On Mon, 23 Feb 2004 21:12:44 +0200, Paul Keinanen
wrote:


If you have a very high-Q tunable front end filter, you usually get
away with a mediocre IP3. However, if wide open (1 octave) front end
filters are used, you would benefit from a high IP3 if you are using a
decent antenna.

If you try to receive the 7000 .. 7100 kHz amateur band in Europe,
with 100-500 kW broadcast transmitters every 5 kHz starting at 7105
kHz, the input IP3 number would have to be at least 20 dB higher than
the strongest broadcast signals or even 30 dB higher if you try to
receive QRP stations.

Attenuating the whole antenna signal with a passive attenuator will
also rapidly drop the IP3 products, but sooner or later the weak
desired signal will be lost in attenuator thermal noise.

Paul OH3LWR

for 40m band even an SBL1 may be found inadequate and the solution is
- as you say - a high-Q input selectivity using a 7040kHz ceramic
filter which passes 7,0-7.1MHz within -6dB bandwidth.

I've been told that my FT-902 with schottky ring mixer is worse than
FT-901 with DG-mosfet 1st mixer, but I have no idea what is the main
problem.
I tested a mosfet mixer on 136kHz and found it adequate in most
respects apart from IF leakage, and ended up with 74HC4066 mixer -
used more for reason of being curious about it than that it was really
needed, see http://home.online.no/~la8ak/L1.htm , but it couldn't be
used on 60MHz as the orignal request was for, it might work on 40M
with HEF4013 LO divider on +10VDC. Again you are fed with lot of crop
from persons who don't understand the difference between what is
optimum and what it the most expensive, so I expect somebody to telll
me the nonsense of using FST3125 instead of 74HC4066 without
understanding that the latter has almost 6dB higher IP3, and it is
more fun to use an inexpensive device in spite of having the house
filled up with lot of other better and expensive devices you won't
have time to use in a lifetime.

73 JM

----
Jan-Martin, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/