Congratulations Farhan on a job well done. You will find over the upcoming
years -- theexperience will prove to be your greatest award. As you now know
nothing feelsas good as saying I did this myself. Every time you make a contact
or look at theunit sitting on your operating table, take a deep breath and feel
the emotionof a job well done.You’re a winner in my book.Stan Wilson AK0B
On Wednesday, May 20, 2015 2:14 PM, Ashhar Farhan <farhanbox@xxxxxxxxx>
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.
Here 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 metersI 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 ampThe 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 requiredGiven 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 systemI 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 discrete 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 tuningEach 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
