I noticed an Arduino Nano too…is that the way you plan to go. If so, I like it
as it’s only about USD$3.50 delivered from Ebay vendors. The ‘5351 would be
very nice, though I understand the problem with sourcing parts like this in
some places. Still, the Si570 is also not widely available and it costs about
5x the ‘570. Personally, I’d like to see it go that way (5351), but I do
understand what you’re trying to accomplish.
Two more suggestions:
1) I had previously suggested synthesizing the sidetone by using the PWM
to generate a sinewave according to a sine lookup table. I’ve done this for
other applications and it can produce a very clean sinewave with only RC
filtering.
2) If you run the pre-volume control audio into an analog input on the
Nano, it opens the possibility to implement digitally synthesized AGC by
sampling the audio at a high rate, peak detecting and generating a gain control
function. The gain control function could be fed forward to a slightly
different version of the TDA amplifier that has a gain control input. This gain
function would also act as an S-meter, and everything could be reported and
controlled via the USB port.
Once there’s a good ucontroller with a USB port available, the sky’s the limit
for CAT features.
Joe
W3JDR
From: minima-bounce@xxxxxxxxxxxxx [mailto:minima-bounce@xxxxxxxxxxxxx] On
Behalf Of Ashhar Farhan
Sent: Wednesday, May 20, 2015 3:43 PM
To: minima@xxxxxxxxxxxxx
Subject: [minima] Re: something fresh - a long long post
joe,
i have used the same ckt for bfo as before. with just a small change. i used
two 1N4007 reverse biased as varactors. lemme clean up the circuit in the next
few days and post it as a regular diagrma.
- f
On Thu, May 21, 2015 at 1:10 AM, Joe Rocci <joe@xxxxxxxxxx
<mailto:joe@xxxxxxxxxx> > wrote:
Nice work Farhan!
What are you planning to use for the BFO? An Si5351 for both LO and BFO would
be a nice choice.
How much output do you get on Xmit?
Joe
W3JDR
From: Ashhar Farhan <mailto:farhanbox@xxxxxxxxx>
Sent: Wednesday, May 20, 2015 3:13 PM
To: minima@xxxxxxxxxxxxx <mailto: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
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