and Q using only software operating on the monaural audio signal from
the receiver. From the documentation:
---- COHERENT BANDPASS FILTER ----------------------------------
DSP Blaster provides a special narrowband processor for
CW reception. The coherent bandpass filter phase-locks to a CW
signal and splits its power into in-phase (I) and quadrature (Q)
components. A phase-stable signal appears only in the I
component. Uncorrelated noise and QRM appear equally in I and
Q, so the signal-to-noise and signal-to-interference ratios are
enhanced 3 dB in I. Select this output for quieter reception of
weak CW. Select the Q output to evaluate signal coherence and
phase-lock quality (AGC gain won't increase while monitoring Q).
The CBPF uses a rounded bandpass filter with gradual
rolloff. Its corners aren't sharp like those of the input BPF
because the CBPF is intended to reduce noise rather than fight
QRM. When a passband with sharp corners is narrowed, the
corners aggravate noise perception. You can narrow a rounded
passband much further without audible problems. This makes the
CBPF very useful as a final noise-shaping stage following the
wideband NR filter.
BP is the ordinary, noncoherent bandpass output. Use it
as a reference when evaluating coherent S/N enhancement in I.
BP also provides a handy rounded passband when it's too much
bother to carefully tune for phase-lock or when the signal has
bad chirp.
S is a special stereo output. The I component is
applied in phase to the left and right channels and appears
spatially centered. Q is applied out of phase and appears
nondirectional and diffuse. This provides striking spatial
separation of signal and noise. S does not reduce noise power
since it uses the full Q component, but the spatial separation
isolates the signal and greatly enhances perception. S allows
you to recover and perceive incidental noncoherent signal power
while focusing on the coherent part. It lets you hear
transmitter and propagation anomalies that are monophonically
inaudible. The stereo presentation is most effective when using
headphones. High-quality earpieces with matched phase
characteristics will enhance stereo imaging.
S provides a normal stereo image. The W output boosts Q
6 dB to widen the image. You may find W useful when using
closely spaced speakers. The N output attenuates Q 6 dB to
narrow the image. You may prefer N when using headphones. N
also provides 2 dB of noise reduction.
8
The CBPF provides a novel way to separate signals on the
same frequency. Sometimes it can even resolve severe multipath,
including simultaneous short-path/long-path propagation, 'round-
the-world echos, and backscatter. Listen in stereo to spatially
resolve multipath components.
To facilitate accurate phase-locking, the CBPF uses
automatic fine tuning (the same algorithm the peaking filter
uses).
Because the CBPF must phase-lock to a signal, it cannot
enhance multiple signals simultaneously. The phase-locked
signal appears at its incoming tone, but an off-frequency signal
will heterodyne with the phase-locking local oscillator and
split into two tones, one per ear in stereo mode. Neither tone
will be spatially centered. This presentation sounds strange,
but it is effective at separating the desired signal and QRM,
particularly in a pileup. (In a pileup you may need to disable
AFT once it has locked to the desired signal.)
The CBPF draws a polar scatter plot that shows I/Q
samples from the last few seconds. When the CBPF is phase-
locked to a strong, phase-stable signal, the dots form a clean
ray that extends upward from the polar center. Frequency offset
causes a phase error that tilts the ray. Phase instability or
noise disperses the dots. The scatter plot lets you monitor
signal coherence and phase-lock without selecting the Q output
and losing audible signal.
--- In [email protected], "i2phd" <[EMAIL PROTECTED]> wrote:
>
> Recently some questions have surfaced on the real need and the
> benefits (or lack thereof) of feeding a SDR program with quadrature
> (I/Q) signals. Let me summarize here my thoughts on the subject.
>
> - Case 1) You have a classical radio, with a last IF of, let's say, 15
> Khz (or a similar frequency up to about one half the sampling
> frequency of your sound card). The radio has a crystal filter which
> has already taken care of the unwanted sideband. E.g., your radio has
> a first IF at 8.3 MHz (like some Japanese rigs have), then the second
> IF is at 455 kHz, and you have added a further conversion down to 15
> kHz (like many do for DRM reception).
> If this is the case, you can feed the same signal to both the left and
> right channels of the sound card. The 15 kHz signal is further brought
> to baseband inside the software, using a quadrature software NCO with
> a complex mixer followed by a complex passband filter centered at 0
> Hz, and the I/Q components are generated inside the program, then used
> for the demodulation process, which in any case needs them. You can
> fully use the filtering and the demodulation capabilities of the
> software, but you will lose the use of a tunable band equal to the
> sampling frequency, you will be limited to just a half of it.
>
> - Case 2) You are implementing in hardware a Direct Conversion
> receiver, with a hardware Local Oscillator at a frequency which we
> will call LO. Then, if you want to be able to separate the wanted from
> the unwanted sideband, you *must* do the conversion to baseband (or
> near baseband, as the final fine tuning is done in software) using a
> quadrature mixer, fed with an analytic LO signal, i.e. with I/Q
> components. Ideally, also the RF should be separated in I and Q
> components, but the advantage of doing this is just the elimination of
> a couple of low pass filters, so normally it is preferred to feed the
> same RF signal to the complex mixer, and have just the LO in
> quadrature form. A couple of low pass filters is much more easy to
> implement than a quadrature hybrid that keeps the phase angle at 90
> degrees over a rather large span of RF frequencies. Also the LO should
> keep the phase angle between the components at 90 degrees,
> irrespective of the frequency. So, solutions like the RC network used
> in Softrock V5 can work as the LO signal is generated by a quartz
> crystal, so at a fixed frequency. I would have preferred to see the
> possibility to tweak the values of the RC network, to achieve a near
> perfect phase angle, but small differences can be taken care of by the
> software. If you use instead a DDS, I am afraid the RC network can
> work only on a limited range of frequencies, in this case a Johnson
> counter fed with a frequency 4 times that wanted, seems to be the best
> solution.
> Returning at the topic of this message, if you are in Case 2) the need
> for I/Q conversion is dictated by the need of suppressing the unwanted
> sideband, otherwise unattainable in a DC receiver. An added plus is
> the fact that now the tunable range goes from LO - fs/2 to LO + fs/2
> where fs is the sampling frequency, the double than in Case 1).
>
> This is my view on the subject. If I haven't been clear enough, just
ask.
>
> 73 Alberto I2PHD
>
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