Re: Harmonic performance - SSB vs CW
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Allison and Scott
Thinking old-school raises the question about using a broadband transformer to
raise the MOSFET output impedance up to around 5K and then use a Pi-network
as was done in ancient times to match almost anything.? Pre-aligning the Pi-net
on receive mode should allow sneaking up on desired power output without over-
stressing the MOSFETs.? Real problem might be in finding suitable variable
capacitors for the pi-network.? Younger hams probably have never experienced
the pain of an RF Burn.
Arv
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On Sat, Aug 4, 2018 at 12:39 PM Scott McDonald via Groups.Io <ka9p= [email protected]> wrote: There is a good example of such an old school preselector on the AA3SJ Progressive Receiver Page, I think originally designed by W3TS (?).
Pretty much exactly as Allison describes, 2 bands, total HF coverage.? I¡¯ve built a couple that I use for receiver front ends, and other than getting the matching toroids to track and finding a 2 gang tuning cap of 300 of/sec or greater, they are pretty quick and easy to build, especially when compare ty winding 10 bands of toroids.
Scott Ka9p On Aug 4, 2018, at 1:19 PM, ajparent1/KB1GMX < kb1gmx@...> wrote: >>>>Has anybody looked at making those multi-band filters into tunable BPF designs?
That would add some more knobs and possible operator errors but it is an?
interesting thought.<<<
Old school:
A pre-selector.? The form for tha tis a double tuned circuit with top coupling between
elements and link coupling to the adjacent 50 ohnm circuits.? ?It would require a
multi-gang capacitor as varicap diodes may be modulated by RF at that level
(testing required) and switching as you would need a few maybe two bands to
do 3-30mhz.? ?At most one knob as the Raduino can do the band select.
Its adjustment would be fairly uncritical as it would be in the RX path poor RX
set the pre-selector.??? However If a real cap is used a dual section of the 30-365
form is the minimum but would cover 3-30 in two bands with switched coils.?
Varicap diodes do not have that tuning range so 3 bands possibly 4 using switched coils.
I tried that for the RX with good result for some of the spurs and out of band signals.
since it was in place of the 30mhz filter it did have significantly less spur output
as it was bandpass.? I put it aside for a later day as it made for space issues in
the desired box.? ?That and suggesting it seemed a big complex mod for most.
Allison
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