Field Effect Transistor Question

[Michael Tallent]

Jim:

I would design for an input impedaqnce of 1 meg ohm, then use a 10X
oscilloscope probe for the pick up lead.  That will give you a 10-30 volt
range with a fet source follower stage.  If you need to got to 100 v then
either a 100X scope probe, or build a 10X resitive attenuator with a 1 meg
input and connect it to the fet stage, this combined with the 10X scope
probe will get you to 100v if the probe is rated that high.  Should be no
problem with the bandwidth, you could put a trimmer cap to comphensate the
attenuator, but for a freq counter this would probably not be necessary.
The fet stage will have a fairly low source impedance, but will not drive a
50-75 ohm termination without some additional stages.  Most any rf type fet
should work. 2N3819, MPF-102, 2N4416.  I have also used a PNP-NPN emitter
follower stage with feedback capacitor to "bootstrap" the input impedance to
very high values.  Advantages over an fet is almost unity gain and can
handle higher voltage, 10-20 v p-p.

Mike W6MXV  in KY
Started designing video stuff in 1964, retired in 1992, now have no time to
do anything.

----- Original Message -----
From: "James W. Miller" <JimWMiller at AOL.COM>
To: <BOATANCHORS at LISTSERV.TEMPE.GOV>
Sent: Tuesday, October 30, 2001 12:02 PM
Subject: Field Effect Transistor Question


> Hello,
> I've received a number of interesting suggestions on how to interface a BA
rig to a modern Freq Counter. Thanks!
>
> (Unfortunately I accidently deleted some email from Barry on this subject.
Barry please send ur email address so that I can respond.)
>
> The Circuit requirements:
> 1. High Z in, Low Z out.
> 2. Accepts 0.5 to 100+ VPP input.
> 3. Clips (without loss of High Z) to protect freq counter.
> 4. Works up to 30 Mhz.
> 5. Must be designed on K.I.S.S principle (Keep It Simple Stupid).
>
> Apparently there is no perfect ckt that meets all requirements exactly but
will design and test a working interface this weekend. I'll post the ckt.
Comments, questions and (especially) alternative circuits would be fun and
appreciated.
>
> Thanks & 73,
> Jim
> WA2UMP
>
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