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My email connection crashed and I had to forward a copy to the list.<br>
<br>
jeff millar wrote:
<blockquote cite="mid44E331E6.1030907@adc.com" type="cite">
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I have some comments on this. But rather than assuming mixers and
multiple conversion, the comments apply to the Mercury Board with
direct sampling of the HF spectrum.<br>
<br>
<a class="moz-txt-link-abbreviated" href="mailto:k3bu@optonline.net">k3bu@optonline.net</a>
wrote:
<blockquote cite="mide076dfda3560f.44e0759c@optonline.net" type="cite">To
add to the picture of super front end:
<p>1. In case if there is a problem controlling the gain of RF
preamp, the step and programmable attenuator should be used on the
front as outlined by N1UL. </p>
<p>2. Use the bandpass filters. They could be shared with TX in the
transceiver arrangement, switchable/selectable when needed.<br>
</p>
</blockquote>
Good idea to share filtering with the Tx.<br>
<blockquote cite="mide076dfda3560f.44e0759c@optonline.net" type="cite">
<p>3. Use of tunable band filters on the front and behind of RF
preamp. Hi-Q circuits or variation of Q multiplier/notch. Selectable if
needed. <br>
They are needed especially on low bands 40/80/160, when very strong
signals within the band are present and one is digging for weak ones in
close proximity.</p>
</blockquote>
Tunable filters will do nothing for strong signals in close proximity.
We have to rely on a combination of very strong signal handling
capability in the A/D and the front end buffer/attenuator. On
160/80/40 meters, the band noise rises to the point that the front end
to the A/D can has no gain or even negative gain.<br>
<blockquote cite="mide076dfda3560f.44e0759c@optonline.net" type="cite">
<p> </p>
<p>4. RF preamp switchable in/out. In case signal levels are
sufficient (high gain antennas) switch it out to improve IP3. Have
items 1, 2, 3, available as needed.</p>
</blockquote>
The A/D needs a low noise buffer amplifier between it and the antenna
for ESD and lighting protection. The buffer need very high IP3, >50
dBm if the A/D has 100 dB SFDR. Then put a digitally programmable
attenuator in front of the buffer amplifier. With the programmable
attenuator set to minimum attenuation, the buffer and A/D combination
has low enough noise figure to work on dead band 10 meters (and 6M?).
With some attenuation set, the IP3 rises to meet worst case low band
conditions. It probably doesn't need more than about 20 dB attenuation
with modern A/Ds such as the LTC2208.<br>
<blockquote cite="mide076dfda3560f.44e0759c@optonline.net" type="cite">
<p>5. AGC continuously adjustable - time constant (attack,
release) and gain (DSP derived?). <br>
</p>
</blockquote>
Suggest that the receiver have two AGC loops. First a loop to control
the programmable attenuator. The receiver starts out with minimum
attenuation. When the DSP detects A/D output that approach the clip
point, about -2 dBFS, the digital loop insert 6 dB of attenuation and
compensates by adding 6 dB of gain in the DSP. The end to end gain of
the programmable attenuator, A/D, and front end DSP remains
constant...which simplifies later DSP, AGC, metering, etc. The first
AGC loop simply adjusts the dynamic range band automatically to keep
the signals within the optimum range. The attack time of the AGC
occurs in 10s of nanoseconds because as soon as a single sample exceeds
-2 dBFS, the attenuator bumps up 6 dB. The AGC release occurs after
about 10 microsec of no signals approaching -8 dBFS.<br>
<br>
The use of 6 dB increments in the attenuator enables the digital gain
compensation to use a simple shift left by two to restore the net
system gain. Using an attenuator with finer step size reduces IP3 and
adds complexity.<br>
<br>
The design needs to consider two overload scenarios. <br>
<blockquote>1) A single strong signal causes clipping in the A/D.
But
that single signal has to exceed 0 dBm on the low bands.<br>
2) Multiple strong signals add together creating voltage peaks that
clip. <br>
</blockquote>
Case number 2) probably will cause the most instantaneous A/D clip
events because thats the way hams operate, pileups, etc. The peaks
occur momentarily and rarely as the phase of many carriers happens to
coincide at the A/D. It's not easy to calculate the statistics, but
with 2 carriers spaced 10 KHz, then the voltage peaks occur at the beat
frequency, 10 KHz, and for a small fraction of the beat interval, about
10 usec. As more more strong carriers get added to the the overload
scenario, the probability of a peak drops and the interval between
overloads gets longer and longer, but the period of the overload
remains inversely proportion to the frequency spread of the strong
carriers.<br>
<br>
Watch out for commercially available digitally switchable
attenuators...they have their own problems with strong signal
handling. With the design outlined above, the front end attenuator can
use 3 stages of 6 dB each, possibly implemented with discrete
components, PIN diodes, etc. for the very high IP3.<br>
<br>
The second AGC loop operates at traditional modulation rates.<br>
<blockquote cite="mide076dfda3560f.44e0759c@optonline.net" type="cite">
<p>6. High level mixer with clean injection signal followed by
selectable Xtal filters - steep skirted and/or low ringing banks.
Option to take the wide band or filtered bandwidth for
bandscope/waterfall or filtered signals.</p>
</blockquote>
With a 90-100 dB SFDR A/D converter, HF receivers can't really use
mixers and buffer amplifiers. Partly because it doesn't need them and
partly because mixers and buffers with 100 dB SFDR don't practically
exist. The modern 16 bit high speed A/Ds have dynamic range
performance that exceeds all but the most extreme buffer design.<br>
<blockquote cite="mide076dfda3560f.44e0759c@optonline.net" type="cite">
<p>7. Wonderful world of I-Q processing and controlling the above. </p>
<p>Unless we have super overload mixer that can tolerate and
eliminate the above, the preamp and filtering use if needed is highly
desirable and should beat anything out there. Right now we have to use
variations of outboard gadgets. Incorporating them into the RX front
end design and be able to control switching and settings as needed by
various antennas and situations would make the ride ultimate and not
available presently in any of the rigs AFAIK.</p>
</blockquote>
Mixers, Roofing filters, and low IF frequencies help present day
receivers reach high dynamic range by filtering out at many strong
signals as possible before presenting a narrow band to a low dynamic
range 2nd IF. When the 2nd IF moves to DSP, dynamic range limits go
away, the whole reason to use mixers goes away, and the reason to use
filters changes from steep skirts at IF to preselector at RF.<br>
<br>
Present receivers have dynamic range specified at about 20 kHz and 2
KHz, with state of the art SFDR around 95 dB and 75 dB respectively.
The close in dynamic range measurement shows the limited performance of
the 2nd IF within the passband of the roofing filter. With a Mercury
style SDR, the close in dynamic range never changes, the design gets
close 100 dB all to way to zero offset. <br>
<br>
The new SDR specification will look more like 130 dB SFDR at 10% off
frequency (due to the preselector) and 100 dB SFDR any offset from 0 to
about 1-3% of frequency (in the passband of the preselector).<br>
<br>
jeff, wa1hco<br>
</blockquote>
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