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?

Expert insights always welcome.

73s - Jeff - KA9S