[R-390] Multicouplers - active vs. passive
2002tii
bmw2002tii at nerdshack.com
Wed Jan 16 15:44:33 EST 2013
Bill wrote:
>As far as the preamp design goes, I believe the Ten-Tec RX-331 is
>similar, if not exactly the same.
That is correct. The part numbering is different, so the parts I
referred to in my prior e-mail are numbered differently on the 331
drawing. The operation of T4 (331 = T2) is more obvious on the 331
drawing. T3 (331 = T1) is the TEM-mode transmission-line
transformer. Note that you need to match the polarity of C117 (331 =
C111) to the DC level at the input.
This is a rather common design for a low-noise, high-dynamic range
broadband amplifier, often seen as the output amplifier for mixers
(the 340 uses an almost identical amplifier following the first
mixer, and a 4-FET single-ended version following the second
mixer). I pointed to the Ten-Tec implementation simply because the
schematic is easy to find on the web. Others have used more or fewer
parallel FETs, as well as push-pull parallel BJTs, along with many
other minor differences.
Ten-Tec used L59 and L60 (331: L64 and L63) to roll off the top
end. These give slight peaking at 30 MHz (~ 1dB) and a -3dB point
around 70 MHz. Without them, there is no peaking and the -3dB point
is ~150 MHz (highly dependent on the transformers and the physical
layout and shielding). I prefer no peaking, so I previously
recommended omitting L59 and L60. However, they do provide some
attenuation of broadcast FM and commercial VHF signals, so choose for
yourself if you build one. (I have two monstrous FM signals and one
very strong 150 MHz paging system near me, and have no problem
without L59 and L60.)
The performance of any individual unit will be very dependent on the
three transformers and the physical layout and shielding.
Because the output impedance of the amplifier is well-defined, and
the input impedance of a passive splitter is also well-defined
(assuming that all outputs are properly terminated), it is easy to
put passive filters or attenuators between the amp and the splitter
if you so desire.
I have a legible, one-page copy of the 340 RF amp I can send to those
interested. (Respond OFF-LIST, and if you "Reply To" this message,
remove "[R-390]" from the "Subject:" line of your e-mail so you go
into the right mailbox.)
Best regards,
Don
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