[minima] Re: A New Approach
- From: Joe Rocci <joe@xxxxxxxxxx>
- To: minima@xxxxxxxxxxxxx
- Date: Fri, 12 Dec 2014 21:16:40 -0500
Allison
I can see where the required 27 mhz LO (high side injection) would land right
on the 27 mhz image frequency, but I don't understand your math ( 22mhz _
5mhz=27mhz range). Please explain.
Sent from my tablet
-------- Original message --------
From allison <ajp166@xxxxxxxxxxx>
Date: 12/12/2014 7:14 PM (GMT-05:00)
To minima@xxxxxxxxxxxxx
Subject [minima] Re: A New Approach
There was something I'd forgotten...
For 17M radio an IF of 9MHz is extremely problematic. I know this from adding
17M
to an old Tempo-one hybrid radio. I was able to but the LO had to be above
the output
( 22mhz _ 5mhz=27mhz range). That and the Mixer (single ended pentagrid tube)
was
pure pain. I won but there is now one Tempo One with a balanced mixer in the
TX path
using sheet beam tube (6AR8) to do it. Its not obvious that a 9mhz if would
end up in
the 18mhz output but the mixer was important.
I try to keep the IF non-harmonically related to the output and far enough away
to
avoid filtering issues.
Allison/Kb1GMX
On 12/10/2014 09:53 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.
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.
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.
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.
4) A product detector made with another FST3257. It also accepts Si570/Si5351
drive levels.
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.
6) Detectors for audio-derived Rx AGC and RF-derived Tx ALC. Both of these are
optional.
7) Should be possible to use the Minima control system as a control foundation
with relatively simple changes
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.
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.
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.
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.
Joe
W3JDR
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