Dear LF Group,
That is a nice feature of only having to cover a narrow range of
frequencies - I use a similar approach on transmit to generate the Jason
signal - I mix 4.5kHz output from the sound card with 133 kHz to get
137.5kHz - the correct sideband is selected by a 4 pole bandpass filter
which covers the full 136kHz band - unwanted mixing products are down by
65dB or greater, without requiring the added complexity of phase shift
networks and I/Q mixers. It does depend on having higher "audio"
frequencies than usual.
Now in my receiver I use 4 tuned
circuits, with strong high frequency attenuation (46 dB at 200 kHz and 80 dB
from 500 kHz to up). This configuration attenuate MW broadcasting signals.
front-end also attenuate the image frequency.
I have a little problem. My CW BFO is not locked to the 5MHz reference as all
the other oscillators in the RX are so it is quite useless for narrowband
I have two options, either to use SSB filter and find some way to remove the
harmonic response from the vectorscope mixer (becomes a problem at
about 1000 Hz vectorscope LO and below) or to use CW mode with SSB BFO
which is not fun at all because of double sided reception (+3dB more noise at
best). Should I build a preselector BPF for the vectorscope or should I build
a 199 or 201 kHz phase locked BFO for use with the narrow CW filter?
A simple 1kHz bandpass filter on the input of the vectorscope circuit to
prevent harmonics of the LO frequency getting in would be a quick answer -
but I would think using a modification of your DDS source to generate the
BFO signal and then have the I/Q mixers operating directly at the 200kHz IF
frequency would be the best idea, because it would remove restrictions on
the use of the filters. With a 32 bit phase accumulator and 5MHz clock, you
could generate about 800kHz, then use the ring counter circuit to get
quadrature outputs at 200kHz - this would give you sub-millihertz BFO
resolution, which should be enough for most things, together with the
stability of the 5MHz reference. It would also make a very nice tuneable
BFO for the RX, and there are probably other things you can do with the I/Q
outputs... There is no particular upper frequency limit on the mixer
circuit - as shown it should be OK to a couple of hundred kHz, although you
might want to use faster op-amps for the phase splitter, or a push-pull
transformer. Using the 74HC4053 instead of the 4053 should extend it to
several MHz. Or use a couple of SBL-1 mixers up to VHF - so long as the
output is DC coupled the result will be the same.
The circuit as shown is only an initial experiment - it could be improved a
lot for low noise, high speed etc. The selectivity of the system is defined
by the single RC low-pass sections, which could usefully be replaced by
higher order active or passive low-pass filters. The cut off frequency
obviously needs to be high enough to pass the sidebands of whatever type of
modulated signal it is being used with.
Cheers, Jim Moritz
73 de M0BMU