[time-nuts] Firmware and antenna for Stanford Research FS700 Loran C frequency standard

[Dr. David Kirkby (Kirkby Microwave Ltd)]

On 18 July 2015 at 09:45, Ole Stender Nielsen <olstni at mail.tele.dk> wrote:

> I had a few LM394CN left and used one for the pre-amp.
> I would try to use another low-noise transistor. Furthermore, you do not
> necessarily need a matched pair. The transistor on the right in the
> schematic is merely used for biasing the amplifying transistor on the left.
> The prices for matched pairs can get ridiculously high, so I too would try
> to look for alternatives. If you still want a matched pair, there's the
> SSM2212 from Analog Devices, which is priced lower than the MAT12. You may
> also be able to find the earlier SSM2210, which now is obsolete. Another
> alternative is the THAT300 from THAT Corporation, which has 4 low-noise
> NPNs in a 14-pin DIP (great for experiments...)
> Useful single NPN transistors I can think of would likely be the BC337 or
> perhaps the 2SC3324 which both have low Rbb, and they are quite cheap.
>

Hi,
I ordered the parts for this, including a MAT12. I do consider some
arguments put by others - the first on the mailing list, the second
privately.

1) There are cheaper low noise devices - that in itself has generated into
another thread.

2) The atmospheric noise at 100 kHz is high

But given

* A MAT12 is not a fortune from reputable semiconductor suppliers like
Farnell (I would not consider eBay, due to fakes)
* I am only building one, so cost is not a huge issue.
* The MAT12 is specified for low noise, without me wasting time selecting
devices.
* MAYBE the argument about atmospheric noise is valid - I tend to agree
with it, but again due to the one-off nature of this, I am going to ignore
that argument.

So I bought a MAT12. I'm *not* going to change that - it is bought, paid
for and here.


>
> The "Ca. 3 Ohm" was indeed the estimated input resistance, including the
> series resistance of the input capacitor as I recall.
>

If the input resistance of that common-base amplifier is 3 Ohms, does it
make sense to have the loop resistance 3 Ohms too? For maximum power
transfer one wants the source and load to be the complex conjugate, but I
know for microwave devices, you do not design the input match for maximum
power transfer, but generally minimum noise.

I just checked my HP 4284A precision LCR meter, which covers from 20 Hz to
1 MHz in 8000 and something discrete steps including 100 kHz. I can set the
voltage as low as 5 mV, so I could measure the input impedance (R + j X) of
the amplifier at 5 mV RMS. So while Ole's estimate is no doubt good, I can
actually measure it with the MAT12 in place.

I have 95 m of 2.5 mm^2 mains cable, with a PVC (?) jacket. The copper
conductor which should have a DC resistance of about 0.7 Ohms. I'm
wondering if I would be better

* Having a longer piece (more resistance, higher cost, a larger thermal
noise voltage, but better impedance matching)
* Use thinner wire, which would be less costly, but again aim for a DC
resistance of 3 Ohms - or whatever my LCR meter says in the input Z at 100
kHz.

In the short term I am just going to put on the 95 m, as I have that here,
but longer term I wonder if I would be better increasing the number of
turns, to put the DC resistance closer to the input resistance of the
amplifier.

Any comments?

Dave