[minima] Re: A New Approach
- From: "Joe Rocci" <joe@xxxxxxxxxx>
- To: <minima@xxxxxxxxxxxxx>
- Date: Wed, 10 Dec 2014 12:44:09 -0500
Allison
To answer your question about the op-amp:
It's a TL974, $82 ea at qty 10, which is the minimm I buy of anything I think I
might reuse in another project.
Noise: 4nv/sqr(hz); plenty good for this application.
----- Original Message -----
From: Joe Rocci
To: minima@xxxxxxxxxxxxx
Sent: Wednesday, December 10, 2014 11:57 AM
Subject: [minima] Re: A New Approach
Allison
Doubly balanced first mixer, with the caveat that the term is somewhat
meaningless when using bus switches. There is no 'balance' to speak of in with
respect to the LO signal. It's just 'isolated'. The mixer is a quad switch
configuration, similar to the FET mixer Ed Oxner developed years ago. Balanced
transformer coupling on input and output.
The PD is also a switching architecture with bus switches. It's transformer
coupled on the IF side and direct, single-ended coupled on the audio side.
The higher TX drive level is made possible by the high amplifier output
capability; eases post-amplifier design with about 10 db less gain requirement
in the cascade.
Op-amps are some cheap, rail-to-rail, low current, low noise quad I found
searching Digikey. I'll get that info for you.
In my version, I plan to use a single Si5351 for LO and BFO, but dual Si570's
could also be made to work for slightly better noise & spur performance (also a
whole lot more current).
My vision is actually a small, full-featured rig that can be battery powered
for portable outings.
I started to put together the block diagram and schematic for the whole gizmo
and will post it to my site when available. Meanwhile, I can post schematic
fragments here. The IF amp is really one of the keys, as are the mixer/product
detector, which have already been presented here. I can post them as
attachments on request, one-by-one if you'd like. What I don't want to do is
create a major schism on the present Minima effort. I think it's really
important that it gets 'finalized' and a reproducible PCB designed.
Joe
----- Original Message -----
From: allison
To: 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
Other related posts:
