[time-nuts] Fury Interface Board: How about TI OPA277? -- Very Long Reply

[Bruce Griffiths]

xaos wrote:
> Hello everyone,
>
> Needless to say, but I have also been following this discussion closely.
>
> First, I must confess, I started this project without making clear
> what my requirements were.
>
> Personally, I knew what I wanted. However, when I asked the members of this
> list for their feedback, I did not properly explain the details.
>
> My apologies, to everyone here, for any mix-ups.
>
> The idea was this:
>
> What I have is this:
>
> 1. A Fury GPSDO.
> 2. Quite a few HP OCXO's. Some are 10544A's and others are 1081-xxxx types.
>    Over the years I've collected these from 5328A's or 5370A/B's.
> 3. I have some Crystek CO27VS12DE-02-10.00, OCXO's.
> 4. A bunch of different power supplies. Lambda's are really nice.
> 5. OrCAD Capture/Layout/PSPICE
> 6. Tools and test equipment.
> 6. Some free time.
>
> What I'd like to get:
>
> A MTI OCXO. The 230 series types.
>
> What I am trying to do:
>
> 1. Experiment with different types of 10MHz OCXO's and find one, with best
>    frequency stability and accuracy.
>
> 2. Use the Fury to discipline the OCXO and generate a 10 MHz signal.
>    I need this to feed all my test equipment.
>
> 3. Design a proper system and build a board.
>
> 4. The board must handle the different OCXO's I have. It must take the EFC
>    signal, from the Fury, and translate it to the EFC input of the 
> different OCXO's.
>    The translation circuit must handle 0-(+5VDC) input and have the option
>    of producing:
>   
>    [0-5V] or [0-10V] or [-10 - +10V]
>   
>    This will, not only, handle my current OCXO's but leave room for 
> future expansion.
>    Between these 3 voltage ranges, I think I have most industry OCXO's 
> covered.
>
> 4. While I am making a board for myself, make a few more for
>    anyone here who would be interested.
>
> 5. Have fun.
>
> Why:
>
> My primary, secondary, tertiary and quaternary frequency standards are
> the HP Z3801A's. They are not getting any younger. I need something new 
> here.
>
> That's it.
>
> As expected, the members of time-nuts have been just great with their 
> feedback.
>
> However, the discussion has become heated at times.
>
> This was not a surprise. We are all techies and we do this for a living
> and we'll be damned if we give up a point without a fight.
>
> I do not want this to be a deterrent to the discussion. I want to hear
> from every person who has something to offer.
>
> If I mean, if anyone is getting beat up here, it's me... right? My 
> original design
> is pretty much scrapped.
>
>   
Having more than 1 person is indeed useful in picking up errors that we
all make from time to time.
> The thing is, I expected this to happen. It happens all the time in the 
> real world
> and this is how really good systems are made. By consensus.
>
> Anyway, moving on to more technical details...
>
> Over the course of this discussion the following have become clear.
>
> a. The power supply must have 3 outputs and they must be isolated from 
> each other.
>
>    +24V for the HP OCXO oven.
>    +12V for the OCXO supply
>    +/- 12V for other circuits.
>
> b. The +/-12V, for the EFC translation circuit, might need to get +/-15V 
> from the
>    power supply. If the output is to swing to [-10V] - [+10V] then this
>    might be too close to the 12V rails.
>
>    Does anyone know of a quiet voltage regulator for that range?
>    If I add more filter caps to fix this, would this be enough? Might be 
> bulky.
>
>   
A 723 based regulator with an RC filtered reference is a good benchmark.
With isolated supplies either output can be grounded so that a 723 will
suffice for both positive and negative regulators.
One can always use one of the better engineered variants of the so
called super regulators favoured by some of more technically competent
in the Audio fraternity.
These use a 3 terminal regulator (317 or 337 as appropriate) as a
tracking preregulator and a low noise, low drift zener reference.
> c. In order to handle the different voltages required, the only option 
> is a 24V power supply
>    capable of producing + and - 24V.
>
> c. The 3 grounds must be of a star configuration (from the power supply 
> ground) to avoid noise.
>
> d. The EFC translation circuit is going to be a challenge. The right 
> OPAMP + reference selection
>    is extremely critical. I am currently working on the simulation of 
> the circuit that Bruce has provided.
>
>    I know what Said is saying about, "how much EFC voltage deviation is 
> really required".
>    However, I don't want to worry about that.
>
>    If I have a Crystek, today, I might have brand-X tomorrow. I don't 
> want to play
>    games with the OCXO EFC range. I must be able to handle the entire range.
>
>   
One possibility is perhaps to use a multiplying DACs or equivalent to
set the EFC voltage span and offset.
Some OCXOs have 0-6V EFC ranges.
Some OCXOs have unipolar EFC voltage ranges some have bipolar EFC
voltage ranges.
Diode clamping specific to the OCXO in use may be useful.
> e. The 10MHz output from the OCXO must be connected, via an isolation 
> transformer, back to the Fury.
>    This will isolate the boards. I am thinking of the Mini-Circuits 
> TMO-1-1+ or TMO-1-1T or T1-6-X65.
>
> Questions that I still have:
>
> 1. Besides creating a star configuration, for all 3 grounds, what else 
> should I do to minimize noise
>    and maximize isolation between them?
>   
Use RF transformers for any auxiliary outputs and carefully isolate the
interface board from earth so that the EFC cable shield forms the only
ground connection.
The Fury board connects to ground via its RS232 circuit so it may need
to be grounded.
> 2. In the case of the HP 10544A an isolation amplifier might be needed 
> to connect to the Fury OCXO 10MHz
>    input. The other OCXO's can connect to a 5Ohm load. Is this true?
>   
>    If this is true then shall I just feed the output of all OCXO's 
> through the isolation amplifier?
>    In other words, shall I also do this for the OCXO's that can handle 
> 50 Ohm loads?
>
>   
Yes, you can use the same isolation amplifier buffer for the sinewave
output OCXOs which can drive a 50 ohm load.
For a logic level output OCXO may need a different buffer.
Just use a high input impedance isolation amplifier and have the option
of terminating the input in 50 ohms.
Some OCXOs which can drive a 50 ohm load can also drive a high impedance
load.
To get the isolation amplifier design right just need to get have an
idea of the output levels from the various OCXOs.
So far an E1938A with a +4 dBm output seems to be the lowest and an
1000B with +13dBm output the highest.

