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

[Peter Gottlieb]

Alright, did a bunch of reading and pretty much get it regarding filter slopes, 
phase and so forth.  Life on the edge of a filter. It looks like I can use an 
all pass or bandpass but I may need several poles to gen the phase response 
slope necessary to get a full signal with the 5 kHz or so deviation used on 
comms channels. I'll continue to read, concentrating on circuits used for 
similar kinds of deviation detection.  I'll also download the latest LTSpice so 
I can simulate before building.  As the filters are plug-in I can make up my own 
small PCB so I don't have to change anything existing.

The information and leads you are giving me are invaluable.

Peter


On 11/1/2015 10:38 PM, Charles Steinmetz wrote:
> Peter wrote:
>
>> The 2N5432 is interesting, while non stocked it is "available" from Digikey 
>> for only $46.76 each. Yikes.  There are some sellers of them on ebay, one guy 
>> has them for $4.50 each plus $1 each shipping.
>
> All of the 2N5432s listed on ebay are almost certainly counterfeit -- don't 
> bother with them.  Crystalonics sells small quantities directly, so it's worth 
> a call to inquire about both 4447s and 5432s.
>
>> Costly, but if that's what it takes to fix this I would do it.  Of course I 
>> could re-jigger the circuit to simply use something like 2N7000s which are 
>> dirt cheap.
>
> Enhancement-mode MOSFETs are unlikely to work without major redesign.  
> Depending on the circuit, they may not work at all because of the substrate 
> diode.  2N4447s are high-current JFETs of the type often recommended for 
> chopper circuits or other switching duty.  As I said, the J106 should work 
> fine but you might need to replace all 8 (and check the pinout before you 
> solder them in).
>
>> The biggest step will be to design a filter which will give good phase 
>> response for the narrow bandwidths I want' for example 10 kHz.  That may be 
>> tricky.  I suppose maybe using a crystal filter?
>
> Oh, heavens no.  Crystal filters (any filters with a sharp cutoff) are the 
> very antithesis of linear phase and will cause greatly increased distortion.  
> As you observed, the filters in the FM demodulator have very gentle slopes.  
> They aren't being used to pass some frequencies and reject others -- they set 
> the conversion gain of the detector, similarly to how an I-Q detector works. 
> They could well be all-pass filters.  The passband is set elsewhere, probably 
> in the IF stages (and that filtering also needs to have a linear phase 
> characteristic -- Bessel filters are often used).
>
> You may want to read up on the theory of FM demodulation with a balanced 
> mixer, so you have a firm understanding of what to expect when you change things.
>
> Best regards,
>
> Charles
>
>
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