[CQ-Contest] RX Heresy?
s56a at bit.si
Mon Mar 1 12:37:46 PST 2010
K5ZD wrote yet another K3 "made in USA" hype: No pops or beeps or phantom
There are laws of physics governing electrical signals mixing and Collins
interpreted them in 1970's on noisy HF with IF at least 3 times of the
highest frequency received. They used 105 MHz overtone crystal filters
about 20 kHz wide which also helps noise blanking without streched ringing.
Hamradio used 45 and 75 MHz versions. Cold war ended, satelites are
numerous, optic cables everywhere and HF bands became quiter. Hams around a
world got cheap military power amplifiers and begun causing QRM to
themselves. The rescue came from USA manufacturers reinventing the wheel
with low IF radios using sharp CW ladder filters produced from cheap PC
timing crystals. Low frequency VCO have less wideband noise, PLL with DDS
is cheap and shouldn't generate beeps. N7VE reinvented Oxner pasive FET
mixer for SDR and they provide nice +40 dBm IP3 although we need only +20 dB
even in EU on 40 m. ARRL begun measuring IP3 with strong 2 kHz separated
signals occuring only in contests and poor DX expeditions. Now even Yaesu
is forced to go back to 9 MHz IF! But we can't change the IF leak and image
RF signals into "phantoms". Cheap SMD RF chokes are good enough for 50 Ohm
RX filters saving Flexradio from blocking. DSP AGC has matured so no more
pops. Even HB ADAT-200 100% digital radio is available!
Disclamer: This is NOT April 1st posting but I wonder how many contesters
understand my Serbo-English joke?
LP MMM S56A
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