RF & AF Gain Operating Recommendation for 565/566/588
From the Ten-Tec Reflector - January 31, 2008
"OK, I brought this up before, and was specifically told that "low RF gain and high AF gain," as is traditional in analog radios, does not apply to the O2."
I am going to get some flak here (which is why I read this reflector less and less these days) for what I am about to add… but here goes:
I must emphatically disagree with the comment quoted above. Why? I actually operated with the same thought for nearly two years and in that time I learned to hate my Orion a bit more each day…. much for the same comment regarding CW readability in a pile up, but also to weak signal audio distortion, extreme AGC compression and clicks in the speaker and headphones. Regarding this particular reflector thread….There is just too much AGC compression taking place in the Orion if the RF Gain is at maximum. However like most hams I had gotten use to not touching that control. Fact many transceivers don't even have an RF Gain control (and generally sorely missed).
About three months ago I was talking (complaining) to one of my engineering friends at Ten-Tec regarding 'clicks' in the audio when I key. It was one of many things that continued to annoy me… and was really on my mind as I had just completed a CW contest and my ears were in pain. Anyway, I was hoping that there might finally be a 'fix' for this problem. At one point during our conversation he asked me how I set my RF GAIN, PREAMP, and ATTEN during normal operation. I explained that I normally didn't adjust down the RF Gain control unless maximum attenuation and no RF Preamp still resulted in too much signal (i.e. 40 or 80 meters at night with S7 or above noise floor). I could hear a 'frown' forming itself over the phone… and then he (a long time CW operator) explained what my engineering background had me consider a bunch of times but eventually ignored. Basically he said, to NOT use the attenuator unless I was in a genuine overload situation and that I actually would want to have the RF Preamp in under most conditions…. and that I should advance the audio output as high as possible and use the RF Gain to reduce the final audio level to what was pleasant to my ears. Sounded a lot like 'pre product detector days' to me. But frankly I was just about to give up on my Orion 565 (latest version 2.x) at that point…. already doing a lot of K3 research. So I decided to listen and maybe learn.
We discussed the A-D converters in the Orion and that it was their practical sampling limit…. or max range of ability…. that made it important to keep the Preamp IN. The object there is to make sure that there is sufficient signal available to the A-D converters so that the conversion produced a better sampling overall (think of early audio CDs). If the RF energy applied to the A-D converters is too weak then the conversion process generates too few samplings… resulting in continual sampling errors which many of us experience as 'distorted' weak signal audio (I certainly had). Then it was explained (rather nicely) how the DSP derived AGC should be minimized…. even partially defeated by the Orion's RF GAIN control and that leaving the RF Gain MAX would result in extreme AGC compression and initial AGC overshoot…. even overdrive the following audio stages momentarily (resulting in the clicks and yet another situation of discernible audio distortion). The high AGC compression on the CW signals coming through the same passband lessened my ability to sort through CW pile ups and created the ever present key clicks (especially in the headphones) as a side effect. Think about it…. to discern one CW signal from another you need audible 'markers' to help you separate them from each other. Since most of today's CW transmitters output essentially pure notes (no chirp, drift, hummm, etc.)so what we have left is separation in note… differing audio levels… and differing keying characteristics (speed, hand key, keyer, keyboard, dot-dash ratio, etc.). So eliminating differing audio levels (extreme AGC compression) eliminates one normally important factor that helps us separate. And if you further consider the eventual conversion back from digital to audio you can see the advent of more audio 'error' since the digital signal that is being converted is merely the sum of all digitized RF signals through the bandpass at any particular moment. No matter how good the digital to audio conversion at this point it will generate more apparent error when compared to a well engineered, fully analog signal path. This apparent conversion error adds to the ear's confusion when working overtime to separate so many signals…. so closely spaced… during extreme reception conditions… i.e. CW contest.
Finally…. learning to operate with the RF gain used to control the audio output (Audio GAIN at or near maximum) produces the best signal-to-noise ratio in the Orion's receiver. And isn't that what we usually want?
So before you completely dismiss what I have written why not do what I did…. I turned off my automatic 'disbelief' button for a few weeks and started considering my Orion as a unique receiver that simply didn't provide its best performance when driven in a conventional manner. I began driving it as recommended and soon afterwards I began to develop a new respect for the Orion's abilities. And no more audio clicks… no more extreme AGC compression… no more weak signal distortion…. and I could more easily separate the piles of CW signals during a contest… or simply during the hunt for new DX.
73,
Jerry, KG6TT
Fairfield, CA
Note: The above recommendation does not apply to Orion running version 1 firmware.
