Receiver Comparing - IC-756proIII and TS-2000, Others

First off, this is by no means a scientific comparison. Just my own observations as I perceive them to be.

I was over at Universal Radio today for the monthly QRP meeting. I decided to play around with the IC-756proIII and TS-2000 and compare them with the same signals. Both radios have a 15kHz first IF filter so I wasn't holding my breath that the IC-756proIII would realistically be THAT much different. The Twin Pass Band Tuning (PBT) and 3 filter choices on the IC-756proIII are DSP.

I believe my hunch was right - in my comparison the IC-756proIII was not noticeably different in receive sensitivity (on 40m CW). The signal to noise ratio between the signals I tuned to on both radios was very very similar. I think the IC-756proIII sounded better overall, but for a price difference of $1000 it really isn't worth it.

Now, if there is a trick feature outside of playing with the filters that can be adjusted or turned on that I didn't find in the DSP or filtering that would noticeably change the performance then that may very well push the IC-756proIII over the edge in to the higher performance class. As it sits, and as I played with it, it does not impress me.

Now I am itching more than ever to get my hands on a radio that IS noticeably different than my TS-2000 and compare the two. At this point I think that would be (current radios) an Elecraft K2, K3, Yaesu FT-2000, FTdx-9000, Tentec Omni VII, Orion II, Icom IC-7700, or IC-7800. Unfortunately, the majority of those listed are obnoxiously expensive mega high-end radios - except the Elecraft radios. The FT-2000 and Omni VII are borderline.

If you have any more information on other options, please e-mail me: kc8qvo@yahoo.com. Having never experienced the "others" (at least with the same mindset I have now - KILLING NOISE) the following is my take on following the specs of some of the others.

The Ten Tec Omni VII's specs say it is a 3-stage IF. The selectivity at the first IF is 20kHz (WOW! I thought 15kHz was high). Then the specs go on to say the second IF stage is 20, 6, 2.5 kHz standard, 500 Hz, 300 Hz, optional. This is getting better, though there is already some crap in the system that flowed right on past the first IF. I am sure that would make a big difference, though. The third, and final, IF is DSP filtering - down to 200Hz. That isn't a huge difference beyond the optional 300Hz and wouldn't affect the noise level (crap) as much as the optional 300Hz filter.

The Elecraft K3 has provisions for roofing filters down to 200Hz. I have not found which IF stage this takes place in.

I could not find any information on the K2 specs. However, from what several people have said that I have talked to in person the K2 is a VERY respectable radio.

Another Learning Phase - IF Stages and Roofing Filters

If there is one thing that Ham radio has to dive in to it is a never ending sea of "new", whether it is new technology or just new to you. You can wrap your brain around all kinds of aspects of Ham radio and never know it all - the more you dive in the deeper it gets.

Well, I have just deepened my sea even further. I wrote a couple days ago about how I compared the KX1 to my TS-2000. In that moment of enlightenment I realized how poor of a radio my $2000 TS-2000 was for HF. First off, the reason I bought it 4-5 years ago (as opposed to an FT-847 or IC-746) is it does 100 watts on 2m and 50 watts on 70cm, all modes. The HF and IF DSP was a big bonus, but I could have had the HF with any of the other options at the time.

In the past couple days I have tried to learn about receiver design a little bit to educate myself on why some are better than others. Even if I had someone teach this all to me 8 years ago when I got in to the hobby I probably wouldn't have followed along anyway. Now that I have been around for a while and know a lot about Ham radio I understand the benefit in knowing more, and more accurately at that.

What I have discovered (and if you are knowledgeable in the subject, by all means, please critique and shine more light on the subject - kc8qvo@yahoo.com) is there are many different kinds of receivers!

WOW! What a realization.

What I knew before was the difference between IF DSP/filtering and AF DSP/filtering in that IF is "intermediate frequency", before the crap gets in to the radio, and AF is "audio frequency" , after all the crap has been processed and before it comes out your speaker (but still usually sounds like crap). What I didn't know about receivers is the (sometimes) many stages. It isn't just one stage that takes the RF and converts it in to signals you can hear. In some cases it is, but apparently most of the time it is NOT.

