[minima] Re: A New Approach
- From: allison <ajp166@xxxxxxxxxxx>
- To: minima@xxxxxxxxxxxxx
- Date: Fri, 12 Dec 2014 19:14:40 -0500
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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