to aid us in our decisions to buy or not to buy, to restore or reject. Beyond that, we can learn from adventures like this one, and pass on to those coming up in the ranks that are just starting on the path to building and growing their Ham Radio collections.

This past week Gil shared his experience with the Group regarding a pre-owned antenna tuner he picked up in his travels. What's good about this situation is it shows the method and approach used when the fix-it guys get their chance to open up and do their thing on their bench. It shows what they look for - stuff the rest of us non-fix it guys can keep on file when we're rummaging through used equipment we might soon own from swap meet tables and ham fests tents.

So here goes. The advantage you have here is that you can already see what the surprise was inside. Let's take a look at what the fix it guy said in his notes when he donned his soldering gun, sparked his meters and headed in.

The Saga of the

relays water

Soggy Tuner

All three tuners pass bench test and have been re-assembled. You will recall that I mentioned that the screws on the older unit were all loose and I suspected that was the reason for the water contamination. When I began to re-assemble this unit, I discovered that the screws were actually the wrong size, 4-40, instead of the M3x.5x20 Metric required by the SGC case. I replaced all the 4-40 screws and the case now has a much tighter fit. Here is a summary of the work done.

SG-230 S/N 51472176

Discovered L6 coil loose on one end and easily removed from board.

Repaired cold solder joint on L6 and reflowed solder on all other coils.

 

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coils

Discovered L6 coil loose

Repaired cold solder joint

red line group 7155

 


..Press