[Yaesu] FT-1000MP Sub-receiver Spurious Response

[Rob Hinz]

Hello All,

I have discovered a rather obnoxious problem with my FT-1000MP. I suspect 
that this is a design problem rather than a defect in my particular radio. 
So, I would like to toss this out to all of you and see if anyone else has 
heard the same thing or can reproduce it. Here it is: Main RX tuned to any 
band 160-15M (this just changes the input band pass to something other that 
10M), Sub RX tuned to 28.040 AM, I hear a strong local FM station at 
approximately 103 MHz. In fact, I can hear strong mixing products from this 
station from 28.000-28.075. This occurs with nothing plugged into the radio 
except the power cord and I have added a ferrite to the power cord. The 
attenuator, pre-amp and antenna settings have no effect on this problem. 
The received signal strength increases when I touch the chassis of the 
radio. I cannot hear anything on the main RX so this is not a "real" 
signal.

My initial thoughts on this problem are as follows. One of the worst 
spurious responses from a high level mixer is the 2Flo-Frf response (that 
is the 2*LO frequency minus the RF signal frequency). This response is only 
down 35 dB from the desired Flo-Frf response (see U. Rohde, et al, 
Communications Receivers 2nd Edition). The sub receiver first IF is at 47 
MHz. The LO switching the first stage mixer has a frequency range of 
47.21-77.21 MHz. When the Sub RX is tuned to 28 MHz this LO is 75 MHz 
(75-28=47). The interfering signal is at 103 MHz. Twice the LO is 150 MHz. 
So, 2Flo-Frf is 150-103= 47 MHz. This is the first IF frequency of the 
sub-receiver. This, I think, is why I hear the FM station.

The potential for this type of problem exists in any receiver design. 
Normally, this problem is solved by making certain that strong out of band 
signals never make it to the RF input of the mixer. It appears that strong 
VHF signals are making it to the input of the sub-receiver first stage 
mixer in my FT-1000MP.

So, has anyone else seen this problem? Can anyone reproduce it? If  you try 
to reproduce it remember that you may not have a strong local FM station on 
103 MHz so you may have to use a different frequency. Sub RX tuned to 29 
MHz will tune 104 MHz etc. Any thoughts on my analysis of the problem or on 
possible solutions?

73, Rob Hinz - N7ATM



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