allison n the crewi suppose i must toss in a bandpass filter anyway.. those
who need it (me!) can build it other can skip it on the first pass.- f
------ Original message------From: allisonDate: Thu, 21 May 2015 8:22 amTo:
minima@xxxxxxxxxxxxx;Subject:[minima] Re: something fresh - a long long post
HI Farhan,Very nice. I've bern working on a setup if near identical topology.
Ichose different gain stageformat and the IF is 25mhz as that is a common
crystal. The filter at25mhz works well. However I didn'tgive up 10M. in
addition to the low pass filter I used a switchablebandpass for 10M (I skipped
12M).Your choice of 24mhz is a good one. One item you may find the TX gainis
likely low. I neededabout 6-8db (including filter losses) to get the balanced
mod level upto near with the mixercan take as that gives better signal to
carrier leakage with low IMD. For that LO and those diodesa input level for
signal is around 3-5dbm before compression sets in andthat is much stouter
thana TUF3 or SBL-1 commercial part.I arrived at the same problem with the KISS
mixer its single balanceddesign means at leastone port will must have too much
of the wrong stuff. I also found outearly on notch filtersare a pain to tune
and never selective enough 20 if was just to close tothe 15m ban
d.I did experiment with 12mhz and that did work as the notch was narrowenough
but the2nd and 3rd order mixer products were daunting.The mixer is the same DBM
save for a minor change. The RF port (as yourschematic has it)which is usually
the IF port has the winding spit and a 100 ohm portwith the wiper going to
thecap and the low pass ( and mine has bandpass for 10M). Makes it easy
toachieve higher ordersof balance (TenTec uses it in many of their radios) and
the diodes are1n4148. Rather than thesingle balanced modulator I went with a
repeat of the mixer with the IFport wiper being theaudio in/out carrier balance
is easilly better than 50db with looselymatched diodes.SI570 for LO and crystal
based carrier osc as its easy and switchablefor USB/LSB/CW.The current version
only has the Arduino doing the Si570, as most of theother functionsare easily
done with random logic implemented with transistors. I'mtrying to
avoidanything like menus. Main functions (USB, LSCB, CW, atten
uator,preamp(rx), andpower level) will be buttons with led annunciators. I
may do a variantof another radioI have where there is an up and down button and
a digit select button. Up and Downchanges the selected digit and the digit
select high lights (blinks theselected digit)the desired digit for increment or
decrement. That makes largeexcursions andsmall ones easy.Still working on the
TX amp and its testing and running with a smaller3w. The goalis a switchable
5W and higher like 10 to 20W with high efficiency. Theexperimentalversion of
that amp is single ended driving push pull bipolar, drivingpush pull
bipolarlike MRF406 or 2sc1945s as I have some I have some. Maybe RF16HHF.
I'dconsidered IRF510s but at 12V they are not optimal (at 24V they workvery
well).The goal for that stage is to fit it in a 2x3x1.5" volume including
afinned heatsink.First break I get to assemble a schematic I will post it but
for themost part nothingunusual.AllisonOn 05/20/2015 03:13 PM, Ashhar
Farhan wrote:> comrades,>> i know that it has been a very long time since i
posted anything> substantial on this list. but i have been quite active
measuring,> thinking and trying out various things for the minima. I have
finally> arrived at some conclusions that i'd like to present to you all. >> I
have produced a new version of the Minima that is substantially> simplified,
easy to work with. However, it comes at two major> trade-offs (that is why we
are engineers, not scientists : we work to> a budget). First, the transceiver
works from DC to 21 MHz. I had to> drop 10 meters - a personal favourite.
