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

[Chris Codella, W2PA]

More observations on the notch operation.

The notch circuit is a bit tricky to figure out, but aside from the IF 
frequency, it works as you surmised.  Here's a little more detail:

The IF unit schematic is confusing because there is a pin on P8901 
labelled "NOTCH" that runs over to Q2056, the 12 MHz oscillator - but 
that is not the notch on/off control line.  The notch control line comes 
in on pin 3 of P4 and switches the the notch in and out using Q2020.  
When the notch control line goes high, Q2020 turns on, turning off D2056 
which forces the signal through the notch tuned circuit (when D2056 is 
on, it routes the signal around this circuit).

L2013 and C2084 form a series resonant circuit to ground at 503.3 kHz, 
well above the 4th IF.  Although T2011 has no value indicated, it seems 
to me that together with C2088, C2086 and C2087, they "pull" the 503 kHz 
notch down into the 4th IF, partly under the control of VR2003 (which is 
not the front panel control).  It is this circuit that gets bypassed 
under control of Q2020 and D2056.

The front panel control, VR8901-1, connects to Q2056 on the IF unit 
through a pin on P8901.  It looks to me like here it does what you 
surmised; namely it shifts the 12.280/283 oscillator.  The net result is 
everything coming out of the 4th mixer slides up and down relative to 
the notch at the 4th IF of 100 kHz (see previous posting).

Now to look at the detector stages.
I'm either making progress or digging myself into a hole.  Either way 
it's educational.

73,
Chris, W2PA


KA9S - Jeff wrote:

>I would like to understand the frequency scheme of the 4th mixer, 4th 
>IF / notch filter, and product detector of my FT-1000D. 
>  
>