[minima] Re: Linear Output Filter Switching

  • From: Mark <mark.g0mgx@xxxxxxxxx>
  • To: "minima@xxxxxxxxxxxxx" <minima@xxxxxxxxxxxxx>
  • Date: Sat, 1 Feb 2014 06:48:25 +0000

Thanks to everyone for the suggestions and ideas.

If we go with 4 filters have we scrapped top band?

I'll try and run some designs for the four suggested filters and build some 
prototypes and stick the results on the wiki.

We can then decide and discuss what we think we should do.


-----Original Message-----
From: "Ashhar Farhan" <farhanbox@xxxxxxxxx>
Sent: ‎01/‎02/‎2014 05:00
To: "minima@xxxxxxxxxxxxx" <minima@xxxxxxxxxxxxx>
Subject: [minima] Re: Linear Output Filter Switching


I have been mulling over this ... and I am trying to balance quite a few things 

1. Can we make do with just four filters? I have attached a screenshot of a 
simulation I ran (both these programs are available on the EMRFD CD). As you  
can see, a filter can be cut for 7 and 10 mhz. The 7 mhz's second harmonic at 
14 is down by 25db. Which means, if we don't distort the linear too much, we 
can get more 43 db suppression of the harmonics. If  you guys agree to this , 
then, I propose as follows :
 1st filter for 3.5 MHz
 2nd filter for 7-10 mhz
 3rd filter for 14-18 mhz
 4th for 21-28 mhz
These will have to be three section filters, which is alright. A 1-of-4 decoder 
like 74LS139 could do the switching with just 2 lines.

2. The switching of the main filters is at variance with this scheme. so, we 
will have to use a dedicated line (as we are doing now) to switch these filters.

3. There is a third line A0 (pin 23) that is unused. this can be used to switch 
in a higher bandwidth filter (more than 10 Khz). with this filter, the 
broadband audio can be fed to an 'SDR' back-end like linrad. The serial port 
can provide the tuning control.

- f

On Sat, Feb 1, 2014 at 7:01 AM, Dan Reynolds <on30ng@xxxxxxxxx> wrote:

I'm not any kind of an expert but could you use a Microchip MCP23017 I/O 
expander on the 5 V side of the I2C bus. I was thinking something like I saw on 
the Tronixstuff page here: 
Instead of a LED an I/O line could drive a transistor to drive a relay(?).
Then the code could "watch" the frequency and as it changes the output line for 
that particular LPF could be switched.

Or could you use a 74HC595 to accomplish the same thing.

I was going to approximate the Minima front panel with a bare bones Arduino and 
some parts and see what I could dream up. 
I don't really know what I'm doing. I'm just muddling through trying to patch 
bits and pieces together...

Dan -- KB9JLO

On Fri, Jan 31, 2014 at 6:10 PM, Wayne McFee <nb6m@xxxxxxx> wrote:

Hi Mark,

If the three outputs were set up as you suggest, a Picaxe chip could be 
programmed to control either individual relay driver transistors or a relay 
driver chip like the 2903(?).

I've been using that arrangement for filter selection  in a homebrew rig.

Or, if one of those output lines could be programmed to output a stepped 
voltage such as that provided by the FT817 for bandswitching info, that makes 
the programming even easier.

Wayne NB6M

> On Jan 31, 2014, at 1:24 PM, Mark G0MGX <mark.g0mgx@xxxxxxxxx> wrote:
> Farhan & Team,
> I can see that we have provision in the schematic for 3 digital outputs for 
> band selection, but can only see one of these (BAND_HI - pin 11 - Arduino D5) 
> being used within the function "setBandswitch". It is this function that 
> currently selects the LPF used based on frequency.
> We could set the state of the three outputs based on frequency something like 
> this:

> <Filter Ideas.png>

> But someone far cleverer than I would need to explain how to create relay 
> on/off switching based on the logic states of  the three lines - I have no 
> idea how to do that!
> If we were to implement something like this was the intention to use these 
> additionally switched filters in RX as well as TX?
> I would be interested in your thoughts and ideas on how you saw this working 
> before I head down the wrong road with my construction!
> Many thanks,
> Mark

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