Hi Joe. Plots for 30 and 40 MHz filters very interesting. If you have
crystal characterization data or LT spice model of the 40 MHz filter, would
be very helpful to me abnd probably others.
Thanks,
"Digital Steve", K1RF
_____
From: minima-bounce@xxxxxxxxxxxxx [mailto:minima-bounce@xxxxxxxxxxxxx] On
Behalf Of Joe Rocci
Sent: Thursday, May 21, 2015 12:43 PM
To: minima@xxxxxxxxxxxxx
Subject: [minima] Re: something fresh - a long long post
....and here's the plot of the 40 mhz filter. Unfortunately I don't recall
what the full scale reference was, but loss was pretty close to the LTSpice
model. Maybe I can find that.
https://www.freelists.org/archives/minima/11-2014/gifyb1LX9F1E_.gif
From: Joe Rocci <mailto:joe@xxxxxxxxxx>
Sent: Thursday, May 21, 2015 12:37 PM
To: minima@xxxxxxxxxxxxx
Subject: Re: [minima] Re: something fresh - a long long post
Steve,
Here's the plot I posted some time back for the 30 mhz ECS xtal filter
https://www.freelists.org/post/minima/30-Mhz-Xtal-Filter
joe
From: Steven B. Dick <mailto:sbdick@xxxxxxxxxxxxx>
Sent: Thursday, May 21, 2015 12:23 PM
To: minima@xxxxxxxxxxxxx
Subject: [minima] Re: something fresh - a long long post
Great job!! In order to preserve your beloved 10 meter band, I came across
some fundamental mode 40 MHz crystals made by ECS, part number
ECS-400-20-3X-TR. See Digikey:
http://www.digikey.com/product-detail/en/ECS-400-20-3X-TR/XC1786CT-ND/267665
0 costing only 56 cents, quantity one dropping further with quantities.
Could be used to make a 4 pole quasi equiripple filter, as you like to use.
I purchased some but have not been able to get to characterizing them yet as
I have so many projects ahead of it. They have a specified ESR of 40 ohms.
Not great, but could possibly be used to allow 10 meter operation.
Regards, "Digital Steve", K1RF
_____
From: minima-bounce@xxxxxxxxxxxxx [mailto:minima-bounce@xxxxxxxxxxxxx] On
Behalf Of Ashhar Farhan
Sent: Wednesday, May 20, 2015 3:14 PM
To: minima@xxxxxxxxxxxxx
Subject: [minima] something fresh - a long long post
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
No virus found in this message.
Checked by AVG - www.avg.com
Version: 2015.0.5941 / Virus Database: 4347/9832 - Release Date: 05/21/15
