[time-nuts] DMTD was: high rev isolation amps

Bob Stewart bob at evoria.net
Wed May 11 23:44:11 EDT 2016


Hi Bruce,

I was thinking of using the 125s as the zero crossing detectors.

Won't the DDS spurs cancel out across the 2 DBMS?

Bob
--------------------------------------------
On Wed, 5/11/16, Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:

 Subject: Re: [time-nuts] DMTD was: high rev isolation amps
 To: "Bob Stewart" <bob at evoria.net>, "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
 Date: Wednesday, May 11, 2016, 10:33 PM
 
 For
 DMTD service with an offset frequency of 100Hz or less the
 flicker phase noise of the mixer is more important than its
 wideband PN floor.The
 RDP series phase detectors are amongst the lowest PN noise
 mixer/phase detectors commercially available. They are
 considerably quieter than the TUF series mixers.To
 minimise noise both the RF and LO ports should be
 saturated.The
 specified IF port termination at the sum frequenncy should
 be used  to avoid degrading the isolation between the RF
 and LO ports.
  For
 even lower flicker noise using a mixer assembled using
 2N2222A's as diodes is even quieter as NIST have
 shown.
 125
 gates have large flicker noise and using these in the front
 end of the zero crossing detector will increase the noise
 substantially over that obtainable with a well designed zero
 crossing amplifier chain using opamps.Using
 a DDS to generate the offset frequency raises the ugly
 sepctre of unwanted close in
 spurs, unless one chooses one of the magic frequency outputs
 that is free of such spurs.
 Bruce
 
 
 
    On Thursday, 12 May
 2016 3:00 PM, Bob Stewart <bob at evoria.net> wrote:
   
 
  Hi
 Bob,
 
 OK, I can see that. 
 I had been wondering about how to build it out, but it seems
 like each module should be on its own PCB for isolation. 
 That keeps the costs down, except for the numerous SMA
 jumpers - and the eventual big box.
 
 Bob
 --------------------------------------------
 On Wed, 5/11/16, Bob Camp <kb8tq at n1k.org>
 wrote:
 
  Subject: Re:
 [time-nuts] DMTD was: high rev isolation amps
  To: "Bob Stewart" <bob at evoria.net>,
 "Discussion of precise time and frequency
 measurement" <time-nuts at febo.com>
  Date: Wednesday, May 11, 2016, 8:44 PM
  
  Hi
  
  For extra added “Time Nut” grade fun:
  
  Clip the collector lead off
 of
  the SOT-223 packages (between emitter
 and base leads). You
  probably will gain a
 couple of db of isolation. (running
  common
 base).
  
  Bob
  
  > On May 11, 2016, at
 8:24 PM, Bob Stewart
  <bob at evoria.net>
  wrote:
  > 
 
 > Hi Bob et
  al,
  >
 
  > I'm
  finally
 drawing up a schematic for a DMTD and I thought
  I'd use the NBS circuits for a lot of
 it.  So, how
  about PZT3904 transistors in
 the isolation amps? 
  They're in
 SOT-223 with the heat sink tab.
  > 
  > I figure to use some
 
 version of the TUF-1 for the DBMs.  The first version
 will
  be a bit breadboardish.  I'll use
 the PRS-45A as the
  reference oscillator and
 the 8640B as the offset
  oscillator.  If
 it's workable, then I thought I'd
 
 put an Analog Devices DDS onboard with a small PIC to set
 it
  up.  I haven't gotten as far as the
 zero-crossing
  detectors yet, but I was
 thinking of using 125 gates.
  > 
  > Bob
  >
 
 --------------------------------------------
  > On Tue, 3/29/16, Bob Camp <kb8tq at n1k.org>
  wrote:
  > 
 
 > Subject:
  Re: [time-nuts] high rev
 isolation amps
  >
  To:
 "Discussion of precise time and frequency
  measurement" <time-nuts at febo.com>
  > Date: Tuesday, March 29, 2016, 6:08 PM
  > 
  > Hi
  > 
  > There were (and
  maybe still are) SOT-89 versions of the
  >
  2N3804 and 3906. They
 will handle more
  >
 
 power than most of the other versions. That gives you
  better
  > Vce on the
 string. They also
  have less
  > package inductance which
 
 helps tie the base to ground. If
  >
 you
  are building some of those circuits,
 they
  > are worth looking for.
  > 
  > Bob
  > 
  >> On Mar 29,
 2016,
  at 6:47 AM, Charles Steinmetz <csteinmetz at yandex.com>
  > wrote:
  >> 
  >> See below for schematics of the
 NIST
  isolation
  >
 amplifiers from 1990 and
  1997.  NIST
 reported the
  > isolation as
  >120dB.
  >> 
  >> I built isolation amplifiers
 similar
  to these (with
 
 > lower-noise power
  supplies and biasing
 tinkered slightly for
  > better dynamic
 range), and with careful
  construction
  > achieved isolation
 
 substantially better than 120dB (see my
 
 >
  post of 11/25/14 for more details).
  >>
  
 
 >> Best regards,
  >> 
  >> Charles
  >> 
  >
 
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  >
 
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