Well said, Bill!
This little radio has come along way in the past few months.
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On Sep 11, 2018, at 19:44, Bill Cromwell <wrcromwell@...> wrote:
Hi,
Just to be clear, Warren IS talking about SSB operation when talking about the IMD products. That is a remaining issue. But with rearranged low pass filters and the additional 45 MHz filter, CW operation is healthy enough to operate.
It will probably not be very long before the IMD issue gets resolved. That is how open source projects are supposed to work we have been seeing this open source project do exactly that.
73,
Bill KU8H
On 09/11/2018 05:53 PM, Warren Allgyer wrote:
With the PA filters removed from the motherboard and constructed
externally with two relays per filter and better layout and isolation,
the harmonics on all bands are compliant with the existing filter
component values. With the addition of the R27 45 MHz filter with a 12:1
impedance transformer between the filter and the following mixer, my
uBitx is also now compliant for spurs. What remains for compliance is
confirmation of appropriate levels of intermodulation distortion.
There is no FCC or ITU standard for intermodulation distortion. There is
a standardized measurement procedure. Chapter 25 in the ARRL Handbook
details it. But what comprises an acceptable level is left to equipment
suppliers and specifiers. IMD is measured by putting 2 audio tones
within the 3 KHz SSB passband and generally 1 KHz apart at equal levels.
The resulting sideband signal is examined on a spectrum analyzer and the
level of the mixed products of the two tones is measured and compared to
the tone peaks themselves. It is generally accepted that the 3rd and 5th
order products should be a minimum of 24 dB below the level of either
tone. Military specifications often require as much a 36 dB difference.
IMD generally improves as you reduce power and gets drastically worse
with over driven RF stages. I generally consider the power level which
gives at least 24 dB of 3rd and 5th order suppression to be the maximum
useable level and this is the test standard used by many equipment
manufacturers.
With that as background I set about testing the now compliant radio.
Unfortunately there seems to be no power level at which 80 and 40 meters
can achieve 24 dB in my radio. On 80 meters the best I could achieve as
I varied the power level from 100 mW to 10 watts was 19 dB. And that was
at 100 mW. Audio input in this case was 30 mVrms and reducing it by as
much as 10 dB did not improve IMD. It only limited the maximum power
that can be achieved. Power level in these tests was varied with RV1.
On 40 meters the best IMD achievable was 12 dB.
20 meters was the first band where I could achieve a 24 dB reading and
that was at 1.5 watts out. At any higher power level IMD rose
dramatically. At 5 watts it was 16 dB.
I stopped my tests at this point. Such levels are not useable in my
opinion and only get significantly worse as external amplifiers are added.
WA8TOD
--
bark less - wag more
