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With Allison's permission, here is the message to EMRFD regarding the BITX.
This is good information from a master RF engineer.
john
AD5YE
"Some thoughts on improving the bitx...
First so there is no bad feelings Frhan did a bang up job with the
design and its persistence
speaks loads of the reproduceability and utility of the design.
The yabut..
The BITX was not intended to be the high performance transceiver only
low cost and
its avoidance of hard to get parts. So there are compromises, a hard
fact of engineering life.
Things that may help or improve it based on the 40M version I built years ago.
The mixers using 1N914/4148 diodes are not level 7 mixers (nominal
7dbm drive) like commercial DBMs.
the reason for this is Schottky diodes have a lower turn on threshold
than silicon junction diodes with the difference being .2V compared to
.65V. This means you need more LO drive for the terminal impedance
of all ports to be 50 ohms. It also means they are higher level
mixers by about 3db. So enough drive is important to intermod and
overload.
Second the terminal impedance of the crystal filter and the various
RF/IF amplifiers were optimized for 200 ohms the mixers are better at
50 ohms and there is no impedance matching to correct for that. This
means that the DBMs are compromised in performance again for overload
and IMD. The fix here is to insure all ports
especially the IF port is matched to 50 ohms.
It many radios I've built the DBM to IF amp has a Diplexer. Th reason
for this is to insure the IF port sees a wide band (dc to 100mhz)
match and only the desired IF pass though it. This keeps reflected
signals from reentering the mixer and adding to the possible products.
Choice of LO frequency. Unless there is a defining reason I use a LO
that is Higher than the IF as in 40M
with 12mhz if that would be 19mhz. For VFO that would be drifty but
for a NCO (Si5351) its no big deal.
Why? When mixing signals there are two known players (IF and LO) and
many live signals in the band
and they all mix so you get sums and differences and then harmonics of
all those making sums and differences. A program like spurtune can
list them all out and show what the result and likely strength
of each will be. When the LO is lower than the IF the likely
possibilities and their harmonic mixes are
more numerous at or neat the RX pass band and IF passband. Again a
low pass filter between the
LO and the mixer can help sometimes.
RF amplifier... For bands below 10mhz its likely not needed or needs
to be very low gain as there are an abundance of strong signals. RF
selectivity before the preamp is an aid in this and even adding
switchable
attenuators (I use 6 and 12DB so I can get 6/12/18db of attenuation)
and in a strong signal cases that can
help. Lowering the gain of the RF amp (for RX) can help as well.
Another item is if the RF amp is not robust enough it can easily
overload before the mixer, at that point all is lost to IMD. What
high end radios do is use lots of current in the RF amp so its not
easily overloaded then use a mixer that can tolerate that as well as
its no sense having the RF amp be clean and overload the mixer.
Its important to point out that for a given RF amp design and DC bias
level there is a maximum signal level
that will exceed it distortion capabilities. Differently said that
amp has a maximum undistorted power out
that must be spread over all the accepted signals often that means the
amp must be very robust. An
example is a RF amp I used in a radio that had to withstand 10dbm or
more at the input in band
and not overload. The amp ran at 160ma and could deliver .4W of two
tone signal undistorted (better than 35db down). It actually used RF
power devices (2xMRF584) to get that level of capability. Of course
the next stage had to deal with that. The end result was a crunch
proof radio but would be unforgiving about power
used (RX needed 1.5A with .4A in the low level RF sections). It is
sometimes easier to attenuate the
offending signals (as well as desired) as a strategy. Why? because
even if the RF amp is good enough
and the mixer then the first IF (and maybe even second) need to be
able to handle all that signal. In the end it tend to be a very
"system" level problem rather than point solution.
Other tricks are front end preselection using narrow tuneable band
pass filters (loss is tolerable)
to reject the offending players. Notch filters as well though at
higher frequencies they may not
be effective enough.
Consider the case:
Offending signal of -25dbm which is very strong. Add 17db of RF gain
to that and its now -8dbm and any
mixer below level 17 (50mw LO drive) will be overloaded. To make
matters worse if the RF amp is running
less than a maybe 10ma it will be overloaded itself as I've seen this.
So we omit the RF amp and try again
and a -25dbm signal is right below the limits for distortion for a
level 7 (5mw lo drive) mixer. At this
point 6DB of attenuation of RX is an aid as then the signal is down to
-31dbm. Even without the RF amp
the RX is sensitive enough to hear most likely signals your going to
talk to and if need be you can even up
the audio gain to compensate to a point.
They key is managing the levels of all the signal passing through the
RF and mixer or overload will be
a very negative result. Excess gain often deemed desirable are not
always helpful.
My first pass with mine was the described switchable two step
attenuator and then the ability to switch
out the RX RF amp completely. Note that the Elecraft K2 (which has a
very good receiver) took this
path. FYI: small sugar cube relays are handy for this as they can be
placed close to here needed and
powered from a front panel switch.
The BITX is a great experimenters radio and this is one area where
experimenting can be useful of not
required."
Allison