Use
of quartz tuning-fork resonators to provide a
sensitivity peak well above audio frequencies on
narrow-band opto-diode nanowave receivers.
Here's
a mail to the UK nanowave group that usefully introduces
some thought on the subject:
'Another direction that the light box has
allowed easy evaluation of is the use of a quartz resonator to
null out the opto-diode self capacitance. Whilst only
appropriate for very narrow band modes (well, unkeyed carrier
operation really) it could be useful for evaluating cloud
reflection paths, for example, and allows you to move away from
all that 50/60 Hz optical crud, not forgetting the many
harmonics of this type of QRM. The problem so far had been that crystals considered were
of the AT type cut and these have a much lower parallel
resistance at resonance than the 40 M ohm or so dark resistance
at the usual opto-diode/FET gate interconnection. A 32.768 kHz
quartz tuning fork resonator data sheet, though, looked more
promising. Even the equivalent series resonant resistance was
quoted as being about 50k ohm, so on the basis that the parallel
resonant resistance is potentially 50k times the square root of
the quartz Q, that gives 5 M ohm, which still falls short, but
is a lot closer... Simply shunting the opto-diode/FET gate junction of a
Finningley receiver with a 32.768 kHz crystal from the junk box
down to ground produced a good peak with a bandwidth of about
0.5 Hz. In comparative tests with a 1 kHz tone, s/n was down by
10 to 15 dB, depending on the quartz sample. So this looked
promising. A 75 kHz resonator performed about the same, as did some
100 kHz units that Farnell currently have in stock. It seems
easy at this location (Cambridge/Newmarket area) to ensure that
there is no direct electrical signals at these frequencies
leaking into the front-end, so the SpectraVue display that was
being used to measure the sensitivity remained completely
un-cluttered, with just the test signal visible (OK, there's
still some high frequency optical QRM from SMPU fed lighting,
but the wanted signal is still much more obvious than when it is
down at baseband frequencies). The other potential feature that a quartz resonator can
provide is a degree of impedance matching. So putting the live
end of the opto-diode to a series connected resonator and
feeding the FET gate to the other side of the resonator,
produces a step-down of the diode impedance. Shunting to ground
this second side of the opto-diode with capacitance allows a
degree of control of the step-down ratio. With the resonator
samples that I have and a gate to ground shunt resistance of
220k, the best s/n was achieved with between 33 and 68pF. You
could also easily see a change of loaded resonator Q as you did
this (ie, noticeably lower Q with no shunt capacitor). With this arrangement, there is one obvious advantage (the
lower amplifier input impedance is more sociable) and one
disadvantage (you can't apply opto-diode reverse bias without
using a sensitivity reducing resistance connected to the high
impedance point, as far as I can see). After all this, does it get us anywhere?. I'm not sure -
but it's not far off, and the question should really be 'is
there another form of quartz resonator that has an even higher
parallel equivalent resistance?". Again, I don't know the answer
to that. Watching the SpectraVue display slowly decay when you
remove the light source is fun. Likewise, the slow build-up as
you re-apply the light source.'
There was a zero response to this mailing! - perhaps because very
few people want a receiver that is only suitable for CW reception,
and CW with no keying applied at that, given the extremely low
sub-Hz bandwidth. I just thought that it might be fun to try......
Initial evaluation
This couldn't be simpler:
A measurement of tuning-fork parallel capacitance on one sample
produced a value of 0.5 pF, which is about one twentieth the self
capacitance of an un-biased SFH213 opto-diode, implying that the
addition of a shunt tuning-fork resonator will not degrade the opto
receivers sensitivity at other frequencies in any meaningful way..
Going through a batch of various brand resonators from the junk box
and applying them in the circuit above, resulted in receiver
sensitivities at 32.768 kHz that were within 10 dB of that at 1 kHz.
Two samples were within 3 dB.
