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rickman

On 11/16/2014 5:08 AM, wrote:
On Sun, 16 Nov 2014 03:47:44 -0500, rickman wrote:

On 11/16/2014 3:18 AM, wrote:
On Sat, 15 Nov 2014 22:17:38 -0500, rickman wrote:

On 11/4/2014 6:29 PM, rickman wrote:
I am working on a project for receiving a very narrow bandwidth signal
at 60 kHz. One of the design goals is to keep the power consumption to
an absolute minimum. I'm trying to figure out how to run a
pre-amplifier on less than 100 uW. So far I have found nothing. Any
suggestions?

I haven't seen the original post, but are you building some type of
clock receiver ? Those work for a year with a single battery.

Yes, it is a radio controlled clock.

What kind of antenna are you using ? Do you really need a preamp ?

I was planning on a loop antenna made from RG6 cable, but if I have to
add an amplifier I may use a ferrite loop.

Are you going to use a big (several meters) loop with the RG-6 center
conductor as a loop and cutting the shield at the top and using the
rest of the cable shield as a grounded static shield and using a small
coupling loop into the receiver ? With the main loop resonated by a
capacitor to 60 kHz, you should get quite decent signal without
preamplifier.

That's the general idea but in an 8 turn 2 foot loop. I may add another
50 foot of RG6 (helps with the split) to boost the signal further.
"Decent" must be defined. This signal is not so strong, 100 uV/m and
this loop will only give 26 uV counting a Q of 90 which might not fully
materialize by the time it is plugged into the receiver.

As I think about this (I do more thinking than designing sometimes) I am
becoming less and less convinced I can do this without a preamp.

For anything smaller, a 5 cm ferrite bar is quite adequate due to the
high band noise, even if the ferrite antenna gain might be -40 dBi or
even -60 dBi.

Not sure why you would compare the ferrite antenna to an isotropic
antenna, but when compared to the 2 foot loop the equations show the 2
foot loop provides a stronger signal. Now that I am considering a
preamp I may return to the idea of the ferrite loop antenna. Lol, if I
do that I can explore the joys of Litz wire.

Do you have room for a tank circuit (L/C) on the collector/drain ?

Room should not be a problem. But what is the point of a tank?

1. if you do not have a frequency selective antenna, this tank circuit
will provide the selectivity. Since this stage has a low gain at
unwanted frequencies, this reduces the risk of IP3 distortion, which
becomes critical at low collector/drain currents.

The antenna is already highly tuned to the frequency of interest.
Unlike voice broadcasts the bandwidth of this signal is just double
digit Hz so a Q as high as feasible is useful. That is why RG6 was
picked, with a center conductor that pushes the skin effect the Q will
be as good as practical (without using Litz wire, lol).

2. you get at least twice the voltage swing compared to the battery
voltage. Tapping the inductor or capacitor chain will provide nice
impedance matching avoiding the need for a cascaded stage.

Impedance matching to what? My "receiver" is an FPGA with a rather high
impedance input, measured in Megaohms in parallel with single digit pF.

I don't need to worry about Vcc (or Vdd) limiting voltage swing even
with the amp the voltage is low. If the tuned circuit will boost the
voltage otherwise I would consider it. Would a tank circuit be put in
series with a resistance? Otherwise how is the DC point established? I
am using a source resistor with bypass cap to bias the gate-source
voltage (reminds me of tube circuits) but a resistor is also needed in
the drain connection, no?

--

Rick