My TS-2000 is a great example. It takes a whopping 4 Intermediate Frequency stages ("frequency conversions", if you will) before the signal comes out the speaker/headphones.

The KX1 on the other hand is ONE. Just one. It is a single-conversion superhet receiver.

Something I have known for some time is that the longer the path a signal takes (the more stages, devices, what have you) the lower the quality of the signal. In the case of a receiver it is noise. Every stage of the receiver adds in noise - it is the old sayings "you don't get something for nothing" or "there's no such thing as a free lunch".

Next, we move on to the "crap". The way this works is there is all kinds of RF crap floating around. It comes to us in the form of that white noise, or SHHHHHHHH, we hear in our radios. The higher the intensity of this noise the higher the receiver threshold is yanked up. All receivers are sensitive to signals. However, they are naturally sensitive to the most obvious or strongest signal. The strong signals will raise the sensitivity "bar" so that, for another signal to be detected, it must be HIGHER (or stronger) than the bar to be detectable. The further the signal is above the noise the higher the signal-to-noise ratio = the easier it is to copy.

The signals we are trying to hear, on HF, are in the form of voice (Single Side Band, or SSB of which there are two), CW (or Morse Code), and Digital. The bandwidth of the signals range from upwards of 3kHz for a voice signal on SSB down to just a few tens of hz for a CW tone.

Now Ill get in to filtering. All the crap that comes in to the receiver doesn't necessarily have to. The only signals we are concerned with are the communications signals taking place - at most about 3kHz. This is not a very wide sliver of bandwidth, in the grand scheme of things. The crap is everywhere in comparison. So, by narrowing what the receiver hears one can limit the amount of crap.

This process is done in one of two ways, as I already explained up at the top - in the AF (or audio frequency) or IF (intermediate frequency) stages. Again, the less crap that makes it in to the receiver the better. So AF is pretty much worthless for this discussion. The receiver has already processed and "made use of" the crap, in an undesirable manner.

So if AF isn't good that leaves one option - IF! Well, it isn't that simple and I still don't understand it.

What I do know is the closer you can get the filtering to the RF input the better off you are. That leaves less crap to get in to the radio to raise the noise threshold (IE - the "noise floor"). The higher this level is up front the less sensitive the receiver is and the more noise introduced by later stages.

I have been told that the TS-2000 has a 15kHz filter. It does have variable IF DSP filtering down to 50hz, however I am not sure which stage this takes place in. I am also not sure where the 15kHz filter is. I am guessing it is in stage 1 or stage 2, and a guess on the IF DSP is it is in stage 3 or 4 (an educated guess - based on my blatantly obvious noise problem).

Back to bandwidth for a second. If SSB (the widest mode commonly used on HF) has a bandwidth of about 3kHz (max?) WHAT GOOD IS A 15kHz FILTER??? If I did my math correctly here... that would be 5 times wider than an SSB signal. That also means 5 times more CRAP for the receiver to process = a high noise floor.

Even though the TS-2000 has variable IF DSP filtering the receiver is susceptible to high noise with the 15kHz filter (wherever it is). The DSP filtering can narrow the passband down to 50hz on CW, but that doesn't cut out the noise floor that has been already raised by the crap flowing right on through everything else.

The only reasoning I can come up with for a 15kHz filter is for FM (not used on HF except for a small portion of 10 meters). That still doesn't explain why there isn't some kind of sharper filter (or even the OPTION of) for SSB and CW. I think someone had their head stuck in the wrong place.

I think the term "roofing filter" applies to any filtering ahead of the receiver = the kind that filters out the crap. This leaves me with a question about how the optional filters for some radios (IC-718 and FT-857D for example) are classified - roofing or passband?

So much to learn. Now my hunt starts for a better radio or improving what I have.