Second, I have switched to a> diode mixer resulting in a drop of IIP3
performance down to around> +15dbm. This is still superb. But not in the same
league as before. >> Finally, I have spent the last two days using the rig. It
is a really> sweet sounding transceiver. Easily the best I have used. The
circuit> is simplified to the extreme. It is even simpler than the BITX.>> Her
e is my long story about it :>> Measurements>> I realized that I didn't have
the equipment to actually test and> measure IIP3, loss, etc. Hence, I spent a
few months building> equipment. I now have a spectrum analyzer, entirely
home-built that> has excellent dynamic range. It is based on the same Arduino +
Si570> combo as the 1st oscillator. The rest is an evolution of the W7ZOI's>
spectrum analyzer. Along the way I learnt to sweep VHF filters, and> measure
IIP3. This analyzer can step in 1 Hz steps (thanks to you guys> for having
developed a better Si570 library for radiono) and I have a> narrow 500 Hz and
wide 300 KHz filters. But that is an entirely> separate topic for another
article.>> I also made a two-oscillator setup by pulling 14.318 MHz crystals>
apart by 20 KHz. And combined their buffered output in a 6db hybrid> combiner
and took the output through a an LPF cut for 14 MHz. Thus I> had the ability to
measure loss, intercept, band-pass. I still lack> the ability to
measure noise figure due to a lack of calibrated noise> source.>> Equipped
with this, I set out to hack the KISS mixer. I have spent a> great deal of time
trying to build them with discrete devices. I tried> everything : from 2N3904s
through 2N7000s to J310s. I tried five> different biasing schemes. I have
documented it all in my notes. The> summary is simple : the KISS mixer lacks
enough suppression of the LO> to be used in the middle of a passband. It is an
excellent mixer for> high performance receivers. One could add some narrow band
filters to> the Minima and a Linrad backend to beat the living daylights out
of> K3S. Btw, I measured more than 30 dbm IIP3 on the KISS mixer, original>
version. I say 'more than' because my -10dbm per tone signal source> was
hitting the noise floor on the specan.>> 1. Giving KISS a miss>> So, the KISS
mixer has to be parked aside for the a minmal rig like> the MInima. That leaves
us with the old favourite : the diode ring> mixer. I built a
diode ring mixer with 1N4148 diodes that measured> 15dbm IIP3. This can be as
good as any of the higher performance rigs.>
(http://www.elecraft.com/K2_perf.htm#Main RX Table)>> However, the standard
mixer circuit took the IF from the center tap of> the tranformer that was
driven by the LO. This leaked the LO to the IF> (which we use as the RF port).
By grounding the center tap of the LO> transformer and taking the IF from the
center tap of the other> transfomer (the one connected to the RF port), the LO
dropped> substantially. It went down by almost 57-60dbc (below the carrier). >>
The diodes will have to be matched to the last millivolt : easily done> with a
two dollar DVM.>> 2. Dropping 10 meters> I had written earlier that i was
fooling around with 24 MHz crystals.> These commonly available and inexpensive
too. By moving the IF to 24> MHz, we achieve a number of things. First, an 4
section LPF cut for 21> MHz will receive everything from DC to 21 MHz. Second,
it offers>
reasonable attenuation to IF. It comes at the cost of dropping the 28> MHz
band. (We can add an 'extra band' with relays that provides a> BPF based
narrow band coverage of any one other band (the diode mixer> will mix from 144
MHz to 28 MHz). >> 3. Post IF amp> The diode mixer needs a robust termination
to work well and the> crystals of 24 MHz were quite lossy. Both these factors
lead to adding> of a post-mix amplifier ahead of the crystal filter. I know, it
is> kinda 'old world'. But look : sensitivity is up, crystals need not be>
expensive.>> 4. IF amp - not really required> Given that we have gain ahead of
the crystal filter, we really dont'> need much gain before the audio detector.
Hence, just an emitter> follower to buffer the signal from the crystal filter
to the audio> detector.>> 5. Simpler audio system> I replaced the
three-transistor W7EL style audio preamp with an old> fav from the BITX20 using
a single transistor. The power audio> amplifier is no longer the di
screte power amp. Instead, I am using a> TDA2822 power amp chip. The original
Minima audio needed more gain.> This chip has 40db of voltage gain.>> 6.
Improved tuning> Each time I used the original Minima tuning i wanted to kill
the guy> who wrote it. I rewrote the tuning system. Now, it tunes like a
normal> tuning knob for 100 KHz in 100 Hz steps. However when u hit the band>
edges it starts to 'scan' first in 20 Khz steps, then 100 KHz and> finally in
500 KHz steps. It works well. I must add some visual alert> when it starts to
scan. But that apart, the system is quite workable.>> I am attaching a very
rough and incomplete picture of the circuit from> my lab notes that shows the
changes.>> - farhan
