jack, joe
i had ordered five nano - boards. they have become my favourite. i start
using the Arduino boards primarily because of their built-in USB ports. the
Nano boards are the price of french fries on the side and they have two
extra pins as well. Though I'd like to keep our design independent of the
board used (or just an ATMEGA328P).
The transmit side spurs are all below 50 db below the carrier. I get the
same 50 mv peak (100 mv peak to peak) RF output as the previous KISS
output. It could be driven harder but the spurs go up.
I am pretty confident that throwing away the 28MHz and the 24 MHz bands is
worth the trade-off of having an excellent transceiver for rest of the
bands. The 28 MHz has been a favourite band here. I'd like to preserve the
switchable two filter setup as an option. However, we can make the second
filter a band-pass filter. That way we can accommodate almost any band :
even 2 meters. The band pass filter can have it's own two-way preamplifier
built-in. We could just mark this as a set of relays and the other band
pass filter can be a optional outboard thing.
joe, you had suggest the Arduino AGC earlier as well. One could probably
combine the AGC function with even a noise-blanker. this would make the
Arduino lock to AGC. One could time-slice it and still monitor the
PTT/KEY/Tuning. hmm ... it is worth the trick. Do you have any suggestions
for a readily available DC controlled audio amplifier? We will still be
left with the challenge of producing the DC from the PWM output of the
Arduino.
I did try the PWM option of generating the sinewave. In my case, the
filtering required was so extensive that I gave it up and reverted to a
simpler RC phase-ring oscillator. Lemme know if you could sketch out
something on paper and push it on the list.
- f
On Thu, May 21, 2015 at 1:37 AM, Jack Purdum <dmarc-noreply@xxxxxxxxxxxxx>
wrote:
While I sorta have a vested interest in the Arduino family of processors,
I'm looking hard at the Teensy 3.1. It is smaller than the Nano, has 256K
of flash and a HUGE 64K of SRAM, is 5V tolerant on pins, 12 bit DAC, 3
Serial, 1 USB, 2 I2C, and 1 SPI ports, and has 34 I/O pins. True, it costs
more ($19.80), but packs a lot of horsepower in a very small package. It
has an Arduino style IDE. It's worth a peek.
Jack Purdum, W8TEE
------------------------------
*From:* Joe Rocci <joe@xxxxxxxxxx>
*To:* minima@xxxxxxxxxxxxx
*Sent:* Wednesday, May 20, 2015 3:56 PM
*Subject:* [minima] Re: something fresh - a long long post
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> 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 <farhanbox@xxxxxxxxx>
*Sent:* Wednesday, May 20, 2015 3:13 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
------------------------------
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