GM3WOJ - GM2V - ZL1CT website March 2011
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Section 1 - HF RX performance information from Geoffrey Mackenzie-Kennedy GM4ESD
Section 2 - INRAD and Elecraft roofing filter plots from Gavin Taylor GM0GAV
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Section 1 - the following notes are compiled from a series of e-mails from Geoff :
During the past two decades (roughly) there has been a "horse race"
between Direct Sampling Software Defined Receivers and Superhet Receivers, the
prize
being which platform offers the better performance in terms of 3rd Order Dynamic
Range and Noise Figure. So far the superhet platform is still ahead,
but some direct sampling receivers are closing the gap. At this time a modern
design of superhet can yield a larger dynamic range and lower Noise Figure
than that of the direct sampling SDR- for example an IMDDR3 of >120db at 2kHz
spacing using a 500 Hz roofer and > 110db with both tones inside the
passband, together with a HF Noise Figure of <10db for the superhet without a
preamplifier.
The quality of crystals used in the superhet's roofer, and their matching, is of
paramount importance.
From a paper analysis of the K3's receiver, the IMDDR3 would appear to
"droop" from < 95 db ( @ 2 kHz spacing) to approximately 70db - 75
db when
both tones are within the passband, i.e. a "droop" of roughly 20 -25db
from the delta 2 kHz IMDDR3. This compares with a "droop" of
approximately 10db
from >120db to > 110db as seen in a modern superhet, the result from the
modern design is not phase noise limited
Some people would argue that a large dynamic range (IMDDR3) within the passband
is not useful, because key clicks and/or phase noise generated by
transmitters operating on close frequencies would become the dominant problem. I
cannot agree with this argument after using receivers whose
in-passband IMDDR3 is large for the past 16 years, which includes using them for
working SSB DX on 40m from this Scottish QTH amongst the BC stations
whose carrier levels could reach +5dbm - +10dbm at peak time - before they moved
(allegedly)! Any difficulty in this case was caused by the BC
modulation sidebands, not the BC transmitter's phase noise.
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The AGC applied to the J309 comes from two sources via the op amp U3A.
One source is the HAGC detector D29/ D30, which receives its input from the 15
kHz IF amps U15A and U15B. I do wonder whether or not this HAGC detector
introduces any IMD into the 15 kHz IF, being as it is not isolated in any
way.
Control from the second source appears on the line VIF GAIN 1, which drives the
op amp U3B which in turn drives the op amp U3A
The output of the op amp U3A acts on both the J309 and the diodes D21/ D22 to
control gain. In this scheme the gain of the J309 is changed by reducing
its Drain current, i.e. by changing its operating point, which could increase
any IMD generated by the J309 just at the time when one wants an
improvement. I have similar reservations about the IMD performance of D21/D22.
The problem with the SA612 is its very low Intercept, fine for some portable QRP
kit if large signals are not encountered, but a very poor choice of mixer
for use in this type of receiver.
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1) The LO feed to the receiver's second mixer and the transmitter's first
mixer, derived from a divide-by-six divider, are not buffered but are simply
tied together.
2) The receiver's first mixer could generate noticeable IMD, because unlike in
the H-Mode mixer the Sources of the FETs are not connected
directly to ground.
3) Are the cores used in the bandpass filters, and elsewhere, large enough so
that the IMD that they contribute is of no concern? Cores of size T94 are
required in a H-Mode receiver.
4) How much 'blow-by' exists around the roofers?
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Whether or not the Group Delay Variations are in fact partly responsible for
the "mush" can only be determined, I believe, by some comprehensive
testing
of the filters - and their effect on pulsed signals. Nevertheless it does appear
that the designers were only considering raw selectivity, perhaps for
reasons of cost. I also wonder about the quality of crystals
used in the filters.
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If an input signal to a filter consists of a series of pulses such as a keyed
CW signal, and the filter exhibits any delay, then both the rise time
and the decay time of each pulse as seen at the output of the filter will not be
the same as that of the original input. The decay time in particular
is related to the Q of the filter and can be significant in narrow bandwidth
filters.
* * In the case where a filter exhibits significant group delay variations
within its passband and there are many close spaced keyed CW signals
entering the filter, then a situation can arise where the stronger signals
entering the filter do not appear to be the stronger signals at the output
because they take longer to reach their maximum amplitude than their weaker
companions. * *
When the signals are weak, noise compounds the problem because noise spikes can
be viewed as signals.
Although the effects of group delay variations can be calculated, the
maths becomes very tedious if many CW signals keyed at different speeds are
involved. Dynamic tests are a better option!
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Chris, the test that you suggest would be suitable provided that the odd
order IMD products generated by the combiner used between the generators and
the K3 are well below the K3's noise floor, and the isolation between the
combiner's input ports is adequate. Of course it would be useful if both the
amplitude and p.r.f. of the signal from each generator could be varied on an individual
basis.
The problem as I see it is to identify which element or elements in the signal
path is/are responsible for creating the "pileup mush". Could someone
tell me please whether or not the "pileup mush" becomes less of a
problem, or ceases to exist, if a wider bandwidth roofer is used, e.g. a 1800 Hz
8
pole vs. a 400 Hz 8 pole, while keeping the DSP filter's bandwidth constant.
If the "mush" does disappear when the wider bandwidth roofer is used,
then this would suggest that the group delay variations found by Gavin in the
narrow bandwidth roofers is the prime suspect. If the "mush" does not
decrease, then something else in the signal path is creating the problem.
There is also the middle ground where the "mush" decreases but is
still heard, which would suggest that both the narrow bandwidth roofer and the
"something else" together are creating the "mush" problem.
Not to be forgotten is the two crystal filter between the J309 IF amp and the
second mixer. If it is tuned properly and uses matched crystals, then
its group delay variations should not be large enough to create
"mush".
At the moment I do not suspect the DSP's ADC. However if the delays in the
narrow bandwidth roofer are causing half cycles to be lost, then this would
most likely affect the ADC.
I note also that you have heard the "mush" not only when signals are
weak, but also when signals are quite strong. This raises the spectre of
improper
filter terminations, and whether or not the application of HAGC affects
the output termination of the roofer. I suspect that it might, but I have not
run a SPICE simulation to confirm my suspicion.
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Section 2 - these filter
plots were received from Gavin GM0GAV following his tests on the roofing filters
in his K3 S/No.2203.
Gavin is going to test the roofing filters from his second K3 a.s.a.p.
These plots did not display 100% correctly when converted to HTML, so please click here to download these as a PDF document (156kB)
Geoff GM4ESD made some comments about the results of Gavin's tests - click here to download these as a Word document (574kB)
2nd March 2011 - more information to follow when available ...
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