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CW/SSB Quarz filter with tunable bandwidth #ubitxcw #cw filter #ssb filter #ubitxcw #ssb #cw

karl-heinz@...
 

Dear uBITX enthusiasts.

recently my uBitx transceiver has been finished. I find this project ufb because it is an excellent learning platform for me.

As with many others, a number of modifications are on the agenda now after I installed the uBitx into a case.

One of these modifications concerns the construction of a CW filter.

Up to now mostly audio filters have been described, whereby active filters are described with operational amplifiers or DSP filters with microcontrollers. I can understand that these two solutions are easy to integrate into the transceiver.

Looking at the circuit diagram of my CW transceiver HB-1B from Youkit I noticed that this transceiver uses a quartz filter whose bandwidth can be continuously changed with capacitance diodes (synonms are varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode). A quick search with Google gave this inofficial link to the circuit diagram of the HB-1B: https://matfyz.estranky.sk/fotoalbum/hb1b.-.html

I didn't find much information about this type of SSB/CW crystal filter. But I am impressed by the acoustic result of the simple circuitry of my HB-1B and wonder why this alternative has not been discussed in the uBitx modifications so far.

TenTec has patented this filter: https://patents.google.com/patent/US5051711A/en, which describes the passband curves of this filter.
On a German website (http://funkamateure-dresden-ov-s06.de/index.php?article_id=174 ) I found very recent measurement results on the bandwidth of these filters, which match the data from the TenTec patent.

Based on this little information the possibility to build a tunable quartz filter seems to be very interesting. Since I only deal with electronics as a hobby, I would like to know if there are any arguments against this filter.

And: does anybody know how it performs when compared to the audio filters?

Many greetings

Karl-Heinz
DF9RU

HB9FIH
 

Hallo Karl-Heinz

I also have studied about this Filter made from Quarz. (I also own a HB-1B) - and soon an assembled uBITX (end Nov back in HB9)

I let them with the Quarz Filters in mind. First I build the uBITX as Original, then add some Features.

Also the CW Audio - Filter from SOTABEAMS - which has a vy sharp Bandwith from 300Hz. When I see it could be more narrow then I think about the Quarz Filter.

If you do the Filter - then post a report, maybe othes are interested.

73 Erich

 

 

Thierry Avelino
 

@ DF9RU

Hello dear OM,

would you please have a look to Tasa's site : http://yu1lm.qrpradio.com/bp%20yu1lm.htm

His tunable quartz filter also seems to meet your requirements. A friend of mine is building such a filter, but I don't know if he succeeded, yet. I think it's worth to have a try...

Best 73,

Thierry / F1HSU





Le ven. 25 oct. 2019 à 20:25, <karl-heinz@...> a écrit :
Dear uBITX enthusiasts.

recently my uBitx transceiver has been finished. I find this project ufb because it is an excellent learning platform for me.

As with many others, a number of modifications are on the agenda now after I installed the uBitx into a case.

One of these modifications concerns the construction of a CW filter.

Up to now mostly audio filters have been described, whereby active filters are described with operational amplifiers or DSP filters with microcontrollers. I can understand that these two solutions are easy to integrate into the transceiver.

Looking at the circuit diagram of my CW transceiver HB-1B from Youkit I noticed that this transceiver uses a quartz filter whose bandwidth can be continuously changed with capacitance diodes (synonms are varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode). A quick search with Google gave this inofficial link to the circuit diagram of the HB-1B: https://matfyz.estranky.sk/fotoalbum/hb1b.-.html

I didn't find much information about this type of SSB/CW crystal filter. But I am impressed by the acoustic result of the simple circuitry of my HB-1B and wonder why this alternative has not been discussed in the uBitx modifications so far.

TenTec has patented this filter: https://patents.google.com/patent/US5051711A/en, which describes the passband curves of this filter.
On a German website (http://funkamateure-dresden-ov-s06.de/index.php?article_id=174 ) I found very recent measurement results on the bandwidth of these filters, which match the data from the TenTec patent.

Based on this little information the possibility to build a tunable quartz filter seems to be very interesting. Since I only deal with electronics as a hobby, I would like to know if there are any arguments against this filter.

And: does anybody know how it performs when compared to the audio filters?

Many greetings

Karl-Heinz
DF9RU

Ashhar Farhan
 

These are a variation of the min-loss cohn filters. As only the coupling capacitance is varied without varying the terminating impedance, we must expect high ripple at all setting except one. What does this mean? It means that tha filter.will exhibit ringing and phase delays.
A better option would a smooth Butterworth response with minimum ringing at a fixed frequency. You can vary the bfo for shifting the audio tone. A 400 hz bw will be narrow enough and yet offer a brightness that we miss in more aggressive designs.
Wes wrote a paper on this. It is on his website www.w7zoi.net.
- f

On Sat 26 Oct, 2019, 12:40 AM Thierry Avelino, <thierryavelino@...> wrote:
@ DF9RU

Hello dear OM,

would you please have a look to Tasa's site : http://yu1lm.qrpradio.com/bp%20yu1lm.htm

His tunable quartz filter also seems to meet your requirements. A friend of mine is building such a filter, but I don't know if he succeeded, yet. I think it's worth to have a try...

Best 73,

Thierry / F1HSU





Le ven. 25 oct. 2019 à 20:25, <karl-heinz@...> a écrit :
Dear uBITX enthusiasts.

recently my uBitx transceiver has been finished. I find this project ufb because it is an excellent learning platform for me.

As with many others, a number of modifications are on the agenda now after I installed the uBitx into a case.

One of these modifications concerns the construction of a CW filter.

