Joe
----- 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
