If you are going to put a pre-amplifier in front of the QSD that I would
suggest a better pre-amplifier than a MMic chip. They don't have the best
IP3 figures in the world and can have a lot of noise.
A Norton amplifier will be low noise and can have a very high IP3, for
best results a Norton amplifier configured in a push-pull configuration
will add even harmonic cancellation to boot.
At 09:12 PM 10/28/2005, Bob Hillard wrote:
Consider using the SR40 as an
'IF strip', preceeding it with a MMIC
preamp, some bandpass filtering, and a Diode Balanced Mixer
using an AMQRP DDS module as the LO source.
I have done that very thing with excellent results.
I've converted my SR40 to 30 meters, and use 10.138 mHz as the IF
center frequency. I've located the LO above the incoming signal in
order to reduce image frequencies.
The output of the DDS module ranges from 11.9 mHz on 160 meters to
31.6 mHz on 15 meters. By replacing the DDS module with the newer 60
mHz model when it becomes available, coverage will be increased to
include the 12, 10 and 6 meter bands.
With the LO above the incoming signal, the radio operates on the
lower
sideband. However this can be corrected by software for bands that
require USB operation.
Band switching is accomplished by programming my DDS VFO controller
() so that each of
the
eight programmable configurations represent a band. This gives me a
bandswitching VFO with wrap-around, up/down frequency control or
direct entry, variable VFO steps, sweep and scan functions, and 20
memory cells per band.
Presently I am switching bandpass filters manually, but that will
change with the addition of a band pass filter selection circuit
similar to that found in the 'Software Defined Radio for the Masses'
article. The bandpass filters are also similar to those found in the
QEX article. One of the filters, the one I use most oftenly, covers
the 15, 17, and 20 meter bands.
By using this scheme, the SR40 circuitry is not taxed by trying to
stretch it's operation past the limits of its design since it is
always operating on 30 meters. However the radio itself can operate
from 160M to 6M, with the exception of 30 meters (the IF frequency).
Likewise, since the DDS frequency does not have to be divided by
four,
the useable range of the DDS module is extended to include all bands
through 15 meters.
?
What's next? Add a QSE module, a xmit DBM, some driver amps and
WALLA,
an all band transceiver. Then add an HC908 controller to take care
of
housekeeping, and a DSP module for operation independent of a
computer; a nice winter project.
I'll upload a block diagram of the radio.
Bob WA6UFQ
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Cecil Bayona
KD5NWA
I fail to see why doing the same thing over and over and getting the
same results every time is insanity: I've almost proved it isn't; only a
few more tests now and I'm sure results will differ this time ...
