[time-nuts] GPS Antenna Feed Line Decision

Charles Steinmetz csteinmetz at yandex.com
Sun Sep 3 18:41:52 EDT 2017

Attila wrote:

> The supplies for LNAs are usually quite benign given two constraints:
>    *   *   *
> 1) Low frequency (0Hz to bandwidth of signal) noise is low
>    *   *   *
> But 1) is a bit harder as it also includes 1/f noise, temperature,
> (upstream) supply and load effects.

The LT3042 is better in this regard than anything short of a heroic 
effort with discrete components, IF you use a larger-than-normal bypass 
capacitor on the SET pin.  With Cset = 22uF, the noise density is 
<20nV/sqrtHz down to 10Hz.  220uF is somewhat better, but not by a whole 
decade because of the 1/f noise of the error/output amplifier.

The down side of a large Cset is that the power supply takes longer to 
reach its final voltage.  A 5v supply reaches 90% voltage in about 25 
seconds, and 99% in about 40 seconds.  A 10v supply takes twice as long, 
and a 15v supply, three times.  The LT3042 does have a "fast start" 
mode, which can reduce these times considerably (to ~0.5sec, ~1sec, and 
~1.5sec, respectively) -- but still longer than usual for a regulated 
supply.  For applications like continuously-on LNA power, there should 
be no problem with any of these times.

The LT3042 is a bit of a pain to use, with its buried ground/heatsink 
tab, but it is very good, and is the lowest noise fully integrated 
regulator available, AFAIK.

Best regards,


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