Observing the ammonia resonance


Andy G4JNT
 

Oh dear, if anyone replies, please clean-up my typo in the subject line :-(
(Done here)



On Mon, 20 Sept 2021 at 19:04, Andy Talbot <andy.g4jnt@...> wrote:
On the Timenuts Group there's currently a discussion on, amongst other things, the ammonia resonance and it's possible use as a simpler frequency reference.
There is a narrow resonanance of the ammonia molecule at 23,870,129,007 Hz +/- 10 Hz  (ie just below our 24GHz band)

The resonance will be a Hz or so wide, and it ought to be possible to see this with relatively simple equipment.    First of all, get a length , several metres, of waveguide for 24GHz.   This needn't be 'proper' WG, just squashed water pipe will do to allow just the TE01 mode to propagate.    Fill this with ammonia (obvs. make the transitions gas tight)

Use an ADF5355 synth, and drive it's reference from a 48 bit DDS loike the AD9852.   Anything to be able to tune the 24GHz signal in sub-Hz steps, and to know what it is.   That chip will leak enough 4th harmonic from its VCO; launch this into the waveguide.   At the other end stick a 24GHz receiver tuned suitably.
Vary the input frequency a few Hz either side of the resonance and look for a dip.

One snag - if you use GPS locking, the residual variation of the GPSDO will cause it to drift across the resonance over a few tens of seconds.

Work out a way of feeding this dip back and controlling the synth reference in a locking loop and you have your own atomic frequency standard.   It's potentially  the same performance as cesium 

It seems in years gone by that caesium was possibly adopted instead of ammonia, simply because its resonance at 9.19GHz was easier to generate using ex-WW2 equipment that was 23GHz back in the 1940s when the first Cs standard was developed.


Barry Chambers
 

Hi Andy

an intruiging project. Any idea of the gas pressure in the waveguide? Presumably you have to get rid of the air first?

73 Barry G8AGN


On 20/09/2021 19:06, Andy G4JNT wrote:
Oh dear, if anyone replies, please clean-up my typo in the subject line :-(
(Done here)



On Mon, 20 Sept 2021 at 19:04, Andy Talbot <andy.g4jnt@...> wrote:
On the Timenuts Group there's currently a discussion on, amongst other things, the ammonia resonance and it's possible use as a simpler frequency reference.
There is a narrow resonanance of the ammonia molecule at 23,870,129,007 Hz +/- 10 Hz  (ie just below our 24GHz band)

The resonance will be a Hz or so wide, and it ought to be possible to see this with relatively simple equipment.    First of all, get a length , several metres, of waveguide for 24GHz.   This needn't be 'proper' WG, just squashed water pipe will do to allow just the TE01 mode to propagate.    Fill this with ammonia (obvs. make the transitions gas tight)

Use an ADF5355 synth, and drive it's reference from a 48 bit DDS loike the AD9852.   Anything to be able to tune the 24GHz signal in sub-Hz steps, and to know what it is.   That chip will leak enough 4th harmonic from its VCO; launch this into the waveguide.   At the other end stick a 24GHz receiver tuned suitably.
Vary the input frequency a few Hz either side of the resonance and look for a dip.

One snag - if you use GPS locking, the residual variation of the GPSDO will cause it to drift across the resonance over a few tens of seconds.

Work out a way of feeding this dip back and controlling the synth reference in a locking loop and you have your own atomic frequency standard.   It's potentially  the same performance as cesium 

It seems in years gone by that caesium was possibly adopted instead of ammonia, simply because its resonance at 9.19GHz was easier to generate using ex-WW2 equipment that was 23GHz back in the 1940s when the first Cs standard was developed.

-- 
73
Barry, G8AGN


Andy G4JNT
 

Don't know any of the physics but I doubt air would give a problem.   The absorption is specific to the ammonia and should be unaffected by impurities that don't impede microwave propagation.  You may get extra resonances, but they'll be way-removed from 24GHz, let alone the narrow resonance band.

The Time Nuts thread have cited some references, but they're scanned-in books that'll need some study - and I don't like reading books on line !



On Mon, 20 Sept 2021 at 20:38, Barry Chambers <b.chambers@...> wrote:

Hi Andy

an intruiging project. Any idea of the gas pressure in the waveguide? Presumably you have to get rid of the air first?

73 Barry G8AGN


On 20/09/2021 19:06, Andy G4JNT wrote:
Oh dear, if anyone replies, please clean-up my typo in the subject line :-(
(Done here)



On Mon, 20 Sept 2021 at 19:04, Andy Talbot <andy.g4jnt@...> wrote:
On the Timenuts Group there's currently a discussion on, amongst other things, the ammonia resonance and it's possible use as a simpler frequency reference.
There is a narrow resonanance of the ammonia molecule at 23,870,129,007 Hz +/- 10 Hz  (ie just below our 24GHz band)

The resonance will be a Hz or so wide, and it ought to be possible to see this with relatively simple equipment.    First of all, get a length , several metres, of waveguide for 24GHz.   This needn't be 'proper' WG, just squashed water pipe will do to allow just the TE01 mode to propagate.    Fill this with ammonia (obvs. make the transitions gas tight)

Use an ADF5355 synth, and drive it's reference from a 48 bit DDS loike the AD9852.   Anything to be able to tune the 24GHz signal in sub-Hz steps, and to know what it is.   That chip will leak enough 4th harmonic from its VCO; launch this into the waveguide.   At the other end stick a 24GHz receiver tuned suitably.
Vary the input frequency a few Hz either side of the resonance and look for a dip.

One snag - if you use GPS locking, the residual variation of the GPSDO will cause it to drift across the resonance over a few tens of seconds.

Work out a way of feeding this dip back and controlling the synth reference in a locking loop and you have your own atomic frequency standard.   It's potentially  the same performance as cesium 

It seems in years gone by that caesium was possibly adopted instead of ammonia, simply because its resonance at 9.19GHz was easier to generate using ex-WW2 equipment that was 23GHz back in the 1940s when the first Cs standard was developed.

-- 
73
Barry, G8AGN