[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


Copyright (c) 2013.  Not for redistribution








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