EMRFD Message Archive 11493

Message Date From Subject
11493 2015-08-12 19:09:28 jwolczanski "Diplexer" from a 1980's homebrew DC transceiver
All this mixer talk....

I had a home-brew 40 meter direct conversion transceiver (one watt) that served me well for many years.  I eventually tore it apart to "improve it" to my everlasting regret.  It was a compilation of various circuits

The  DBM was home-constructed with trifilar wound ferrite's and the 4 diodes are labeled GE DHD100.  I remember it needed a fair bit of LO drive.

Now- the diplexer was a .01 capacitor in series with a 50 ohm resistor to ground - - followed by a passive audio low-pass filter taken from p77 in "Solid State Design for the Radio Amateur".  It was a pi configuration with an 88mH toroid and a .1uFd cap on the mixer side...and a .5uFd on the output.

Worked great - but I always used a link-type balanced tuner with tuned feeders....and later, when I moved near a BC station, I put a two-stage passive RF pre-selector in front of the detector.

Question:  Now, looking at very complex diplexers 25 - 30 years later, I wonder how effective this diplexer was?  It sure SEEMED pretty good!



Warrenton, VA

11494 2015-08-14 12:20:45 kb1gmx Re: "Diplexer" from a 1980's homebrew DC transceiver

Question:  Now, looking at very complex diplexers 25 - 30 years later, I wonder how effective this diplexer was?  It sure SEEMED pretty good!

It is good.  For Any RF products the .01 cap directs that to the 50 ohm load.  For audio the 88mh coil
is an effective low pass filter.  Likely enough overlap of the two to catch the high audio products like 
5khz and up. For a DC RX using DBM thats what it has to do.

For IF, at 9mhz you have three bands of interest, 9mhz stuff out to a filter, all the stuff more than a few Khz above the filter (roughly 9.005mhz and up) and the band below (roughly 8.995 and below) need to be dealt with.  

The goal is to make the DBM see a "SWR" of 2:1 or less giver or take for all possible frequencies.  
The more critical and exacting the radio the more critical that becomes.   Also not all ports of DBMs
are impacted equally.  The IF port is by far the most critical.  The LO or RF port tends to be far less 

What happens if the ports are not terminated well, varies!  The worst case is there will be birdies or 
spurs.  Wide band systems designed for lowest insertion loss are more sensitive to that.

11495 2015-08-15 09:55:16 jwolczanski Re: "Diplexer" from a 1980's homebrew DC transceiver
Thanks Alison

I couldn't remember where I got that from, but just looked at W7EL's timeless "Optimized QRP Transceiver" and see a similar diplexer - so that's probably where I first saw (and copied) it. 

I suppose, if one can afford the loss, a resistive pad sounds like a good idea?

W7EL's little transceiver is enjoying a second life, or was that a third - and why not, it seems to have all the elements of a solid DC design.  Roy came to the QRP/Tech table at "Four Days in May" (FDIM) this year with a big grin on his face and that little transceiver in hand.  It's even more amazing in the flesh.  One of the guys in the QRP/Tech group is trying to replicate it using SMT parts.  I think eventually you run out of room for front panel controls.  I certainly never had that problem with MY version of his rig which must be equal in volume a KWM2A.  Har

Roy's rig will work forever - there's no room for the smoke to leak out............

Warrenton, VA