Re: [VLF_Group] Re: E-field rx, front-end protection
- I tested some 1G resistors,
Metal glaze, not sure what that is but I could not measure any
excess noise when used as a bias resistor. Going from 100M
to 1G the thermal noise went down by about the right amount.
Also tested these 100M from the same series,
They worked fine too.
These are now in prx-e1 and the latest circuit is in
Added an LCR network into the front-end to reduce signals
above 1MHz, adds some very worthwhile attenuation (but not
without problems). It may look like an LC filter but in fact
is an RC filter. The L serves only to shunt the thermal noise
of the resistor below 200kHz.
Have decided to drop the cross-coupled line driver and reverted
to the conventional output circuit - the other was too much
of an unknown quantity and didn't add enough value for the risk.
Has one less gain stage than the previous draft. This might
upset the balance of the differential output and the opamp
may have to go back in.
I set a computer to model all the combinations of jumper
settings and a range of antenna capacitances from 30pF to
1000pF and various input cable capacitance. This throws up a
few problems with certain combinations, many of which involve
the L101. The front-end circuit has a long way to go to make
it robust and general purpose.
> When will http://126.96.36.199/prx-e1.m3u be streaming again?Not for a couple of weeks at least. This weekend aim to test
the line driver with various lengths of cat5 and coax, try some
options for line isolation, and test a few other little things.
Next weekend will have another try at forcing some intermod
in the front end: a long long wire as high as I can get it.
It will have to cope with plenty of medium wave and HF because
I'll tune the wire. At last I can use again those big old
vane caps that have sat in the junk box for decades.
- Got a good price for the PCB fabrication from a local
(Todmorden) manufacturer and I added up the parts cost,
In UK pounds for the populated boards:
prx-e1 E-field front end: 23.67
prx-t1 Filter, line driver, PSU: 52.73
prx-r1 Line receiver: 24.42
About 100 quid for the set, not including boxes and
cables, etc, and doesn't include assembly and test.
Well that's a bit cheaper than I thought it would come
to. Will complete the breadboard testing this month
and have a set of boards built up in April and then some
Further back in the pipeline is prx-h1 which is an
H-field front-end developed from the loop receiver I
currently use, and a -p1 isolated power supply.