Hi Bill and Geoff,
thanks for your comments.
I knew I was going to agitate "the elecraft users sea"
but I believe we ham should share ideas, experiences
and knowledge.
I will see to reply to your comments.
Message: 40
Date: Wed, 9 May 2007 08:04:39 +0100
From: "Geoffrey Mackenzie-Kennedy"
<[EMAIL PROTECTED]>
Subject: Re: [Elecraft] Improvement changes to K2 and
K1 transceivers
To: "Bill Coleman" <[EMAIL PROTECTED]>
Cc: Elecraft Discussion List
<elecraft@mailman.qth.net>, Giancarlo
Moda I7SWX <[EMAIL PROTECTED]>
Message-ID:
<[EMAIL PROTECTED]>
Content-Type: text/plain; format=flowed;
charset="Windows-1252";
reply-type=response
Bill,
Gian might be suggesting the use of H-mode mixers to
replace the TUF-1
mixer
and NE602 product detector, but I have not asked him.
The H-mode mixer
has a
much higher 3rd Order Intercept than either the TUF-1
or NE602, but I
would
have thought that the very good IMD performance of the
H-mode mixer
would be
wasted in the K2. As I see it the IMD performance of
the 2N5109 post
mixer
amplifier, variable bandwidth IF filter and MC1350 IF
amplifier (and
probably the front end bandpass filters) would become
the weak link,
and
determine the overall IMD performance of a modified
K2.
> Geoff you are right. Certainly the H-Mode Mixer has
higher IMD performances then TUF-1 and NE602.
Some friends in Italy had problems with K2 (and also
K1) on 40meters during contests using good antennas.
This is how the idea of trying to improve the K2
started.
> Certainly the worst performance is in the post mixer
amplifier and this is the reason why the idea of the
simple 2 xtal roofing filter was implemented. THe
following bandpass xtal filter is protecting the IF
and demodulator stages.
> Unless we do trials on changes we cannot really
calculate if the improvements are really worthed or
not ... but our is an hobby and if one like the "risk"
he can do what he like... then he can go back to the
original.
If one is happy with his equipment it should not touch
it ... the H-Mode mixer has been implemented and
tested in many commercial transceiver, old and new
ones. It Is possible to guess if improvements are
worth or not ... but the "adventure changes calls" are
always around....hi
> To give you an idea, a friend in W-land tried the
suggested mods for his TR7 ... the improvements were
limited by the high phase noise of the PLL ...he then
replaced with an AD9951 DDS ... and the big difference
did show up right away.
> Another friend in Italy tested the H-Mode Mixer and
checked the improvements possible in his IC706 first
mixer mod. The original dynamic range at 20kHz tones
spacing was 85-88dB, with the H-Mode implemented it
went to 98dB. Replacing the original PLL with a xtal
oscillator it went to 105dB... clearly phase noise
differences between PLL and xtal oscillator.
> Although you may have not "enough improvements"
because the stahge is limited by the phase noise ... I
believe it is better than having both the mixer and
the PLL limitations.
Also the
H-mode
mixer requires a 50% duty cycle squarewave LO
injection for best IMD
performance which would require some modification of
the K2's LO and
BFO
output circuitry, although this modification might not
be necessary
because
of the poor (by comparison) IMD performance of the
weak link mentioned
above.
> NO. The H-Mode Squarer has a balance adjustment and
you do not need to put your hands in the PLL. Also the
HMM require 0dBm drive or less, so youmay have less
spurs. The BFO 50-50 balance is not necessary as the
LO frequency is low and the squarer gives quite a good
50/5o outputs.
73,
Geoff
GM4ESD
On Wednesday, May 09, 2007 at 2:46 AM, Bill Coleman
wrote:
Part of the beauty of the K2 is that it is a very
simple radio,
superbly executed. (To paraphrase a comment K9AY said
to me before I bought one)
> The K2 is certainly a simple and good radio. This
does not means it has no limits. Nothing is perfect.
Being only ham radio bands it helps a lot having
narrow input band pass filters. Try to remove these
and you will see something is changing. Listen to
40meter in US and then come to Europe and listen our
40 meters when a contest is on, CW or SSB.
The K2 uses a TUF-1 balanced mixer with close
attention to the
injection levels and impedance matching. You'd be
hard-pressed to
improve the mixer performance for HF.
The H-Mode Mixer is not sensitive to impedances
terminations like a classic diodes d b mixer,
particularly to the LO. Even the IF output when loaded
with a variable load like a xtal filter does not
reduce too much the IP3 (tests give between 5 to 10dBm
changes.. i.e from +40dBm to +35 or +30dBm as second
of type of filter.)
Since the K2 is single-conversion, the primary crystal
filter IS the
roofing filter. The primary crystal filter is pretty
darn good for CW
or RTTY.
> Certainly the filter is a good filter but if IMD is
reduced in the TUF-1 or more in the 2N5109 post mixer
amplifier, the IF filter cannot do much.
> 2)Demodulator replacement.
What would you do to replace the product detector?
Since the dynamic
range of the signals after the AGC of the IF stage is
pretty limited,
the NE602 is a pretty good choice. It wouldn't be easy
to do better,
especially at the price point for the K2.
> As I wrote, the Demodulator changes have not been
applied... they are reported as to experiment with
this stage and see what happen. You have to think that
the NE602 IP3 is around -20dBm while the 1T DBM has an
IP3 of at least +25dBm.
> I know that my ideas can be heritics for the K2 ...
as they seems to try to change the entire K2 receiver
stages... but these are ideas, right or wrong, and one
should not take them as melted gold... hi
> There are friends that have modified a lot of
equipments and are pleased. Some of these are:
Heathkit SB204, TS940, IC751, FT981 (here we
discovered one RF amplifier self-oscillating at vhf),
FT1000MP (2nd Mixer) anmd some others on trial...
> What I believe is also important to share comments
like your as it is part of the learning process and
share knowledge.
73
Gian
I7SWX
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