[Yaesu] Re: FT-1000D 4th IF / Notch and Prod Det - Frequencies? Function?

[KA9S - Jeff]

I would like to understand the frequency scheme of the 4th mixer, 4th 
IF / notch filter, and product detector of my FT-1000D. I would also 
like to know how these stages function. I bought my FT-1000D in 1992 
and it is still my main/most modern rig. It's a workhorse and I 
appreciate that. 

I have the Yaesu FT-1000 Technical Supplement document. It's thick, 
heavy, and full of good information. But it is totally lacking in any 
sort of functional description of the rig. I had been spoiled with 
American boat anchor documentation like Drake and Hammarlund. They go 
to great lengths to describe the function of each circuit in their 
rigs. Yaesu acts like it's some great secret, or puzzle to figure out. 


Here is what I have surmised so far: 


The four local oscillator injection sources are centralized on one 
circuit board they call the "Local Unit". This makes sense as they have 
to coordinate the frequency shift of some injection oscillators to 
realize the IF shift and variable width functions. It's the fourth IF, 
notch filter, and product detector that are not obvious to reverse 
engineer. I have surmised the following frequencies: 


------------------------------------------ 
1st LO - 73.63MHz - 103.62 MHZ (hi side injection to correspond with 
the tuning range of 100 KHz - 30 MHz). 


1st IF - 73.62 MHZ (roofing filter). 


2nd LO - 65.405 MHz +- 1.5 KHz (low side injection +- shift / width). 


2nd IF - 8.215 MHz (first set of filters) 


3rd LO - 8.67112 MHZ +- 1.5 KHz (hi side injection +- shift / width). 


3rd IF - 455 KHz (second set of filters) 


4th LO - 12.635 MHZ ? (High side injection?) 


4th IF - 12.18 MHz? (It seems that this IF exists only to add the LC 
notch filter). 


Product detector oscillator - 12.18 MHZ? 
------------------------------------------ 


It looks like the LC circuit of the notch filter is fixed in frequency. 
It seems that the "tuning" of the notch is accomplished by IF shifting 
the 4th IF. There is an oscillator on the IF Unit (Q 2056). It has two 
crystals (X2002 - 12.280 MHz, and X2003 - 12.283 MHz). I am guessing 
one or the other crystal is active based on the rig being in USB or 
LSB. This oscillator is "pulled" in frequency by a varicap diode D 2080 and the 
front panel variable resistor "Notch" VR 8901-1. This oscillator seems 
to be mixed with the 4th LO though I can't figure any mixer product 
that makes sense to me. 


The choice of 12.18 MHZ for the 4th IF and notch filter seem poor at 
best. (I had to reverse engineer this frequency value as it is not 
documented anywhere I could find.) The achievable Q of a LC notch at 
12.18 MHz is going to leave the notch fairly wide (and it is - a crappy 
notch compared to my Drake R-4C - why bother?). Why they didn't use the 
455 KHz IF to implement the LC notch and leave the receiver a triple 
conversion design is a puzzle to me. 


It seems that the injection signal for the product detector does not 
come from the Local Unit. There is an oscillator Q 3002 with the 
crystal X 3001 - 12.18 MHZ on the AF Unit board. 


Am I even close with this explanation of the 4th IF / notch and product 
detector? Can anyone explain how this is actually working? Is there any 
way to make the notch filter narrower / deeper? 


Thanks - Expert insights always welcome. 


73s - Jeff - KA9S