Idea Box: Passive CW Filter

I have been playing around with some older simple direct conversion transceivers and receivers for CW.

Some examples are the Pixie kits and the Rockmite kits, and some Chinese kits, like the "Frog  Sound".

All these have one drawback, the receiver audio is rather wideband and has a good deal of high frequency hiss.

There are a ton of excellent CW filters both active (more complex) and passive. I wanted something really simple, and took a look at this simple CW filter from YO6DXE: https://dxexplorer.com/passive-cw-audio-filter/

Take a look at his article, and his demonstration of the filter on Youtube (linked in the article). With a total of 6 passive components a simple, yet pretty good CW filter can be made. Two different resonalt frequencies, and variable bandwidth. 

It is a simple series resonant circuit with two switched capacitors for the resonances, and a potentiometer partly bypassing the off resonance frequencies. 

I love the simplicity, and in the video the filter demonstrated a good sounding filter effect. It may take a while, as I am busy with improving my QO-100 setup, but I will get to it.

For the inductor in the filter I have some old multi tap inductors that were used in tone signalling equipment in the 60s, 70s and 80s. sound perfect for my purpose.

Now I need time to build this, and I intend to add a simple (PCB module) audio amplifier, LM386, or the like, so I have speaker sound out of it.

CW Filter for FT817.

 Going through some of my stuff I found something very interesting in a box with crystal filters and ceramic filters: Two 455kHz Collins mechanical filters, bandwidth 500Hz for CW, for the FT817.

One is now mounted in one of my 817s, and is working fine. That one is likely to be used as back-end for the QO-100 receiver system. I think I will use another one for my uplink system. As that is TX only I will run that one without the filter.

I do have a third one with a defective PA. I will likely use that one as a base for transverters, for both home and portable. so I can run 4m and microwaves on that one. I will need to bypass the PA module and make some control circuitry for that one. The second CW filter will likely go into that one.

Mounting the filters is easy, it's a simple plug and play. I like to get those small transceivers running, as they draw much less power than the "big radios", like the IC7600 and others.

Filters for AR8600 and AR7030 Arrived.

Two new Collins Mechanical filters have arrived.

A 2.5kHz SSB filter for the AR8600. This will be mounted in the Mk 2 I have, and is intended for use as back-end for some microwave converters/transverters. Initially the main use will be for the QO100 reception system on 10.489GHz, down converted to 739MHz, and probably switchable to the 10.368GHz down converted to 618MHz.

The original SSB filter in the AR8600 is not very good, the main weakness is a poor selectivity, resulting on poor rejection of the unwanted sideband. The SSB filter should solve that problem.

A quick alignment of the AR8600, especially of the BFOs for SSB, and the D/A converter for the fine frequency steps.

Now I need to make space on the lab desk for doing this, so a bit of tidying is in order. 

The CW 500Hz filter will go into the AR7030 receiver for HF ... well, 15kHz - 32MHz. This is a very nice receiver, albeit with a relatively high current consumption.

The AR7030 also needs a replacement of the backup battery. Yes, with time the batteries go empty. I have a few other radios and other stuff that needs new backup batteries, but this one comes first.

This year, I suspect, will be a year for getting much more of the equipment I have up and running for my propagation monitoring system and some of the microwave activity. So maintenance, repair, and renewal is on my program, as well as some building projects.

Last year's challenge was operation, so I did not write so much about "working a QSO a day" or so. This year I will write more about my progress or completion of many projects, large and small.

AR8600 Filters and Add-on.

As mentioned earlier, the AR8600 receiver originally has a poor SSB filter. 

I do have an old Mk1 and a newer Mk2, and the filters are different. Also, the Mk1 stops at 2040MHz, and the Mk2 continues up to 3GHz, so it is useful for reception on 2400MHz, too. 

I suspect that a preamplifier will be needed for those high frequencies. I can find one, or use a module I have for that.

I made a search on Ebay after looking at a video on updating the receiver.

I did locate one SSB filter suitable filter, and ordered it from Japan. This is an 8-pole Collins SSB filter that should be much better that the original Murata filter in the receiver. The post mentions the AR8600, so I hope that the filter fits in the PCB. I am hoping to receive the filter some time in February.

The filter I ordered looks like a fit for the Mk2, and that suits me fine, as the Mk2 has the wider frequency range.

I suspect that I will have to find an alignment procedure for the AR8600s, as there seems to be some misalignment, at least in the Mk1 I have used until now.

I do have an AR7030, too, and I also ordered a 500Hz CW filter suitable for that one.

When I look into my crystal ball I see some upgrade activity in my future ;)

An added Idea Box entry:

The other part is the audio output of the AR8600. The built-in speaker is very small and not very loud. So I am thinking (ouch! ;) ) of building myself a new audio device with an external audio amplifier, possibly with an equalizer circuit (I have PCB a module for that), and an audio CW filter.

I do have a stereo amplifier suitable for use with at least two radios. With a little mixing trickery that could easily become 3 radios simultaneously, one in the left channel one in the right, and the last one in the middle. With an external switch box (that I already have) more radios could be added to the external audio system, albeit not all radios simultaneously.

Mini #5: Test of low pass filters.

When I picked up the PM2B mentioned in #3, there were two 30MHz low pass filters included in the price.

Further, I had some older low pass filters, a 30MHz  and a 50MHz one, so I got them all tested today:

Out came the NanoVNA F (4" screen). The tests were made with a wideband calibration setting, so the absolute values are not perfect. 

All filters are in good working order.

This means that I now have three 30MHz low pass filters and a 50MHz filter, all fully functional:

The old one I have is a Yaesu filter with a 100W rating. I have had that for decades.

The second one is a Trio/Kenwood filter, 100W rating.

The third one was a bit of a surprise. An old Drake filter rated 1000W.

The fourth one is a BNOS low pass filter for 50MHz. No rating on the filter, but I suspect it is 10W or so. It is a much smaller filter than all the others.

I reckon that all those ratings will be reduced to about 50% of the stated rating if I want to operate 100% duty cycle modes, like digital modes, e.g. FT8.

I opened the 1kW filter, and all looks clean, a beautiful mechanical construction with silver plated coils and high power capacitors, likely silver mica types. Somewhere I have a PA for 10-11m that I suspect has poor harmonic suppression so the 1kW rated filter would likely fit nicely in a system with that one - if it has sufficient linearity for SSB. OK, as I mainly want to use it for CW that is not so much of a problem. FSK digital modes will also work nicely.

The other option would be using this filter with an old IC-M710 that can deliver 150W continuously.

I would hesitate to use this filter with my 1kW power amplifier, even if I use it extremely rarely.

Given that I paid <$100 for the two filters, including the 1kW one, as well as the not fully functional PM2B, I consider this trade as excellent.

I picked up more at that rally, more about that later, as it is a different story.