Multiple CW filters in one radio

[tkamiya]

Looking over new-to-me TS940 documentations, I realized I can have more than one CW filters.  One in different I/F stages.  What I don't understand is, why is this beneficial?  Each filter seems to have very steep skirt.  Having same width filters shouldn't make any difference....

Anyone have any first hand knowledge?  Is it worth it?

[David Hess]

Filtering on earlier IF stages helps improve overload performance but requires higher Q for the same selectivity.  Filtering on later IF stages provides better selectivity for the same Q but has little effect on overload performance.

These high Q IF filters are expensive so they are made optional and not every user will require every IF filter.  For instance CW operators will be interested in 250/500 Hz filters while phone operators will want 2.7kHz and wider IF filters.

[Lord of nothing]

are expensive

€? 

[David Hess]

are expensive

€? 

These were not your typical 25kHz wide 10.7MHz FM and 12kHz wide 455kHz AM filters which were produced in mass quantities for commercial broadcast band receivers so prices were $100 and higher per filter.  Usually the radio comes with an inexpensive standard set of lower performance filters for basic operation.

The optional filters in this specific case were:

YG-455C1 CW Filter 500 Hz [455 kHz IF]
YG-455CN1 CW Filter 250 Hz [455 kHz IF]
YK-88A1 AM Filter 6000 Hz [8.83 MHz IF]
YK-88C1 CW Filter 500 Hz [8.83 MHz IF]

And while these are all discontinued, you can get some idea of the prices here:

http://www.inrad.net/home.php?cat=110

At $200 per filter, some users just did without.

[tkamiya]

With antennas I have, I don't have overload problems.  Selectivity and close-in rejection is what I want.  Then I'd use one in later stages then?

[David Hess]

With antennas I have, I don't have overload problems.  Selectivity and close-in rejection is what I want.  Then I'd use one in later stages then?

Or rely on the variable passband or DSP filtering.

[German_EE]

If possible try before you buy. I used a transceiver loaded with filters once and on CW 500 Hz was nice but 300 Hz sounded like I had a bucket on my head.

[iMo]

I put the #109 inrad into my ic718. I did it because nobody had managed my expectation..

[Ghislain]

The question is also why spend money on analog solutions when the filtering can be done in software (SDR)?

[David Hess]

The question is also why spend money on analog solutions when the filtering can be done in software (SDR)?

If filtering is done late in the mixer chain, then the lack of early selectivity results in more intermodulation products (or overload) and once they are produced, they cannot be filtered out.  SDRs which use wide filters early in the signal chain so they can capture a large part of the band at once pay for this with poor overload performance.

[Ghislain]

The question is also why spend money on analog solutions when the filtering can be done in software (SDR)?

If filtering is done late in the mixer chain, then the lack of early selectivity results in more intermodulation products (or overload) and once they are produced, they cannot be filtered out.  SDRs which use wide filters early in the signal chain so they can capture a large part of the band at once pay for this with poor overload performance.

That is true David although it is my understanding that the 4th generation SDR's all convert the analog signal at the antenna input to digital (Direct Digital Sampling) using DDC (Digital Down Conversion) for the receiver, all that is left are some front end filters before the ADC.
Looking at the test figures published by Sherwood Engineering http://www.sherweng.com/table.html you'll see that the 3rd order dynamic range or RMDR is among the best for the latest generation of SDR radios (compared to analog ones) so they seem to be able to be able to cope with overloading more than adequately.
To be honest I have not looked into how this is accomplished in detail 
 

[borjam]

The question is also why spend money on analog solutions when the filtering can be done in software (SDR)?

The problem is, even most of the radios for sale now are analog designs. Yaesu just released the new FT-818 (a tweaked version of the venerable FT-817) and its optional filter for CW or SSB is still a Collins mechanical one.

My transceiver, for instance, is an IC-7200, one of the first SDRs with the look of a conventional radio. Yep, it's an SDR but the SDR part (so-called IF-DSP) is at the last IF conversion at around 15 KHz. It has a 6 KHz roofing filter in the first conversion.

[tkamiya]

I bought and installed a 500Hz filter on my TS940.  It's the 8830Khz kind.  It is working well.

Why not DSP?  I will probably go to that route if the radio has it.  TS940 does not, as it is at least 20 years old design.  Doing it on AF stages won't do as a strong signal can overwhelm a weak signal way before that.  This radio has a neat feature where I can shift 2 filters relative to each other and cut off high, low, or both.  I tried it and it works, but there are some leakage and slope is shallow.  I have a certain hearing issue, and it just didn't work for me.  CW filter works.  Wonderfully.  Decision on which stage basically came down to price and availability.

By the way, I have been a ham for nearly 40 years and I started my hobby with TS520 series. I just can't get used to looking under menus under menus, just to change one parameter.  I owned and disliked IC703Plus.  Why menu, to change a speed of CW keyer?  Plus, new radios are all surface mount.  I can't fix that.  So I'm staying with older rigs with conventional designs. 

Anyway, thanks for everyone for your input!

[David Hess]

The problem is, even most of the radios for sale now are analog designs. Yaesu just released the new FT-818 (a tweaked version of the venerable FT-817) and its optional filter for CW or SSB is still a Collins mechanical one.

I thought Rockwell shut down the Collins mechanical filter division a couple years ago.  Why would someone produce a new radio which uses them?

http://www.arrl.org/news/rockwell-collins-to-end-mechanical-filter-production

[iMo]

I can still remember the aversion of the broad ham community when the first dsp appear in the IF 15y back or so (ic-756 pro, etc).
It is still a topic, sure..