[time-nuts] FE-5680A's suitability for use as a 10 MHz reference for microwave transverters

[C. Turner]

Hello,

As has been mentioned here before, the output of the "non-tunable" 
FE-5680A's has been noted to have low-level spurs in it - no doubt due 
to the way the various frequency loops are derived within, some using 
DDS techniques.  It is for this reason that when I packaged my FE-5680A 
in its own, stand-alone enclosure I included a fairly narrow band (+/-6 
kHz @ -6dBc) crystal-based bandpass filter in the output.


After more recent testing of two FE-5680A's using two different 10 GHz 
microwave transverters, I've determined that this filtering just isn't 
enough.  At first, it was assumed/hoped that the racket that I was 
hearing was coming from somewhere else - perhaps the switching 
up-converter or some other interaction - or just something "odd" about 
my homebrew 10 GHz transverter, but this is, unfortunately, not the case.


In testing with a DownEast Microwave 10 GHz transverter fitted with an 
N5AC synthesizer, the CW notes sounded nice and clean when locked to the 
Z3801 and there was only a trace of modulation that I'd not really 
noticed before when I used the Efratom LPRO-101, but when the '5680A was 
connected, the incidental PM was bad enough that it was difficult to 
determine where, exactly, zero beat was!  Since the synthesizer uses a 
fairly high reference frequency internally there was little impediment 
to the low-level phase modulation on the reference.


I compared this with my own homebrew 10 GHz transverter.  This unit uses 
an 18.4 MHz Butler VCXO that is multiplied to 110.4 MHz which is then 
fed to a "brick" oscillator with the 110.4 MHz being compared to the 10 
MHz reference using a harmonic mixer, locking to the 400 kHz residual.  
Since this unit has a comparatively low loop bandwidth in the VCXO the 
grunge was considerably reduced, but still objectionable, giving some 
hope that a simple VCXO scheme might make the '5680A usable.


I still have yet to do a more-detailed analysis of the phase modulation 
that is appearing on the 10 GHz signals, but I can clearly hear a low 
frequency modulation source (perhaps the lock-in amplifier) plus a 
myriad of other audio frequency components and their harmonics.  Again, 
with the LPRO-101 was very "clean" by comparison and I could *just* hear 
some similar, very low-level noises in the background that I'd not 
really noticed before.


As it is, the '5680A-based reference is unusable with the N5AC 
synthesizer and its wide loop bandwidth and "almost" usable with my 
homebrew transverter and its comparatively narrow loop bandwidth.  I'm 
now bent on making the '5680A usable as a microwave reference, but my 
current plans are to build a simple 10 MHz Butler VCXO and then lock it 
to the '5680A using a very "slow" loop filter:  In that way, I'm hoping 
that the phase noise will be largely that of the 10 MHz VCXO and its 
cheap CPU-type crystal rather than the '5680A!


Clint
KA7OEI


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Re: [time-nuts] FE-5680A's suitability for use as a 10 MHz reference for microwave transverters

[Bob Camp]

Hi

The phase noise of the FE's is not great, even inside +/- 3 KHz. To do an
adequate job of cleaning them up, something like a < 1 Hz loop bandwidth PLL
would be needed. 

Bob

-Original Message-
From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On
Behalf Of C. Turner
Sent: Tuesday, February 07, 2012 12:48 PM
To: time-nuts@febo.com
Subject: [time-nuts] FE-5680A's suitability for use as a 10 MHz reference
for microwave transverters

Hello,

As has been mentioned here before, the output of the "non-tunable" 
FE-5680A's has been noted to have low-level spurs in it - no doubt due 
to the way the various frequency loops are derived within, some using 
DDS techniques.  It is for this reason that when I packaged my FE-5680A 
in its own, stand-alone enclosure I included a fairly narrow band (+/-6 
kHz @ -6dBc) crystal-based bandpass filter in the output.

After more recent testing of two FE-5680A's using two different 10 GHz 
microwave transverters, I've determined that this filtering just isn't 
enough.  At first, it was assumed/hoped that the racket that I was 
hearing was coming from somewhere else - perhaps the switching 
up-converter or some other interaction - or just something "odd" about 
my homebrew 10 GHz transverter, but this is, unfortunately, not the case.

