Hi dear Homebrewers & Lowfers!
I got a Garmin GPS 12 receiver on my birthday which immediately
resulted in wild plans to build a DGPS beacon receiver ;-) I want to
try a phasing SSB receiver with about 500 Hz output. The reason
for using 500 Hz IF is that there is free PIC software available
for synchronizing onto a "hard-limited" MSK signal at 500 Hz and
converting it into a RTCM-104 data stream that can be sent to a
GPS receiver.
The rejection of unwanted sideband is not as important as in the
case of serious HF work since the DGPS beacon of interest is
always the nearest (and strongest) one. Since the next-to-strongest
beacon cannot be stronger than the strongest one (;-), I guess that
20 dB rejection (or so..) would be sufficient.
The audio phase shift networks I have seen so far (in the R2
receiver etc.) are designed to give a 90 degree phase shift over
the entire speech band which is a frequency decade!
The bandwidth of the MSK beacon signals is just a few hundred Hz
(100 or 200 baud). I guess the phase shifter can be made quite
simple (I want to make the receiver as small as possible).
Well... to the point...
I am searching for a simple circuit that is able to phase shift two signals,
in the range 300 to 700 Hz, by 90 degrees (+/- 1 degree or so) with
respect to each other, without introducing any large amplitude
differencies between the two channels.
The phase, and amplitude, response outside the above mentioned
frequency range is irrelevant, a good bandpass filter will follow after
"the I/Q summing point" anyway.
Any ideas?
73 de Johan Bodin, SM6LKM
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