Double sideband demodulator mk3 squaring loop
After having tried the dsb demodulation using the MC1496
divide by two and the Andren frequency doubler I thought i would
marry up the bits of circuit that gave the most performance gain and build up
what I call the mark 3 dsb demodulator , Hopefully this will cut down the
complexity of the circuit and enable a simple PC board to be
produced .
I used the Andren frequency doubler as a squaring generator as it gave the most
gain to produce the dsb squaring output' from the DSB supplied . The NE592
application with a crystal as the the frequency selective feedback element
, gave the most gain with the minimum frequency bandwidth to enable one to pick
the wanted carrier from the mixer( squarer) output ,Its output was enough
to feed the synchronous oscillator and with its ability to
lock to subharmonics , enables one to achieve a "divide: by two " function
and at the same time derive a suitable level of rf output at the correct
frequency for the NE612 DSB product detector .
The components were assembled together to enable the required results , a double
sided PC board was made as per the photo below . Once again i
used the 1.84 Mhz crystal to achieve the narrow bandwidth to filter
the wanted synthesised carrier , the filter bandwidth was very
narrow which in this receive situation will not allow a lot if drift to occur
before the circuit looses lock , Im sure an IF Frequency of 5 MHz or more
will perform equally as well and enable a larger tolerance of drift in the
receiver and still provide adequate demodulation performance ..
The circuit rail was 12v however the Dc feed to the NE602 had to be
limited to 8 v max with a series resistor and a zener diode bypassed with
a tantalum polarised capacitor .
I am not sure of the input impedance of the frequency squaring circuit so have transformed the 50ohm signal generator output to 450 ohms to approximate a match for circuit evaluation purposes , with a homebrew 9:1 trifilar wound transformer transformer design could be somewhat more relaxed if i was running the circuit at 5Mhz rather than 1.8 MHz . The other minor inconveniences I had was the use of 6 mm neosid slug tuned coils requiring rather a large number of turns and capacitance to achieve resonance at such a low frequency 85 turns of fine wire and 220 Pf to get me near 1,8 Mhz . . and much larger values of capacitance to achieve resonance at 912 Khz in the synchronous oscillator ..
The Finished product
9:1
input transformer
squaring mixer ne592 xtal filter
synchronous osc
ne592 prod detector
more to come later
Double Sideband suppressed carrier demodulation , MC1496 as squaring means
DSSC demodulation by the squaring means diode doubler technique
The next DSB demodulator I want to try is the switching inverting alternate audio type which sequentially inverts the demodulated audio and thus assembles it in the correct sequence , the phase changes in the carrier RF at Zero Crossings on modulation , trigger the alternate switching action This type was patented in 1957 ( applied 1952) by Leonard Khan of am stereo fame . he of course used valves !!
Double Sideband suppressed carrier Demodulation Patents
These are all the References I can find on Double sideband
suppressed carrier demodulation , there are bound to be more , they are
all US patent Office registration No.. The E ones are European
Patent office ,
Have a look on the application of File dates and the Granting dates and some
people had to wait a long time before their patents were granted .
2924706 ,2992326, 2999155, 3047659, 3060383, 3088069, 3108158, 3141066, 3151217, 3252094, 3286183, 3345571, 3457513, 3537017, 3593149, 4253066
4821322, 4475218, 4642573, 4862098,
2979611,2397961, 4130839, 4430620, 4485487, 4510467, EP0291826B1, EP0293828B1,
2193801, 2276863, 2784311,
3375453 , 3430151,
3719903, a novel dsb modulator ,
references : http://adamkar.republika.pl/spread_spec_rx_en.htm http://www.tapr.org/ss_g1pvz.html#part_two-receiver see squaring loop demod