On Wed, 5 Nov 1997 16:47:09 -0500 (EST) W0AH@aol.com writes:
LOTS of SNIPS
>Both KM1H
>and W8JI wrote that the MMIC was a poor choice for a 1.8 MHz preamp.
>I agree
>that it is an odd choice, but my Avantek Semiconductor Data Book shows
>that
>the power gain and other MMIC specs are flat from about 0.1 to about 1
>or 2
>GHz, depending on the specific device. Also, the Avantek power gain
>versus
>frequency chart shows them being flat to DC! Therefore, the device
>should
>be suitable for 1.8 Mhz.
The reasons were covered at length by W8JI and myself in prior posts but
to recap:
The MMIC will amplify fine at 1.8MHz, I never said it did not. It is
however a bad choice in an enviroment subjected to many strong signals.
It does a great job as a lab amp or an IF stage where it is not subjected
to the combined power of a contest type enviroment. In such a case it
becomes an IMD generator. Remember that the MMIC and its xfmr are
receiving every signal from DC to over 1GHz and only limited by the
response of the Beverage.
> KM1H siad there had been a discussion of the ZJ Beverage Box
>which can
>be found in the archives. So more information is available there. He
>also
>said that the ZJ Boxes had been shipped with "at least 4 different"
>MMICS
>including the MAR1, MAR3, and others. He stated that some of those
>devices
>needed different biasing. He also stated the ZJ Box was subject to
>overload
>and IMD. Carl said the "ultraminature 9:1 balun is just as bad."
Yes I did and so did Tom. The reason being is that subminiature balun
core will saturate and generate its own IMD which will then be amplified
by an already overloaded MMIC. I hope you are beginning to understand the
picture Doug. Just reading a spec sheet is only a small part of what it
takes to produce an application specific product.
>W8JI suggested replacing the MMIC with "a regular CATV transistor."
A 2N5109 would be an excellent choice.
> W4MPY wrote that his ZJ beverage box filled up with water and
>that he
>has tried unsuccessfully to get help from ZJ.
> Both W8JI and KM1H stated that a good beverage will not
>generally need
>a preamplifier.
> K0CS kindly sent me a MAR-3 device which had been supplied by ZJ
>as a
>replacement device.
> Looking at the various Avantek MMIC specs (I can not find my
>cross
>reference to compare the Avantek MSA MMICs to the MAR MMICS) I see
>several
>MMICs with very similar specs (in some cases just different plastic
>packages-
>ie. cases). Almost all of them require 4.5 to 5.5 volts DC. I
>suspect the
>several different MMICs supplied in the ZJ boxes have identical specs
>at 1.8
>MHz and that biasing is not an issue. For those who need to replace
>the
>MMIC, a MAR-1, MAR-3 or equivalent should work fine.
A bad misconception Doug. Each MMIC model has very specific biasing
requirements AND they come with different gains. For example a MAR-1 has
18.5dB of gain and requires a bias of 5V at 17ma.
The MAR-3 has 12.5 dB of gain and requires 5V at 35ma. The MAR-7 has
13.5dB of gain and requires 4V at 22ma. It is NOT a simple matter of
blindly swapping parts that look alike.
IN ADDITION, ZJ supplies different wall warts depending upon the MMIC
used.
Would you swap GaAsFets around the same way without matching or at least
confirming bias requirements?
MMICs are listed
>in
>several of my electronic catalogs for about a dollar each.
> I hope the above is useful information and that some of you get
>your ZJ
>beverage boxes back up and running. Unfortuately, mine still is not
>working
>after I replaced the MMIC. I suspect that the ultraminature balun, as
>KM1H
>described it, needs replacement. Not sure I want to proceed until I
>get a
>schematic. Does anyonwe have one?
It takes about 10 minutes to trace the circuit and come up with the
schematic. Then you can fix the ZJ box and sell it.
Once you get a proper transformer at the antenna, then consider a
tuneable pre-selector with a gain control in the shack. The added front
end selectivity can be a big help.
73 Carl KM1H
>See also my next post on beverage transformers.
>Doug W0AH
>
>
>
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>
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