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

Thu May 12 11:29:43 UTC 2016

Hi

Probably the simplest limiter that works at all is one based on a couple of op amps. You
need the active filter function (high pass + low pass) in front of the limiter. You also might 
as well amplify the signal up to a significant peak to peak without limiting. That gets you 
the fastest edge speed with no nasty impact. The old style approach was to feed that 
signal into an op amp with a diode pair as feedback. Net result was a 1.4V p-p signal 
that would drive a counter pretty well. A more modern approach would be a series 
of noise optimized (Collins) limiter stages. 

Bob

> On May 12, 2016, at 1:51 AM, Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
> 
> The input noise of a 125 series gate will be far larger than the noise at the mixer output at the beat frequency. A 125 may be useful as the output stage of a Collins style limiter chain wherein which the gain and bandwidth of each successive stage increases in a way that ensures that the output slew rate increases whilst minimising the output jitter due to amplified noise..
> 
> There will be some degree of spur cancellation between the 2 channels but the cancellation will be far from perfect. Its better to use a cascaded mix/divide chain with a DDS to reduce the amplitude of any spurs by ensuring that the DDS only contributes a small fraction of the offset frequency.  
> At least a modular system will allow these issues to be explored.
> 
> Bruce 
> 
>    On Thursday, 12 May 2016 5:00 PM, Bruce Griffiths <bruce.griffiths at xtra..co.nz> wrote:
> 
> 
> 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 3:33 PM, Bruce Griffiths <bruce.griffiths at xtra..co.nz> wrote:
> 
> 
> 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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