[time-nuts] SMD TADD-1 distribution amplifier

Bob Camp kb8tq at n1k.org
Sat Dec 19 08:52:35 EST 2015


Hi

A biassed CMOS gate (NC7SZ125 is one) is quieter than anything you will feed through the 
amplifier below about 10 KHz. Above that it’s as quiet as all but a very few sources. At less
than 10 cents each, they are hard to beat.

Bob

> On Dec 18, 2015, at 9:46 PM, Li Ang <379998 at qq.com> wrote:
> 
> Hi Charles,
>   I'm making a 1-to-4 distribution amplifier for 10MHz.  Can you give any suggestion? The schematic is attached. The opamps I'm considering are LMH6609 LMH6624 LMH6702. 
>   Does the piezoelectric effect of capacitors need to be considered here?
> 
> 
> 
> 
> 
> 
> Thanks
> BI7LNQ
> 
> 
> ------------------ Original ------------------
> From:  "Charles Steinmetz";<csteinmetz at yandex.com>;
> Date:  Sat, Dec 19, 2015 09:18 AM
> To:  "Discussion of precise time and frequency measurement"<time-nuts at febo.com>; 
> 
> Subject:  Re: [time-nuts] SMD TADD-1 distribution amplifier
> 
> 
> 
> Anders wrote:
> 
>> Far-out PN/AM is still 7dB short of the 6502!
>> Looks like an SNR issue to me, rather than some issue with the linear
>> regulator noise feeding through?!
>> AD8055 in non-inverting circuit with 1+2k7/2k7 gain has 9.6 nV/sqrt(Hz)
>> input-referred voltage noise PSD (if I calculated correctly..)
>> With an ADA4899-1 and lower value resistors I get about -4.5 dB improvement
>> to 3.4 nV/sqrt(Hz) input-referred
> 
> You're the victim of a very unfortunate choice of op-amp.
> 
> The op-amp that the TADD-1 was designed around (MAX477) is specified 
> with 5 nV/sqrtHz (typical) of input voltage noise at 10MHz.  No 
> details are given about its noise performance at lower frequencies, 
> but the fact that the noise is specified at 10MHz suggests that the 
> 1/f corner frequency is probably high, very likely 10kHz or 
> higher.  This further implies that its 10Hz input voltage noise is 
> more than 1,000 nV/sqrt/Hz.
> 
> The AD8055 is specified at 6 nV/sqrtHz at 10kHz, rising to ~150 
> nV/sqrtHz at 100Hz below a corner frequency of ~1kHz.  Extrapolating 
> the curve suggests that the 10Hz voltage input noise is > 1,000 nV/sqrtHz.
> 
> The AM and PM noise you are measuring is caused primarily by noise at 
> baseband, *not* by the in-band noise of the op-amp.  Baseband noise 
> AM modulates the signal, and it is also converted to PN because the 
> fluctuating voltage modulates the bandwidth of the op-amp (by 
> modulating the locations of the second and subsequent amplifier poles 
> with signal-dependent bias changes).  So these egregiously noisy (at 
> baseband) op-amps cause high AM and PM noise.
> 
> Compare those with the following op-amp specs (like the specs above, 
> these are all "typical"):
> 
> ADA4899:       1nV/sqrtHz at 100kHz       10nV/sqrtHz at 10Hz
> AD8010:          2nV/sqrtHz at 10kHz     ~12nv/sqrtHz at 10hz  [note 
> specific bypassing instructions]
> LME49713:      1.9nV/sqrtHz at 10kHz      11.5nV/sqrtHz at 10Hz
> 
> So, at 10Hz, each these three possible choices is more than 100x 
> quieter than the MAX477 or AD8055.  (They are also quieter in the 
> signal band, but not by as much.)
> 
> Best regards.
> 
> Charles
> 
> 
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