RadioBanter - View Single Post

February 23rd 05, 11:46 PM

On 23 Feb 2005 02:27:53 -0800, wrote:

The LSB signal which was transmitted at 50.30 mhz is digitally
downconverted with an IF of 50.25mhz. The complex samples I have is
centered at 50 khz. Now

As someone running a 6m analog phasing rig what your describing makes
little sense to me.

Will not work unless you down covert again to baseband.

The signal you should be looking at is SSB rf source at 50.250 with
a 90 degree I-Q local osc at 50.250 and an audio baseband. Then
sample at greater than 20Ks/S with two channels and process them by
placeing phase delay of 90 degrees on one path and the and summing
them.

I do exactly as described in the KK7B article on phasing receiver[2].
The I and Q signals are downconverted to baseband, and lowpass
filtered. The I channel is delayed and Q channel is passed thru hilbert
filter.

No it is not.

I get the audio output, when the I and Q are subtracted. Fine. But I
get audio even when they are summed together!!! i.e There is no
sideband suppression. I am wondering what the problem is. I looked at
the hilbert transformer freq response. It looks okay (allpass, with
nulls at zero and fs/2), could this null at zero cause a problem,
because the input is already at baseband?

Unless your considering baseband signal desired to be the 50khz signal
then what you've done is created an image reject mixer with a 50khz
output and not an SSB detector. The image reject mixer case means
your signal is in the output passband at LO+ 50khz but not at
LO-50khz. You'd still have to repeat the process again to get from
50khz to recovered audio with a 50khz digitally derived LO and mixing
process. You can do that. In the end your doing something that
needs only to be done once. Look at EMDRF [Experimental Methods in RF
design] doing SSB in DSP is coverd there.

Allison