In testing with a DownEast Microwave 10 GHz transverter fitted with an 
N5AC synthesizer, the CW notes sounded nice and clean when locked to the 
Z3801 and there was only a trace of modulation that I'd not really 
noticed before when I used the Efratom LPRO-101, but when the '5680A was 
connected, the incidental PM was bad enough that it was difficult to 
determine where, exactly, zero beat was!  Since the synthesizer uses a 
fairly high reference frequency internally there was little impediment 
to the low-level phase modulation on the reference.

I compared this with my own homebrew 10 GHz transverter.  This unit uses 
an 18.4 MHz Butler VCXO that is multiplied to 110.4 MHz which is then 
fed to a "brick" oscillator with the 110.4 MHz being compared to the 10 
MHz reference using a harmonic mixer, locking to the 400 kHz residual.  
Since this unit has a comparatively low loop bandwidth in the VCXO the 
grunge was considerably reduced, but still objectionable, giving some 
hope that a simple VCXO scheme might make the '5680A usable.

I still have yet to do a more-detailed analysis of the phase modulation 
that is appearing on the 10 GHz signals, but I can clearly hear a low 
frequency modulation source (perhaps the lock-in amplifier) plus a 
myriad of other audio frequency components and their harmonics.  Again, 
with the LPRO-101 was very "clean" by comparison and I could *just* hear 
some similar, very low-level noises in the background that I'd not 
really noticed before.

As it is, the '5680A-based reference is unusable with the N5AC 
synthesizer and its wide loop bandwidth and "almost" usable with my 
homebrew transverter and its comparatively narrow loop bandwidth.  I'm 
now bent on making the '5680A usable as a microwave reference, but my 
current plans are to build a simple 10 MHz Butler VCXO and then lock it 
to the '5680A using a very "slow" loop filter:  In that way, I'm hoping 
that the phase noise will be largely that of the 10 MHz VCXO and its 
cheap CPU-type crystal rather than the '5680A!

Clint
KA7OEI


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Re: [time-nuts] FE-5680A's suitability for use as a 10 MHz reference for microwave transverters

[John Ackermann N8UR]

I am just finishing my promised stability and phase noise measurements 
on a batch of inexpensive Rb standards; I hope to publish the results 
tomorrow evening.


In the meantime, I've looked at two of the FE-5680s and their phase 
noise is significantly worse than either the Efratom FRS or the Datum 
LPRO -- in particular, there is a forest of spurs all the way from 1 Hz 
on out, most of them at around -80dBc or worse.  By the time you 
multiply that to 10 GHz, that's only about 20 dB below the carrier! 
Apart from the spurs, the noise floor is significantly higher than the 
other two types.


A clean-up oscillator would be an interesting add-on.

John



On 2/7/2012 12:48 PM, C. Turner wrote:

Hello,

As has been mentioned here before, the output of the "non-tunable"
FE-5680A's has been noted to have low-level spurs in it - no doubt due
to the way the various frequency loops are derived within, some using
DDS techniques. It is for this reason that when I packaged my FE-5680A
in its own, stand-alone enclosure I included a fairly narrow band (+/-6
kHz @ -6dBc) crystal-based bandpass filter in the output.

After more recent testing of two FE-5680A's using two different 10 GHz
microwave transverters, I've determined that this filtering just isn't
enough. At first, it was assumed/hoped that the racket that I was
hearing was coming from somewhere else - perhaps the switching
up-converter or some other interaction - or just something "odd" about
my homebrew 10 GHz transverter, but this is, unfortunately, not the case.

In testing with a DownEast Microwave 10 GHz transverter fitted with an
N5AC synthesizer, the CW notes sounded nice and clean when locked to the
Z3801 and there was only a trace of modulation that I'd not really
noticed before when I used the Efratom LPRO-101, but when the '5680A was
connected, the incidental PM was bad enough that it was difficult to
determine where, exactly, zero beat was! Since the synthesizer uses a
fairly high reference frequency internally there was little impediment
to the low-level phase modulation on the reference.

