More ARK4 comments

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From: Mike Czuhajewski (Mike.Czuhajewski%hambbs@wb3ffv.ampr.org)
Date: Thu Jun 09 1994 - 12:17:27 EDT


Some more comments on the new ARK4 from S&S Engineering--
              
Severn, MD, 8 June 1994--Someone recently asked about performance of
the new ARK4 from S&S Engineering. I sent him some brief comments but
The System didn't like the address I used (even though it was
identical to the one on his message--that happens sometimes) so
instead of remailing it I decided to flesh it out a bit and "go
public."
                  
I got a loaner from Dick, KA3ZOW, a couple weeks ago at a local
hamfest, for review. I played with it a bit, mostly off the air,
then loaned it to a friend whose antenna is in the air (unlike mine,
which is mostly on the ground at the moment). I didn't do any
sensitivity or stability tests yet (the latter seems a bit pointless
in a synthesized rig, which is based on the stability of crystal
oscillators). I did notice some things I didn't like in the RX audio
chain, but this is an early iteration of the design and Dick will, I
hope, fix them when brought to his attention.
              
First, in all fairness, I should point out that the ARK4 I have now
is a prototype, and the ARK40 last year was either a prototype or
very, very early production model. (I received both already
assembled.) In both the loaner ARK40 this ARK4 there is a constant
whine or tone in the audio chain, even with volume control all the
way down. Dick told me the ARK40 problem was the LM386 oscillating a
bit. In the ARK4, which will NOT drive a speaker and is not intended
to, there is a much higher tone, about 10Khz (confirmed with an audio
oscillator), with volume at min. That shows it's in the audio chain,
not the main rig. The ARK40 tone was about 2K, as I recall.
                 
Both were low level, probably not easily noticed with volume cranked
up to usable levels, but the point is they should NOT be there in the
first place. Second ARK4 audio bug is the constant low level
sidetone freq heard at all times, even with vol at min. He runs the
sidetone osc constantly and gates the output on and off. It's
probably not easily noticed with vol at working levels. It's gated
on and off by shorting the oscillator output to ground with a 47 uF
capacitor and transistor (2N3904?). At first I thought the
collector/emitter voltage drop of the conducting transistor was
keeping the cap from shorting the signal completely, but later
disproved that. I also thought that the problems might be caused by
the inches-long PCB trace from the volume control to the audio amp
picking up stray signals along the way; I disproved that one, too.
                          
The audio IS distorted as someone recently said of an ARK4 he saw at
Dayton. It finally clears up fairly well at about 12 o'clock on the
vol, but at lower levels it is distinctly distorted and harsh and
annoying. By the way, this one had the optional audio filter.
Here's the audio chain--sidetone and product detector feed a single
op amp section which goes to the phone jack. If the filter is
installed, that op amp section instead goes into another op amp,
where it is attenuated, filtered and passed on to the jack. I
disabled and removed that filter IC, jumped the signal straight over
to the output line, running the audio chain in its "stock",
nonfiltered mode; all problems were still there so can't blame the
filter. (Part of that IC in the filter is used for the audio whether
the filter is turned on or not; switching the filter in or out is
accomplished with +12V, not by use of a DPDT switch as in some rigs.)
  
                    
Transmitter output signal on good quality HP spectrum analyzer--all
spurs and harmonics down WELL over the required 30 dB. Interestingly
it has a spur on the TX signal at the IF freq of 3579, not too
surprising, I suppose, and perhaps to be expected...and it IS about
40 dB down. There is a second spur just below that, at 34-something,
and that second spur moves when you change freq, moving 100 KHz if
you change the rig by 100 KHz (a one to one change). That, too, is
waaaaay down. (I didn't check the ARK40 on the analyzer when I had
it, but would assume harmonics are down similarly.)
                 
One final experiment, which I've always wanted to do on something,
anything, was to look at the output signal on a sweep from 0.1 to 36
MHz, seeing up through the 5th harmonic, both with and without the pi
net filter in place. I'd always heard that harmonics were pretty
strong in a class C amp, and here was the perfect chance to do it.
"Dramatic" is the best way to compare the two different sweeps. I
took pictures of both with a scope camera, and will have them in the
Idea Exchange in the QRP Quarterly in the future. (This is certainly
not an indictment of the ARK4, of course, but a simple fact of
electronics, and will happen with ANY class C amp--and that's why
everything we build has a filter on it!)
          
                
73 de WA8MCQ@WB3V.MD packet and WA8MCQ%hambbs@wb3ffv.ampr.org (which
works just as well as the longer Internet address with my name in it)

--
Mike Czuhajewski, user of the UniBoard System @ wb3ffv.ampr.org
E-Mail: Mike.Czuhajewski%hambbs@wb3ffv.ampr.org
The WB3FFV Amateur Radio BBS - Located in Baltimore, Maryland USA
Supporting the Amateur Radio Hobby, and TCP/IP InterNetworking


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