your thoughts about an undersampling LF receiver are very interesting!
The "tunable crystal filter" solves the problem with multiple alias passbands
associated with undersampling but there are other problems that may
be harder to solve - sampling aperture time and aperture jitter. Perhaps
quantization noise will be a problem too (due to the low Fs ), I don't know...
Anyway, the undersampling idea is very interesting.
The aperture time has to be very small compared to the period of the input
frequency of interest. Many A/D converters, including those integrated on many
microcontrollers, will easily run at Fs=6kHz but the aperture time / acquisition
time is far greater than a fraction of a 136kHz cycle. Some kind of a fast
sample-and-hold circuit has to be inserted before the ADC.
Of course, sampling at or above 300 kHz (to satisfy the Nyquist criterion)
is out of the question, due to the availability of such high sampling rate
ADCs and the huge dynamic range required and also the huge processing power
Not necessarily out of the question.. Hardware decimation chips *are* available.
One example is Intersil (formerly Harris) HSP50016 which has a 16-bit parallel
input capable of >50 MSPS. It has a half-complex mixer and a quadrature NCO
that produce an I/Q signal which can be decimated by a programmable factor down
to 1/64K of the input Fs! Decimation filters are on-chip (working in the digital
domain of course). The output is serial and a lot of formats can be selected,
including a real (single-wire) signal in "Weaver/3rd method" mode.
A simple RX using this chip was described by KC1HR in QEX September 1997.
A suitable ADC for this kind of RX is Analog Devices AD9260. This is a 16-bit
ADC with input BW = 1 MHz. It is "oversampling" at 20MSPS and contains a
Fs/8 decimation filter on-chip. With 20MHz clock, the output Fs is 2.5 MHz which
can be used as clock for the HSP50016. No "nice numbers" though, the NCO
in the HSP50016 is a "power-of-two" thing... :-)