I compared this with my own homebrew 10 GHz transverter. This unit uses
an 18.4 MHz Butler VCXO that is multiplied to 110.4 MHz which is then
fed to a "brick" oscillator with the 110.4 MHz being compared to the 10
MHz reference using a harmonic mixer, locking to the 400 kHz residual.
Since this unit has a comparatively low loop bandwidth in the VCXO the
grunge was considerably reduced, but still objectionable, giving some
hope that a simple VCXO scheme might make the '5680A usable.

I still have yet to do a more-detailed analysis of the phase modulation
that is appearing on the 10 GHz signals, but I can clearly hear a low
frequency modulation source (perhaps the lock-in amplifier) plus a
myriad of other audio frequency components and their harmonics. Again,
with the LPRO-101 was very "clean" by comparison and I could *just* hear
some similar, very low-level noises in the background that I'd not
really noticed before.

As it is, the '5680A-based reference is unusable with the N5AC
synthesizer and its wide loop bandwidth and "almost" usable with my
homebrew transverter and its comparatively narrow loop bandwidth. I'm
now bent on making the '5680A usable as a microwave reference, but my
current plans are to build a simple 10 MHz Butler VCXO and then lock it
to the '5680A using a very "slow" loop filter: In that way, I'm hoping
that the phase noise will be largely that of the 10 MHz VCXO and its
cheap CPU-type crystal rather than the '5680A!

Clint
KA7OEI


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Re: [time-nuts] FE-5680A's suitability for use as a 10 MHz reference for microwave transverters

[Bob Camp]

Hi

Here is a little more on how much of a problem you have.

If you would like the spurs to be down 70 dbc at 10 GHz. They go up by 20 log 
N. in this case N is 1000. That gets you 60 db. Spurs at 10 MHz would have to 
be down at -130 dbc to make it at 10 GHz.  

If you want noise over 10 KHz to be 60 db down, it goes by 10 log BW. That gets 
you to 1 Hz noise at -100. The same 60 db to 10 MHz then applies. You would 
need -160 dbc phase noise at 10 MHz to hit that.

Both of those would be hard to hit with any Rb. Fortunately you can use a multi 
step multiplication chain. Once you make that decision, noise and spurs on the 
Rb are not a big issue.

Bob




On Feb 7, 2012, at 1:52 PM, John Ackermann N8UR  wrote:

