Re: Harmonics


ajparent1/KB1GMX <kb1gmx@...>
 

Jerry,
It referred to your post  #57037
Its the exercise of why DBMs look easy but make people unhappy.
Nowhere is ther e more than one conversion in that discussion or even a receiver.
its about what you get from mixers (some much worse than others) for two simple
signals.

Now to your topic:
>>>>Perhaps my vague suggestion of going back to a single conversion rig.
The thought being, if we are adding all these filters between the first mixer and the power amp,
could make them all bandpass and do away with the 45mhz IF.  <<<<

First be warned its long and goes round some covering a lot of ground for what and why.

Opening:
The problem with single conversion is the choice of IF.  Minima used 20Mhz and 15M was 
too close for simple low loss wide filters.  The Minima issues spawned HF1 which begat 
uBitx.  The underlying desire was for low cost continuous tuning from some low frequency 
to at least 30mhz.   That is its history and its important as it is why some paths and 
choices came to be.

Single conversion does work.  I have 14 radios that mange to do well on on or many
bands though 2m!.  A few are even multiband radios.

Those that come to mind are the Tempo-one (9mhz if) but 17M and 30M are near impossible.
WM20 has an 8mhz IF, which is too close to 40M to do that.

Two of my radios have nice commercial 8 pole filters at 7.8mhz which rule out 40M also too close.

The Atlas-210X a classic 80/40/20/15/10 radio uses a IF of 5.520mhz single conversion.
It might work on 30M (the 2if spur is 10.44mhz but can be trapped). All the other possible
bands are a go save for 60M (way to close to the IF to filter).

FYI the HW101 is dual conversion low IF at 3.395mhz and the second conversions is
hIghly pre-selected in the 8mhz range (tunable IF) to keep birdies down.  This was the
common technique for many radios and also KWM2, S51, HRO others many using
455khz filters in the last if.

There is a lot of history there for single and dual (even triple) conversion
and the key is those known to be great radios were big, well shielded and well 
filtered ( some with frightening mechanics to tune many stages).

At least two of the commercial radios I have Argo505 and the Triton have a 9mhz IF but are
from the 80/40/20/15/10 only era.  So bands like 17M are plagued with 2if (~18mhz) spurs, 
30M requires really good filters to keep 9mhz out of the air.  For those radios 60, 12M
are easy though. 

The only radio I have that has 500khz to 30mhz (+6M) tuning and a 9mhz HF First IF
is the Tentec Eagle first IF is 9mhz but the level of filtering is intense. You cannot tune
close to 9mhz without hearing negative effects.

So a single conversion can work but you have to jump a few hurdles.
IF it were easy everyone would have done it already.

If one is willing to dismiss with all that Phasing radios using digital IFs
are doable now with inexpensive processors and fairly simple RF to
base band conversion. That is other wise known as image rejecting
DC RX/TX where the image is the other sideband!

Allison







Join BITX20@groups.io to automatically receive all group messages.