[minima] Re: A New Approach
- From: allison <ajp166@xxxxxxxxxxx>
- To: minima@xxxxxxxxxxxxx
- Date: Wed, 10 Dec 2014 11:31:12 -0500
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
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