[minima] Minima Roadmap
- From: "Joe Rocci" <joe@xxxxxxxxxx>
- To: <minima@xxxxxxxxxxxxx>
- Date: Wed, 27 Aug 2014 07:47:34 -0400
In an earlier posting I suggested that this new mixer might be a drop-in
replacement for the much-discussed J-KISS. In the interest of moving the
project along, I'd like to suggest that we now have two alternatives, one that
functions (albeit difficult to reproduce and of unproven performance) and one
that tests out very well on the bench. From a functional standpoint, they're
pin-compatible at the block diagram level. I want to suggest once again that
the board developers move forward as though the mixer issue is resolved, but
put provisions on the board so that a 'daughter board' replacement mixer can be
put in if/when it proves to be better. Actually, it wouldn't be bad to make
provisions for doing this at every stage. The actual circuit implementations
are likely to change regularly, but the block diagram is (I think) sound. If
this is an acceptable bridge between present roadblocks and the long-term
roadmap, it will allow us to get a set of boards out that more folks can build
and improve on without fear of obsolescence. If we're going to do this, we
should agree on the number/function of the pins required for each
daughter-board.
Joe
----- Original Message -----
From: Joe Rocci
To: minima@xxxxxxxxxxxxx
Sent: Wednesday, August 27, 2014 7:28 AM
Subject: [minima] Re: New Mixer Design
Farhan
I believe there's no 1st order difference in the placement of the switches,
primarily because it's the same circuit either way, and either way the FETs
have to handle all the 'signal' currents (except the LO drive). It's all a
matter of where you place the AC ground point in the schematic. In the KISS,
the AC ground is placed at the switch common point and the output is taken from
the transformer CT. You could just as well have AC grounded the CT and take the
output from the switches' common point. In this case, it sure looks a lot like
a diode single balanced mixer, doesn't it? This equivalency holds for other
common circuits too. For instance, if you take a bipolar colpitts oscillator
and move the common AC ground point from the collector to the emitter, then
stare at it for a few seconds, you'll realize that it now looks like a Pierce
oscillator. AC ground points are often moved around in the interest of easier
ground-referenced drive or load coupling. In any case, the loop currents in the
circuit remain essentially the same. In the KISS mixer, the output current loop
is formed by whichever switch is on at the instant, the half of the secondary
winding it connects to, and the load. This is the same current whether the
switch is ground referenced or floating....just a difference in the common
point reference. However, in the quad arrangement, you have true 'push-pull'
operation which halves the current that each switch has to handle and improves
balance and symmetry. There might be higher order effects from changing the
reference point, such as IP3 (which is finicky about just about anything), but
from a 1st order point of view, I think they're equivalent.
Joe
----- Original Message -----
From: Ashhar Farhan
To: minima@xxxxxxxxxxxxx
Sent: Tuesday, August 26, 2014 9:49 PM
Subject: [minima] Re: New Mixer Design
joe,
the earlier IMD setup couldn't be used beyond 20dbm. i am attaching a
simpler and better design that i am currently using.
this is excellent work, it points to something that we secretly suspected
all along : the placement of the switches, either in the signal's path or on
the grounding leg of the transformer, isn't going to make much of a difference.
- f
On Wed, Aug 27, 2014 at 2:44 AM, Joe Rocci <joe@xxxxxxxxxx> wrote:
For reference on my previous, here's a scan of my return loss bridge
terminated in a precision 50 ohm load. It's about 50 dB over most of the HF
range, so no questions about the accuracy of my previous RL scans.
Joe
-------- Original Message --------
Subject: RE: [minima] Re: New Mixer Design
From: "Joe Rocci" <joe@xxxxxxxxxx>
Date: Tue, August 26, 2014 5:08 pm
To: minima@xxxxxxxxxxxxx
I quickly hooked the new mixer board up to the test set for some more
scans. The conversion loss looks identical, maybe even a tenth or so better.
All the mixing products up to 100 Mhz look identical at a cursory glance,
except the LO leakage which looks about 10 dB worse at -39 dBm. Still very
respectable but I'll need to find out what changed. I used the transformers
from the breadboard, so it's not them. Possibly something to do with using a
different pair of switches in the IC package.
I attached scans of return loss at both ports. They look almost
identical, with the LO popping up at 34 MHZ....to be expected. This is a good
sign of performance symmetry and also a good sign of very little energy wasted
due to mismatch.
More to come...
Joe
-------- Original Message --------
Subject: [minima] Re: New Mixer Design
From: "Joe Rocci" <joe@xxxxxxxxxx>
Date: Tue, August 26, 2014 1:16 pm
To: <minima@xxxxxxxxxxxxx>
I got a chance today to design and etch the attached PCB, so
hopefully I can get some more data tonight. Note that I changed the usage of
the available switches in the package to make a single-layer PCB easier. Now
we'll find out if the performance is reproducable.
Farhan
I saw on your HFSignals site that you were developing an IMD test
set. Is that working?
Joe
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