Switched CW-SSB Filter for QCX


Cory KG7BBV
 

Hi all

I have a QCX 40 and like to have it on my desk to listen to radio chatter while I'm working. I modded it recently to bypass the 700 Hz CW filter with a switch so I could receive SSB. It works well and I'm able to copy lots of stations. However, I've decided recently it would be nice to have an SSB audio filter to get rid of some of the high frequency audio noise and let me pick out weaker signals. Since my mod just bypasses the CW filter entirely, the audio is wide open and I can hear strong SSB stations often from more than 5 kHz away!

So here's my plan: the QCX CW filter uses three low pass filters and then a high pass filter. I'm thinking I could modify the first two filters to be an SSB band pass pair (I'm thinking 250-2400ish Hz). I could then switch the last two filters in and out of circuit. Since the CW filter is within the passband of the SSB filter, I'd still be able to use the CW filter after the SSB filter. The only potential issue I'm seeing with this is I will have reduced the rolloff of the 700 Hz CW filter, making it a little less selective. I think I can deal with that though.

Does this seem feasible? Does anyone see any potential hangups I may have missed? Thoughts are appreciated.
--
Cory, KG7BBV


Dave VE3GSO
 

Sounds like an excellent plan.  There are three low pass filters to provide a very steep skirt above the CW 700 Hz preferred frequency, and only one high pass to get rid of the low frequencies.

If you were to move one of the low pass filters up to 2450Hz or thereabouts, and move the high pass filter down to 250~300 Hz you might just be able to bypass the two remaining low pass filters for SSB and flip them back into circuit for CW.  Your low pass skirt would not be quite so steep, but I’m sure it would be entirely useable.

Dave


On Jun 12, 2020, at 11:58, Cory KG7BBV <cory.goates@...> wrote:

Hi all

I have a QCX 40 and like to have it on my desk to listen to radio chatter while I'm working. I modded it recently to bypass the 700 Hz CW filter with a switch so I could receive SSB. It works well and I'm able to copy lots of stations. However, I've decided recently it would be nice to have an SSB audio filter to get rid of some of the high frequency audio noise and let me pick out weaker signals. Since my mod just bypasses the CW filter entirely, the audio is wide open and I can hear strong SSB stations often from more than 5 kHz away!

So here's my plan: the QCX CW filter uses three low pass filters and then a high pass filter. I'm thinking I could modify the first two filters to be an SSB band pass pair (I'm thinking 250-2400ish Hz). I could then switch the last two filters in and out of circuit. Since the CW filter is within the passband of the SSB filter, I'd still be able to use the CW filter after the SSB filter. The only potential issue I'm seeing with this is I will have reduced the rolloff of the 700 Hz CW filter, making it a little less selective. I think I can deal with that though.

Does this seem feasible? Does anyone see any potential hangups I may have missed? Thoughts are appreciated.
--
Cory, KG7BBV


Cory KG7BBV
 

So I have the switching figured out, but I'm having trouble figuring out how to change the component values to get the bandwidth I want. Unfortunately, I'm not a EE (in another life...), and I can't figure out from the circuit diagram exactly how the filters are set up. How do I calculate the cutoff frequency of each? I tried using the spreadsheet that other use for shifting the center frequency, but that can't tell me how to change the second LPF into a HPF...
--
Cory, KG7BBV


John Kirby
 

Corey,

Take  lQQk here
 http://www.bowdenshobbycircuits.info/opamp.htm
Drop down to  "2nd Order Opamp Filters" where the example is 200 Hz
...then... plug in your choice of freq for example 2 kHz (WSPR and FT8).

I also have first hand experience here 
>>>>http://www.circuitstoday.com/band-pass-filters<<<<
I plug these  math equations into EXCEL
then built several BPF ... to include a >VARIABLE< CW band pass filter
THAT WORKED FIRST TIME :>))

72, 73
John
N3AAZ