# [time-nuts] HP 11729C versus 11848A

Bob Camp lists at rtty.us
Sun May 9 00:14:14 UTC 2010

```Hi

Just to be picky:

You get 6 db for SSB to DSB when you are looking at a modulation effect (correlated sidebands) and 3 db when you are looking at random noise (un-correlated sidebands). If both oscillators contribute equally, you get a 3 db factor. The total can be anywhere from 3 db to 9 db depending on the situation.

I'm sure glad I'm out of rock throwing range ....

Bob

On May 8, 2010, at 5:33 PM, Adrian wrote:

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