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 <[email protected]>, 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 ____________________________________________________________________________________ Food fight? Enjoy some healthy debate in the Yahoo! 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