[time-nuts] HP 11729C versus 11848A

Adrian rfnuts at arcor.de
Sat May 8 21:33:42 UTC 2010


Minus the 6 dB L(f) conversion factor,
so we get theoretical L(f) noise floors of -176 dBc/Hz and -191 dBc/Hz.

Adrian

Adrian schrieb:
> Thanks to Rick's great hint, I'm now awaiting some AM123 amps to arrive.
>
> Why is it that I just don't get better than -165...-170 dBc/Hz?
>
> Let me try some simple math:
>
> At L and R levels of +7.5 dBm (that's what come out of my 10811's),
> subtract some 7.5 dB mixer conversion loss to see that 0 dBc would be 
> exactly 0 dBm at the mixer output.
> (The calibration is performed with the R signal decreased by 40 db. At 
> the 40 dB LNA output, I'm measuring pretty exactly 0 dBm.)
>
> At room temperature, the thermal noise is -174 dBm/Hz.
> Add some 4 dB for the LNA noise figure and LPF insertion loss, so the 
> system noise floor is at -170 dBm/Hz.
> Give or take a dB, but that's pretty much about it.
>
> Now, with the calibrated carrier level of 0 dBm at the LNA input, the 
> residual noise floor is -170 dBm/Hz -(+0 dBm) = -170 dBc/Hz
>
> A modern RF spectrum anylyzer has a noise figure in the 15 dB range, 
> add 10 dB of 'safety' attenuation, so the analyzer noise floor is -174 
> dBm/Hz -(+25 dB) = -149 dBm/Hz. Remember that the mixer output / LNA 
> input of -170 dBm/Hz is amplified by the LNA by 40 dB, so the analyzer 
> input 'noise signal' from the test set is -170 dBm/Hz + 40 dB = -130 
> dBm/Hz, which is 19 dB above the analyzer's noise.
>
> If we increase the L and R levels by 15 dB, we need a +23 dBm mixer, 
> but we will add 15 dB to the system dynamic range.
> The mixer conversion loss, LNA noise figure and thermal noise floor 
> haven't changed, so the noise floor in dBm is still the same -170 
> dBm/Hz, but we're now refering to a carrier level of +22.5 dBm instead 
> of the above +7.5 dBm (+15 dBm versus 0 dBm at the mixer output).
> That's why we can now measure down to -170 dBm/Hz -(+15 dBm) = -185 
> dBc/Hz.
>
> Adrian
>
>
> John Miles schrieb:
>> That sounds about right to me.  I was guessing you meant 40 dB and 
>> not 30 dB
>> in the previous message, or there was something else causing about 10 
>> dB of
>> loss.  Lots of things to go wrong in this process!
>>
>> -- john, KE5FX
>>
>>  
>>> -----Original Message-----
>>> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
>>> Behalf Of Adrian
>>> Sent: Friday, May 07, 2010 10:38 PM
>>> To: Discussion of precise time and frequency measurement
>>> Subject: Re: [time-nuts] HP 11729C versus 11848A
>>>
>>>
>>> Just the roughly 16 dB of insertion loss caused by the 562 ohm resistor
>>> at the input don't make that filter such a great solution ;-)
>>> So, I brewed something better together...
>>> The 10 kHz beat note is now near 0 dBm when the R input signal is
>>> decreased by 40 dB, which makes a lot more sense than before...
>>> And, I have a noise floor of some -165 to -169 dBc/Hz at +7.5 dBm 
>>> input,
>>> and I'm seeing the 10811A's some 5 dB above that.
>>>
>>> Adrian
>>>
>>>
>>> John Miles schrieb:
>>>    
>>>>> John,
>>>>>
>>>>> I'd say you nailed it.
>>>>>
>>>>> After some more testing, I can confirm that the limiter amp and 
>>>>> the LPF
>>>>> are the culprit.
>>>>> I opened the box and plugged directly into the mixer LO port.
>>>>> And, for the LPF, as a quick 'n dirty solution, I connected the <1 
>>>>> MHz
>>>>> front panel output with the LNA input.
>>>>> Now, at 10 dBm each into the mixer ports, I'm getting a noise 
>>>>> floor of
>>>>> <-145 dBc/Hz at 100 Hz and about -170 dBc/Hz at 10 kHz and above.
>>>>>
>>>>>         
>>>> That's pretty aggressive for 10811s.  The floor on those is usually
>>>> around -165 dBc/Hz.  How's your calibration process -- are you
>>>>       
>>> accounting
>>>    
>>>> for the 600 ohm output Z of the <1 MHz output port?  It'll lose
>>>>       
>>> a few dB if
>>>    
>>>> you try to drive 50 ohms with it, and/or the filter response won't be
>>>> correct.
>>>>
>>>> -- john, KE5FX
>>>>
>>>>
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>>
>>
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>
>
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