Hallo Joe,
Thanks for your advices. When I return from the holyday, I will build the
filter and I will let you know how it came out. Maybe it must be the low
frequency filter.
73,
Frans pa0fmc
2015-06-04 13:28 GMT+02:00 Joe Rocci <joe@xxxxxxxxxx>:
Frans
I built a 40 mhz xtal filter using inexpensive fundamental mode crystals
from DigiKey. Here are the issues you’ll run into:
1) The higher the xtal frequency, the lower the crystal-Q and the wider
the minimum usable bandwidth of the filter will be. I built nice looking
filter with 5-6 khz bandwidth, which is rather wide. I recall the loss of
this filter was in the 3 dB range.
2) If you try to make a filter with 2 khz bandwidth, the loss will
probably be in the 6-10 dB range. You could overcome this with an amplifier
before the filter, but that amp would need to have a pretty high output IP3
intercept in order to cope with all the wideband signals that come out of
the mixer.
3) With the wider bandwidth and lower Q comes slower rolloff in the filter
skirts. This will make it more difficult to filter out the unwanted
sideband.
4) Obtaining good carrier balance in the balanced modulator will be more
difficult at these high frequencies.
5) All of the bi-di IF amplifiers will probably have lower gain at this
frequency.
I think a filter at this frequency would be good for 1st IF ‘roofing’ in a
dual-conversion design, but not for the primary function of single-signal
selection. When all was said and done, I moved away from this idea in favor
of a low frequency IF (say, 8 mhz) with switched bandpass filters to remove
image responses.
Good luck with your endeavor.
Joe
W3JDR
*From:* frans m <framei333@xxxxxxxxx>
*Sent:* Monday, June 01, 2015 5:09 PM
*To:* minima@xxxxxxxxxxxxx
*Cc:* joe@xxxxxxxxxx
*Subject:* Re: [minima] Re: something fresh - a long long post
Good evening everybody,
I would like to ask why you leftf the idea of an IF of 40 MHz; recently I
received a pack of 100 40 MHz xtals and I sorted them. Very boring job that
I performed during our 2m sunday evening roundup of our VERON chapter
Rotterdam, but it is done now.
I did also all the measurements for the xtal parameters and tomorrow I
start the calculations for the filter design.
Also I would like to know why Farhan didnot go on with a mixer like the
FST3257 or something like that.
I really look forward for an answer.
Thanks in advance for spending time for this.
73,
Frans PA0FMC
ps btw I know nothing about Arduino, but that is a pain for the future
2015-05-22 20:55 GMT+02:00 farhanbox@xxxxxxxxx <farhanbox@xxxxxxxxx>:
joe
i posted only a partial as a snapshot from my notebook. i will post the
complete diagram as soon as i get nacl home : i am attending a fat indian
wedding in bhopal. prolly by monday
- f
------ Original message------
*From: *Joe Rocci
*Date: *Fri, 22 May 2015 11:37 pm
*To: *minima@xxxxxxxxxxxxx;
*Subject:*[minima] Re: something fresh - a long long post
Farhan
Among the missing items on the schematic you posted are the mic amplifier
and how/where you in tend to inject audio into the balmod. Also, for CW,
are you planning to stick with the tone method or are you going to
unbalance the balmod?
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 engine ers, 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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