[Premium-Rx] Reaction Instruments 685 AM-FM demodulator

[Charles Steinmetz]

Peter wrote:

>[FM] Detection is done with a MC1496 balanced modulator/demodulator 
>chip.  One input is fed raw IF and the second is after going through 
>one of the four selected filter sets.    *   *   *    The 1496 
>detects the phase difference and produces a voltage proportional to 
>the deviation of frequency from center.    *   *   *    Has anyone 
>else ever seen such a scheme?

You used to see this circuit in IC cookbooks.  Balanced modulators 
can be used as phase detectors, and of course phase and frequency are 
kissing cousins (frequency being the time derivative of phase, and 
phase the time integral of frequency).  That said, using phase 
detectors to demodulate FM is an inherently noisy way to go about it 
and the 1496 is a particularly noisy balanced modulator, so the 
circuit was never popular.

>one set consists of 2N4447 FETs and the second set are 2N2222A 
>bipolars.  You cannot parallel the inputs to these two as the 
>bipolar B-E junction will saturate at about 0.65 volts which is well 
>below the 3-4 volt threshold for the 2N4447, so the 2N4447's will 
>never turn on.

The 2N4447s are depletion-mode JFETs, so they are fully on when the 
gate-source voltage is zero.  You need to pull the gates 2-10v below 
the source to turn them fully off.  Note that you can only 
reverse-bias the 2N2222A's B-E junction by 5 or 6v before you degrade 
or damage the device, so it's possible you wouldn't be able to turn 
the FETs *off* without breaking the bipolars.  (You also don't want 
to forward-bias the FET gate junction, but that is a function issue, 
not a damage issue.)

>In any case I'm going to try and find some 2N4447's and restore it 
>to a design which will work as intended.  Again, comments?

I believe only Siliconix and Crystalonics made 2N4447s.  I don't 
believe Siliconix still does.  My most recent Crystalonics catalog is 
from 2013, and they still listed them at that time.  Expect them to 
be expensive.  2N4447s are similar to 2N5432s (Crystalonics listed 
those, too, and I believe InterFET makes them, as 
well).  Alternatively, the J106 should work (available for less than 
a dollar from Fairchild and others), although you may need to replace 
both sets if they need to match.

Best regards,

Charles