[time-nuts] Firmware and antenna for Stanford Research FS700

[Dr. David Kirkby (Kirkby Microwave Ltd)]

On 18 July 2015 at 04:25, Charles Steinmetz <csteinmetz at yandex.com> wrote:

> Dave wrote:
>
>  1) Do you have any suggestions for a replacement for the LM394CN, which is
>> obsolete and unobtainable from any reputable source? There are plenty on
>> eBay for a few $'s from China, but the probability of them being fakes is
>> greater than 0.99. The MAT12 seems to be one possible candidate for a
>> replacement and while not cheap, is available from reputable sources like
>> Farnell.
>>
>
> The MAT12 should certainly work, but there is *plenty* of DC degeneration
> in the circuit (1v at each emitter), so there is no advantage to matched
> transistors.  A pair of Zetex (Diodes, Inc.) ZTX849 or FZT849 actually have
> significantly lower voltage noise than either the LM394 or MAT12.
>
> Best regards,
>
> Charles
>

Do you really mean the ZTX849? It is a power transistor (5A continuous
current collector current, 20 A peak collector current), with no
specification for noise on the data sheet. It would on the face of it
hardly seem a wise choice for a low noise amplifier, but perhaps you know
something I don't.

The LM394CN or MAT12 are at least a low-noise devices, even if the matching
between the two transistors in the package is not needed.

Dave