An audio amp I like is the NXP TDA7052A. I like it for the following
reasons:
1. relatively cheap at $1.14 from Mouser
2. Has a fixed gain of 35.5db typical
3. Has a wide voltage supply range of 4.5 to 18 volts
4. Gain is D.C. controllable with a control range of 80 dB typical. Needs
about 0.4 to 1.4V. If DC goes below 0.4V, it provides a mute function.
5. I like the fact that it is a bridge type load amplifier as opposed to a
single ended amplifier. A BTL amplifier provides twice the voltage swing to
a load compared to a single-ended stage. This provides 4 times the power
output compared to a single ended output. Why is this important? Because
if the amplifier is swinging close to rail-to-rail output, clipping
distortion will not set in for four times more power output than a
single-ended amplifier for a given power supply voltage.
6. Does not require a Bucherot cell - that's the series R-C across the
output which attempts to compensate for the R-L typical of a speaker. I
never had much luck with those. The ubiquitous LM386 for example could
easily be set into oscillation if that compensation network did not
correctly compensate for the resistance-inductance of the speaker you were
using.
7. Does not require a series output electrolytic capacitor.
6. Short circuit proof, thermally protected. No clicks or thumps on
power-on.
I have used this to make a nice loud keyer tone amplifier in a homebrew
keyer.
_____
From: minima-bounce@xxxxxxxxxxxxx [mailto:minima-bounce@xxxxxxxxxxxxx] On
Behalf Of Joe Rocci
Sent: Thursday, May 21, 2015 7:30 AM
To: minima@xxxxxxxxxxxxx
Subject: [minima] Re: something fresh - a long long post
Farhan
Just to make sure we're on the same page...
When I suggest using a PWM output to generate a sidetone, I'm not talking
about generating a square wave and filtering the daylights out of it. I'm
talking about dynamically adjusting the duty cycle of the PWM output at a
rate that's much higher than the desired sidetone frequency, then filtering
with a very simple RC filter. It can be shown that the only significant
spurious will be at the sampling frequency and above. For instance, assume
we want to generate a 1 Khz sine. We could set up a timer interrupt inside
the Nano to generate interrupts at a 16 khz rate. On each interrupt, the
processor would pull the next phase increment value (where phase values are
stored as values scaled to 100% of the PWM's Vcc voltage) and output this
value to the PWM. The PWM output would then be a 0-Vcc staircase
approximation of a 1 khz sine with the steps occuring at a 16 khz rate. Once
filtered in an RC filter, the major spurious outputs will be at 16 khz and
above. These very high frequencies will be removed by the bandwidth of the
audio amp locally and by the sideband filter for the RF output. The size of
the lookup can be reduced if desired by only storing values for 1/4 of a
cycle. This is essentially a fixed-frequency DDS where the D-A converter is
a PWM and the output filter is an RC. And it wouldn't slow down or otherwise
complicate the main loop code
As to the feed-forward AGC, we once again employ an interrupt so we don't
complicate the main-loop timing trying to do time slicing; maybe the same 16
khz interrupt timer that's used for the sidetone generator. We feed the
detected pre-volume audio signal to an A-D input on the Nano and develop a
software peak detector (PD) function with controllable rise and fall
parameters. We feed that forward, either as a PWM-derived DC voltage
proportional to the PD value or as a pulse train to the controllable gain
audio amplifier. I don't have part numbers in front of me, but in the TDA
family there's a voltage controlled amp and also a digitally controlled amp.
In the latter, there's a pulse-up pin and a pulse down pin (maybe they're
the same pin; I forget) that produce small changes in gain upon each pulse.
In either case, the advantages of a feed-forward system is that it avoids
need for a variable gain IF stage, and eliminates the inevitable servo-loop
tradeoffs of underdamping, overdamping, ringing, oscillating, etc. Also if
the sampled audio inputs are then reproduced by a D-A function using another
PWM, then there's some capability to do real-time gain control, noise
limiting, etc in software right inside the Nano before the audio even gets
to the amplifier stages.
I did the tone synthesis a long ago using a PIC and it works very well.
Unfortunately I don't have code at my fingertips. The AGC is something I've
been dreaming up for a long time and I plan to implement it on my new
Chinese X1M SSB/CWQRP transceiver that I just got. I'll do that by embedding
a Nano inside. More on this will be on my web site (www.w3jdr.com).
Joe
W3JDR
From: Ashhar Farhan <mailto:farhanbox@xxxxxxxxx>
Sent: Wednesday, May 20, 2015 9:19 PM
To: minima@xxxxxxxxxxxxx
Subject: [minima] Re: something fresh - a long long post
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 <mailto: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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