----- Original Message -----
*From:* allison <mailto:ajp166@xxxxxxxxxxx>
*To:* minima@xxxxxxxxxxxxx <mailto:minima@xxxxxxxxxxxxx>
*Sent:* Wednesday, December 10, 2014 11:31 AM
*Subject:* [minima] Re: A New Approach
On 12/10/14, 9:53 AM, Joe Rocci wrote:
For the last several months, I've been quietly and slowly
developing the components for a different approach to the
Minima. I've written about some of the concepts here, but I
never really laid out the whole thing. At the risk of
creating a major fork in this group, I'm going to describe
where I'm going. If there's serious interest, I'll put all
this up on my own web site, along with a discussion forum
for anyone who wants to follow along.
Joe,
I for one am curious.
Here are the key components:
1) A high-level front-end mixer based on my Quad FST3257 bus
switch design. It accepts Si570 or Si5351 drive levels.
Doubly or singly balanced?
2) An IF at 9 mhz, but changeable according to the builder's
tastes. I'm considering an optional dual xtal filter for
SSB/CW, using bus switches for filter switching.
I use 8 or 9mhz most frequently as crystals are cheap and
9mhz is far enough from
most common bands. Its also high enough that minimal
filtering assures no trouble
with images to at least 6M.
3) A totally different BiDi amp architecture. This one uses
a single Hybrid Cascode amplifier with a complimentary
NPN-PNP output stage. It easily develops 30 dB of gain, up
to +10dbm of uncompressed output power, excellent input and
output match, and about 3.5db noise figure according to
LTSpice. It only draws about 18 ma. It has well over 70 db
of voltage-controlled gain range that can be used for TX ALC
as well as Rx AGC. This single amp is turned around from the
Rx direction to the Tx direction with another FST3257 bus
switch. It works very nicely.
I've done similar( HYCAS+ and 4066 switches) with excellent
results. Id expect the bus switches
(15 years ago not available) would work very well. They
really beat diodes for low power needs.
I found with the 4066 that there was a little cross talk and
I had to keep the input and output
on separate packages and used the excess in parallel for
lower Ron (they are cheap).
4) A product detector made with another FST3257. It also
accepts Si570/Si5351 drive levels.
Switching type?
5) Audio amplifier chain that is made with a single package
of low-noise op-amps. This amplifier chain is used for Tx as
well as Rx and is turned around with some left-over switches
in the product-detector bus switch package.
Sounds like a economical way to go.
6) Detectors for audio-derived Rx AGC and RF-derived Tx ALC.
Both of these are optional.
TX ALC is valuable improvement in TX performance. Works
better than compression.
RX AGC is nice even if it is delayed (hearing protector)
7) Should be possible to use the Minima control system as a
control foundation with relatively simple changes
That alone was what initially attracted me to the design.
I've done some preliminary bench testing of the whole RX
chain (minus input filters) and the 10db sensitivity is in
the sub-1uv range. In Tx mode, it's putting out about 3-5
dbm. There are two easy to make binocular core transformers
in the mixer (possibly one can be eliminated) and another in
the product detector. Other than the FST3257 bus switches,
everything else is done with a few 2N3904's & 2N3906's, a
J310 and a pack of quad op-amps. Overall, I don't think the
parts count is much/any higher than the Minima.
That sensitivity sounds more like what I'd expect.
The output power sounds more like that I'd expect. The
Minima I suspect suffers from
drive to the balanced mod as most of my TX with similar gains
do more than -16 by
about 10dB.
I presume by opamps something like 5532, a low noise part.
This basic Xcvr sub-system can be finished off in several ways:
1) The input filters, which will be a separate board, can be
individual switched bandpass filters (my preference) or a
pair of low-pass filters as in the Minima. Based
measurements I'm seeing, I'm pretty sure the LPF approach is
workable with the 9mhz IF, although the IF can be built for
other frequencies. In either case, I plan to use more
FST3257 switches to select input filters.
Good plan. Too many permutations for arrangements and the
base board can be frozen
more easily.
2) AGC/ALC can be left out at the builder's desire.
I'm calling this design the "BSS-1", standing for "Bus
Switch Special". I started to draw the schematic in Eagle
with the intention of making a PCB. Due to my own design
tastes, I'm doing the first layout with SMD components. This
generally results in smaller size and better performance.
;) I like the name and the SMT approach.
Although I fully intend to take this to fruition on my own,
I sometimes get sidelined for long periods of time, so a
group effort would keep things moving. If there's enough
interest, I'll put up the forum I mentioned.
Where can I see the schematic and notes?
Allison/KB1GMX
Joe
W3JDR
