Hello Jim, LF,
Am 19.07.2011 23:11, schrieb James Moritz:
Scale the values of either R or C proportional to the inverse of the
centre frequency (or both - it is 1/RC time constant you need to
scale). Yo might find faster op-amps neccessary at high audio
frequencies.
OK, i did that for a center frequency of 12 kHz but the result was
poor. Then is systematically played with the values and found two
pairs of 1n/5k8 and 15n/2k27 which showed at least 47 dB image
rejection in the LF band, peaking at -76 dB at 12.55 kHz, which will
be a theoretical value of course. I'm going to try these values in a
practical circuit and see what comes out.
Well, it just depends what you are trying to optimise - do you want a
wide frequency range with moderate unwanted sideband supression (this
is what Andy's initial values on the spreadsheet seem to be aimed at),
or a higher supression over a narrow frequency range - you might like
to try C1 1n, R1 5470, C2 4.7n, R2 6790. These are feasible values -
but to get the response shown on the spreadsheet, you might need a
screwdriver with reduction gears for adjusting the preset resistors ;-)
I tried 1n / 5800 Ohm (5k1 and a 1k Pot) and 14.7n / 2300 Ohm (1k8 and a
1k Pot) and the 74HCT4052. After struggling a bit due a hardware mistake
i found that the output waveform looked like nonsene. But this was due
to massive overloading the input of the OPA which runs at 5 VDC (i tool
the LM358 which seem to work reasonable). When setting SpecLab to -5dB
of the full audio level, everything seems to work well for the first
attempt.
This is the board:
http://dl.dropbox.com/u/19882028/LF/TX%20converter%20board.JPG
I recorded the output voltage of the 2 busses. It looks not really like
a sine wave ;-). But i think this is normal since the RF has a much
lower frequency than the LO and a 90 deg shift. So the curve will look a
bit like a staircase. Or am i wrong? This output of the busses:
http://dl.dropbox.com/u/19882028/LF/Pin3and13of74HC4052.BMP
When feeding this differential signal to a transformer ( i took a
FT50-77 and 12 turns primary, 10 secondary, so far) and adding a 100 Ohm
Pot, the output spectrum looks OK for me, for the first attempt, see:
http://dl.dropbox.com/u/19882028/LF/Output%20spectrum%20without%20LPF%20and%20without%20optimising.BMP
This is the spectrum without optimising the two Pots. It shows above 30
dB image rejection and the LO (=125 kHz) is not visible at all :-)
Next week i will add a white noise to the RF input and will optimise in
the region of 11...12.8 kHz. Then i will a simple and uncritical LPF and
will take a look at the spectrum. Then another amp stage will follow.
Then i'll come on air on 137.7000000 kHz (both, the LO and SL is GPS
"locked")...
Any comments please? Do the waveforms look OK for you or is there a big
mistake?
Vy 73, Stefan/DK7FC
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