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 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 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 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
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