> I am just finishing my promised stability and phase noise measurements on a 
> batch of inexpensive Rb standards; I hope to publish the results tomorrow 
> evening.
> 
> In the meantime, I've looked at two of the FE-5680s and their phase noise is 
> significantly worse than either the Efratom FRS or the Datum LPRO -- in 
> particular, there is a forest of spurs all the way from 1 Hz on out, most of 
> them at around -80dBc or worse.  By the time you multiply that to 10 GHz, 
> that's only about 20 dB below the carrier! Apart from the spurs, the noise 
> floor is significantly higher than the other two types.
> 
> A clean-up oscillator would be an interesting add-on.
> 
> John
> 
> 
> 
> On 2/7/2012 12:48 PM, C. Turner wrote:
>> Hello,
>> 
>> As has been mentioned here before, the output of the "non-tunable"
>> FE-5680A's has been noted to have low-level spurs in it - no doubt due
>> to the way the various frequency loops are derived within, some using
>> DDS techniques. It is for this reason that when I packaged my FE-5680A
>> in its own, stand-alone enclosure I included a fairly narrow band (+/-6
>> kHz @ -6dBc) crystal-based bandpass filter in the output.
>> 
>> After more recent testing of two FE-5680A's using two different 10 GHz
>> microwave transverters, I've determined that this filtering just isn't
>> enough. At first, it was assumed/hoped that the racket that I was
>> hearing was coming from somewhere else - perhaps the switching
>> up-converter or some other interaction - or just something "odd" about
>> my homebrew 10 GHz transverter, but this is, unfortunately, not the case.
>> 
>> In testing with a DownEast Microwave 10 GHz transverter fitted with an
>> N5AC synthesizer, the CW notes sounded nice and clean when locked to the
>> Z3801 and there was only a trace of modulation that I'd not really
>> noticed before when I used the Efratom LPRO-101, but when the '5680A was
>> connected, the incidental PM was bad enough that it was difficult to
>> determine where, exactly, zero beat was! Since the synthesizer uses a
>> fairly high reference frequency internally there was little impediment
>> to the low-level phase modulation on the reference.
>> 
>> I compared this with my own homebrew 10 GHz transverter. This unit uses
>> an 18.4 MHz Butler VCXO that is multiplied to 110.4 MHz which is then
>> fed to a "brick" oscillator with the 110.4 MHz being compared to the 10
>> MHz reference using a harmonic mixer, locking to the 400 kHz residual.
>> Since this unit has a comparatively low loop bandwidth in the VCXO the
>> grunge was considerably reduced, but still objectionable, giving some
>> hope that a simple VCXO scheme might make the '5680A usable.
>> 
>> I still have yet to do a more-detailed analysis of the phase modulation
>> that is appearing on the 10 GHz signals, but I can clearly hear a low
>> frequency modulation source (perhaps the lock-in amplifier) plus a
>> myriad of other audio frequency components and their harmonics. Again,
>> with the LPRO-101 was very "clean" by comparison and I could *just* hear
>> some similar, very low-level noises in the background that I'd not
>> really noticed before.
>> 
>> As it is, the '5680A-based reference is unusable with the N5AC
>> synthesizer and its wide loop bandwidth and "almost" usable with my
>> homebrew transverter and its comparatively narrow loop bandwidth. I'm
>> now bent on making the '5680A usable as a microwave reference, but my
>> current plans are to build a simple 10 MHz Butler VCXO and then lock it
>> to the '5680A using a very "slow" loop filter: In that way, I'm hoping
>> that the phase noise will be largely that of the 10 MHz VCXO and its
>> cheap CPU-type crystal rather than the '5680A!
>> 
>> Clint
>> KA7OEI
>> 
>> 
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Re: [time-nuts] FE-5680A's suitability for use as a 10 MHz reference for microwave transverters

[Javier Herrero]

El 09/02/2012 01:40, Bob Camp escribió:

Hi

Here is a little more on how much of a problem you have.

If you would like the spurs to be down 70 dbc at 10 GHz. They go up by 20 log 
N. in this case N is 1000. That gets you 60 db. Spurs at 10 MHz would have to 
be down at -130 dbc to make it at 10 GHz.

As a side note (since I've just asked by), does the phase noise also go 
up by 20 log N when multiplying?


Regards,

Javier


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Re: [time-nuts] FE-5680A's suitability for use as a 10 MHz reference for microwave transverters

[Bob Camp]

Hi

Indeed the phase noise goes up by 20 log N when multiplying. To be precise, 
only the phase spurs go up by 20 log N. If they are incoherent they still 
multiply, but you get a bit of an offset.

Bob
On Feb 9, 2012, at 2:03 AM, Javier Herrero wrote:

> El 09/02/2012 01:40, Bob Camp escribió:
>> Hi
>> 
>> Here is a little more on how much of a problem you have.
>> 
>> If you would like the spurs to be down 70 dbc at 10 GHz. They go up by 20 
>> log N. in this case N is 1000. That gets you 60 db. Spurs at 10 MHz would 
>> have to be down at -130 dbc to make it at 10 GHz.
>> 
> As a side note (since I've just asked by), does the phase noise also go up by 
> 20 log N when multiplying?
> 
> Regards,
> 
> Javier
> 
> 
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Re: [time-nuts] FE-5680A's suitability for use as a 10 MHz reference for microwave transverters

[C. Turner]

Last week I noted that the FE-5680A's "barefoot" output was found to NOT 
be a suitable 10 MHz reference for microwave transverters.  
Specifically, I tested it on two different 10 GHz transverters and found 
there to be objectionable levels of "grunge" on signals caused by 
low-level phase modulation internal to the '5680A and at 10 GHz the 
result of this phase modulation was a racket of audible and subaudible 
noises on CW carriers that made it difficult to find zero beat!  In 
comparison, the 10 MHz outputs of the Z3801, Isotemp VCXO and LPRO-101 
yielded results at 10 GHz that were quite clean.  Related observations 
were also made by N8UR in his web page comparing various units.