The question arises, should provision be made for discipling a 5MHz OCXO
eg FTS1200, OSA 8607 etc?
This can be done if a frequency doubler is used in place of the
isolation amplifier.
A NIST style JFET doubler is one option.
In this case a buffered version of the original 5MHz signal may also be
desirable.
While we are at it, maybe a divider should be available so that a 100MHz
OCXO can be used??
It would also be useful to bring out the OCXO connections on a connector
to accommodate OCXOs like the OSA8607, FTS1200, SC10 etc which use DE9
or other connectors.
>    I don't think I have an option here. If I add a Mini-Circuits 1:1 
> transformer to connect back
>    to the Fury, then some sort of isolation amplifier will be needed.
>
>   
The transformer also allows the SMA connectors on the Fury to be
retained avoiding warranty issues.
> 3. Said said, that a thermistor should be placed between the Fury OCXO 
> VCC and ground. Since I will be
>    breaking out these signals to the interface board, I assume I can 
> connect the thermistor there.
>    Am I missing something there?
>
> 4. Should I be remote sensing the +12V, (from Fury OCXO connection) in 
> order to track my +12V power supply?
>    I get the impression that the Fury controls this voltage as well. Am 
> I correct in this?
>    Bruce suggested this and it sounds like a good idea.
>
>   
I dont remember making that suggestion, the Fury doesnt regulate the 12V
supply, this is the merely the input voltage to its onboard regulators etc.
Thus tracking this voltage may have no particular advantage.
> Well, that's all I can think of now. I've been up all night working on 
> this and I need some sleep.
>
> Before I go, let me just say a big thank you to Bruce, Said, Kevin, John 
> Ackermann and all
> the members of time-nuts for your help.
>
> You guys are great.
>
> Good Night,
>
> -George
>
>
>   
Bruce