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

Bob Camp kb8tq at n1k.org
Thu May 12 17:56:57 EDT 2016


Hi

So, now that the *is* an alternative to ‘125 buffers … What do you need?

You can string enough inverters in series to hit any isolation spec. They are cheap and easy 
to string together. The question is: how good are they? You do a sine to square conversion 
and then split the digital signal. Anything in the first part (sine to square) will be common to 
both channels. That should let it drop out (first order at least). The outputs between the two
split signals should be below 1x10^-13 ADEV at 1 second. Is that good enough? It depends on
what you are doing. It should drop by 1/sqrt(tau) down into the “likely does not matter” range.
One nice thing about the ‘125 buffer is that you can be quite sure of saturating the mixers. Variation
in the level of the offset oscillator will not be an issue. 

Bob

> On May 12, 2016, at 8:24 AM, John Ackermann N8UR <jra at febo.com> wrote:
> 
> I don't mean to hijack the thread, but may be able to avoid Bob having to reinvent the wheel.  A few years ago I built up the PCB for a high isolation, low noise buffer amp based on a design by Bruce Griffiths. The board is all set for production; I just ran out of time.
> 
> The "TNS-BUF" works extremely well, with a phase noise floor below -170dBc/Hz, and isolation of at least 110dB.  It's designed to run from 18V but will operate from 12V with less headroom.  The gain is set by jumpers from about -3dB to +7dB.  Frequency range is about 100kHz - 30MHz.
> 
> It's a 1.75 x 3.75 board using SMT components.  There are no fine-pitch parts, but there are a lot of passives so it's a bit tedious to build.
> 
> Ideally, I'd like to put together a group order through TAPR to do a batch of assembled boards.  To make that economically feasible, we'd need to order at least 50 units.  I haven't re-priced the components lately, but think (no promises) that the price for an assembled board would be less than $100 each at that quantity, and would go down significantly if we could do 100.  If there isn't enough interest to do a production run, I'll post the design files so folks can roll their own.
> 
> I'll try to get a web page put together with schematics, performance results, and a picture.  In the meantime, if you'd be interested in joining a group purchase, please let me know off-line.
> 
> Again, sorry that this isn't a fully baked product announcement, but with the recent discussion about buffer amps I thought I'd toss this out.
> 
> John
> ----
> On 5/11/2016 10:37 PM, Bob Stewart 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
>>  >>
>>  >
>>  <NIST_120dB_isolation_amplifier_schematic_email.gif><Iso_amp_NIST_schematic_with_values_5MHz_1997_email.gif>_______________________________________________
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