In order to clean up the output of the FE-5680A I did the obvious thing, 
disciplining a homebrew VCXO to its output - details are found here:


http://www.ka7oei.com/10_MHz_Rubidium_FE-5680A.html

While the comparison frequency is fairly high (1.25 MHz) the loop gain 
and bandwidth are quite low so it's pretty much the Butler oscillator 
VCXO that determines the phase noise of the 10 MHz output and I can't 
detect any audible artifacts from the '5680A at all.  At the moment I 
don't have the means of generating a "pristine" test signal at 10368 
MHz, but from what I can determine, the resulting CW notes from the 
transverter (being locked to the "regenerated" output of the the '5680A) 
compared to the other 10 MHz sources sound the same.  At some point I 
hope to do a more-detailed analysis.


Had I a low-noise "canned" 10 MHz VCXO around, I'd have probably used 
that rather than go through the hassle of building the oscillator, but 
none of the 10 MHz VCXOs that I *did* have on hand produced as good a CW 
note as the Butler built around a cheap microprocessor-type crystal.  I 
also had on hand some 10 MHz ovenized VCXOs which would have worked 
fine, but not only were these too large to fit in the box, they would 
have added even more current consumption to an already power-hungry 
frequency source - an important consideration when operating from a battery!


There are, no doubt, a number of ways one could do this same thing, but 
it's clear that this simple of a circuit will do an admirable job of 
extracting the frequency stability of the FE-5680A without the 
synthesis-related artifacts.  Of course, the regenerated 10 MHz output 
will have a degree of variable phase offset with respect to the '5680A's 
"barefoot" 10 MHz output over varying conditions (such as temperature) 
but when used only as a frequency reference these rather slow changes 
are unimportant.


73,

Clint
KA7OEI


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Re: [time-nuts] FE-5680A's suitability for use as a 10 MHz reference for microwave transverters

[paul swed]

Clint a good read and a fine approach.
By accident I looked at the original filter schematic first and believe it
has an error for the first output amplifier. The 470 ohm resistors in the
wrong location.
Others on the list have mentioned the same thing about the 5680s output
being dirty. You have that fixed up nicely.
Regards
Paul
WB8TSL

On Mon, Feb 13, 2012 at 4:03 PM, C. Turner  wrote:

> Last week I noted that the FE-5680A's "barefoot" output was found to NOT
> be a suitable 10 MHz reference for microwave transverters.  Specifically, I
> tested it on two different 10 GHz transverters and found there to be
> objectionable levels of "grunge" on signals caused by low-level phase
> modulation internal to the '5680A and at 10 GHz the result of this phase
> modulation was a racket of audible and subaudible noises on CW carriers
> that made it difficult to find zero beat!  In comparison, the 10 MHz
> outputs of the Z3801, Isotemp VCXO and LPRO-101 yielded results at 10 GHz
> that were quite clean.  Related observations were also made by N8UR in his
> web page comparing various units.
>
> In order to clean up the output of the FE-5680A I did the obvious thing,
> disciplining a homebrew VCXO to its output - details are found here:
>
> http://www.ka7oei.com/10_MHz_**Rubidium_FE-5680A.html
>
> While the comparison frequency is fairly high (1.25 MHz) the loop gain and
> bandwidth are quite low so it's pretty much the Butler oscillator VCXO that
> determines the phase noise of the 10 MHz output and I can't detect any
> audible artifacts from the '5680A at all.  At the moment I don't have the
> means of generating a "pristine" test signal at 10368 MHz, but from what I
> can determine, the resulting CW notes from the transverter (being locked to
> the "regenerated" output of the the '5680A) compared to the other 10 MHz
> sources sound the same.  At some point I hope to do a more-detailed
> analysis.
>
> Had I a low-noise "canned" 10 MHz VCXO around, I'd have probably used that
> rather than go through the hassle of building the oscillator, but none of
> the 10 MHz VCXOs that I *did* have on hand produced as good a CW note as
> the Butler built around a cheap microprocessor-type crystal.  I also had on
> hand some 10 MHz ovenized VCXOs which would have worked fine, but not only
> were these too large to fit in the box, they would have added even more
> current consumption to an already power-hungry frequency source - an
> important consideration when operating from a battery!
>
> There are, no doubt, a number of ways one could do this same thing, but
> it's clear that this simple of a circuit will do an admirable job of
> extracting the frequency stability of the FE-5680A without the
> synthesis-related artifacts.  Of course, the regenerated 10 MHz output will
> have a degree of variable phase offset with respect to the '5680A's
> "barefoot" 10 MHz output over varying conditions (such as temperature) but
> when used only as a frequency reference these rather slow changes are
> unimportant.
>
> 73,
>
> Clint
> KA7OEI
>
>
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Re: [time-nuts] FE-5680A's suitability for use as a 10 MHz reference for microwave transverters

