TopBand: 1000MP RX

[w8ji.tom]

I hope Bill doesn't mind this post, but this does apply to 160 meters.
There are a lot of FT1000's in use. 160 meters always requires exceptional
dynamic range when DX signals are weak and the band is active. 

Just some observations on my brand new regular FT-1000 I used in the Stew
Perry. I found it generated internal IMD when exposed to 20 over nine
signals a few kHz away. I was so disappointed I stopped the contest, and
took the receiver out to the barn for testing. 

This type of IMD sound like garbled CW, like the beeps and bloops heard
listening to the novice band on a wide AM (no BFO) receiver. 

When two test signals 40 over nine two kHz apart were injected, IMD was S7
two kHz up or down, and extended out every two kHz at least 12 kHz. Any
narrow spaced test signals more than 10 over nine produced audible IMD. 
  
I located the primary sources of overload. The problem is even with the
noise blanker is OFF, the first amplifier FET in the noise blanker operates
at full gain. It drives the second noise blanker stage so hard it generates
mixing products. These spurious signals couple back through wiring directly
into the 8 MHz IF stage. After correcting the switching by removing forward
bias on the noise blanker amplifier using three diodes and a resistor, IMD
under the same test condition dropped from S7 to S1.

Please note the noise blanker could be OFF, and the problem still occurred
(before the mod).

It now takes a pair of 50 dB over nine signals to overload the receiver,
and the receiver operates fine with the internal RF amplifier on. Not a bad
change for less than 50 cents in parts and five minutes of time (after
spending two or three hours tracking it down stage by stage).

It looks like performance can be bumped up another 5-10 dB by adding an
amplifier after the narrow 8MHz filter, and lowering front end gain 5-10
dB. The LAST thing I would do is add an amplifier to the 70 MHz IF strip or
any earlier stage, because the 70 MHz to 8 MHz mixer is a primary source of
IMD problems. That might be fine for ten meters, but not for 160 with close
spaced strong signals near weak DX if you have a low noise floor.

73 Tom

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