[Bruce Griffiths]

paul swed wrote:

Clint a good read and a fine approach.
   


If and only if the high phase noise pedestal exhibited by the buffer 
amplifier that extends to offsets of a few hundred kHz or so isn't an issue.

The relatively low isolation between the 10MHz outputs may also be an issue.


By accident I looked at the original filter schematic first and believe it
has an error for the first output amplifier. The 470 ohm resistors in the
wrong location.
Others on the list have mentioned the same thing about the 5680s output
being dirty. You have that fixed up nicely.
Regards
Paul
WB8TSL

On Mon, Feb 13, 2012 at 4:03 PM, C. Turner  wrote:

   

Bruce

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Re: [time-nuts] FE-5680A's suitability for use as a 10 MHz reference for microwave transverters

[Rob Kimberley]

Nice job!!

Rob

-Original Message-
From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On
Behalf Of C. Turner
Sent: 13 February 2012 21:03
To: time-nuts@febo.com
Subject: Re: [time-nuts] FE-5680A's suitability for use as a 10 MHz
reference for microwave transverters

Last week I noted that the FE-5680A's "barefoot" output was found to NOT be
a suitable 10 MHz reference for microwave transverters.  
Specifically, I tested it on two different 10 GHz transverters and found
there to be objectionable levels of "grunge" on signals caused by low-level
phase modulation internal to the '5680A and at 10 GHz the result of this
phase modulation was a racket of audible and subaudible noises on CW
carriers that made it difficult to find zero beat!  In comparison, the 10
MHz outputs of the Z3801, Isotemp VCXO and LPRO-101 yielded results at 10
GHz that were quite clean.  Related observations were also made by N8UR in
his web page comparing various units.

In order to clean up the output of the FE-5680A I did the obvious thing,
disciplining a homebrew VCXO to its output - details are found here:

http://www.ka7oei.com/10_MHz_Rubidium_FE-5680A.html

While the comparison frequency is fairly high (1.25 MHz) the loop gain and
bandwidth are quite low so it's pretty much the Butler oscillator VCXO that
determines the phase noise of the 10 MHz output and I can't detect any
audible artifacts from the '5680A at all.  At the moment I don't have the
means of generating a "pristine" test signal at 10368 MHz, but from what I
can determine, the resulting CW notes from the transverter (being locked to
the "regenerated" output of the the '5680A) compared to the other 10 MHz
sources sound the same.  At some point I hope to do a more-detailed
analysis.

Had I a low-noise "canned" 10 MHz VCXO around, I'd have probably used that
rather than go through the hassle of building the oscillator, but none of
the 10 MHz VCXOs that I *did* have on hand produced as good a CW note as the
Butler built around a cheap microprocessor-type crystal.  I also had on hand
some 10 MHz ovenized VCXOs which would have worked fine, but not only were
these too large to fit in the box, they would have added even more current
consumption to an already power-hungry frequency source - an important
consideration when operating from a battery!

There are, no doubt, a number of ways one could do this same thing, but it's
clear that this simple of a circuit will do an admirable job of extracting
the frequency stability of the FE-5680A without the synthesis-related
artifacts.  Of course, the regenerated 10 MHz output will have a degree of
variable phase offset with respect to the '5680A's "barefoot" 10 MHz output
over varying conditions (such as temperature) but when used only as a
frequency reference these rather slow changes are unimportant.

73,

Clint
KA7OEI


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