Up to now mostly audio filters have been described, whereby active filters are described with operational amplifiers or DSP filters with microcontrollers. I can understand that these two solutions are easy to integrate into the transceiver.

Looking at the circuit diagram of my CW transceiver HB-1B from Youkit I noticed that this transceiver uses a quartz filter whose bandwidth can be continuously changed with capacitance diodes (synonms are varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode). A quick search with Google gave this inofficial link to the circuit diagram of the HB-1B: https://matfyz.estranky.sk/fotoalbum/hb1b.-.html

I didn't find much information about this type of SSB/CW crystal filter. But I am impressed by the acoustic result of the simple circuitry of my HB-1B and wonder why this alternative has not been discussed in the uBitx modifications so far.

TenTec has patented this filter: https://patents.google.com/patent/US5051711A/en, which describes the passband curves of this filter.
On a German website (http://funkamateure-dresden-ov-s06.de/index.php?article_id=174 ) I found very recent measurement results on the bandwidth of these filters, which match the data from the TenTec patent.

Based on this little information the possibility to build a tunable quartz filter seems to be very interesting. Since I only deal with electronics as a hobby, I would like to know if there are any arguments against this filter.

And: does anybody know how it performs when compared to the audio filters?

Many greetings

Karl-Heinz
DF9RU

Jim Tibbits
 

Filter mod on my uBitx (uses existing 12 Mhz crystals)
ubitx filter mod.jpg


On Fri, Oct 25, 2019 at 12:10 PM Thierry Avelino <thierryavelino@...> wrote:
@ DF9RU

Hello dear OM,

would you please have a look to Tasa's site : http://yu1lm.qrpradio.com/bp%20yu1lm.htm

His tunable quartz filter also seems to meet your requirements. A friend of mine is building such a filter, but I don't know if he succeeded, yet. I think it's worth to have a try...

Best 73,

Thierry / F1HSU





Le ven. 25 oct. 2019 à 20:25, <karl-heinz@...> a écrit :
Dear uBITX enthusiasts.

recently my uBitx transceiver has been finished. I find this project ufb because it is an excellent learning platform for me.

As with many others, a number of modifications are on the agenda now after I installed the uBitx into a case.

One of these modifications concerns the construction of a CW filter.

Up to now mostly audio filters have been described, whereby active filters are described with operational amplifiers or DSP filters with microcontrollers. I can understand that these two solutions are easy to integrate into the transceiver.

Looking at the circuit diagram of my CW transceiver HB-1B from Youkit I noticed that this transceiver uses a quartz filter whose bandwidth can be continuously changed with capacitance diodes (synonms are varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode). A quick search with Google gave this inofficial link to the circuit diagram of the HB-1B: https://matfyz.estranky.sk/fotoalbum/hb1b.-.html

I didn't find much information about this type of SSB/CW crystal filter. But I am impressed by the acoustic result of the simple circuitry of my HB-1B and wonder why this alternative has not been discussed in the uBitx modifications so far.

TenTec has patented this filter: https://patents.google.com/patent/US5051711A/en, which describes the passband curves of this filter.
On a German website (http://funkamateure-dresden-ov-s06.de/index.php?article_id=174 ) I found very recent measurement results on the bandwidth of these filters, which match the data from the TenTec patent.

Based on this little information the possibility to build a tunable quartz filter seems to be very interesting. Since I only deal with electronics as a hobby, I would like to know if there are any arguments against this filter.

And: does anybody know how it performs when compared to the audio filters?

Many greetings

Karl-Heinz
DF9RU

Curt
 

Karl

This tunable xtal filter with varactors has a notable use beyond tentec, in the elecraft K2. Its schematic and complete manual are on line. I still enjoy my K2. It allows setting up 4 bandwidths for each mode. I have data showing it isn't as sharp as a dsp filter, nor a fixed bandwidth filter as Ashhar suggests. One complication of the varactor tunable crystal filter is that its center frequency varies with the bandwidth setting. The k2 compensates for this by associating a different bfo setting with each bandwidth. Also look up 2n2 series designs of k8iqy on this type of filter.

Folk often scoff at putting the narrow filter at the back of a receiver, but modern dsp receivers do this very thing. It requires maintaining high enough dynamic range, which the ubitx does well. In my on air experience,  the ubitx is well suited for audio analog or dsp filtering.

To consider a rig having separate wide and narrow filters would require some firmware changes to use the ubitx as its platform. But I think it might need brawnier amplifier updates to get the benefit, see K2 circuitry. For casual operating the ubitx does well. For a busy contest, I can perceive the benefit of the k2. But the ubitx is about 1 tenth the cost, affordable by many more people on the planet.

Try an audio filter with your ubitx to conclude yourself the need for a narrow xtal filter.

Curt wb8yyy

karl-heinz@...
 

I would like to thank everyone for the good contributions to the discussion.

I now have many new ideas, which I will read in a quiet moment. But I am sure that I will build and measure a separate quartz filter with Varicap diodes. But I will also build an audio filter for comparison.

Building and modifying is an important aspect of the uBITX. The nice thing about it is that you can start with a working device. Some time ago I bought all the components for the Minima and even had separate circuit boards routed and professionally manufactured for each function module, but I still didn't have the peace of mind I needed to finish building the Minima. My plans were overhauled by the uBITX :-)

Many greetings to all.

Karl-Heinz
DF9RU

kx4om
 

The SSB6.1 transceiver employs a tuneable SSB filter with tuning diodes in place of the capacitors in a min-loss configuration. The rig is a basic SA612-based design.

https://www.qsl.net/yt2fsg/diykits/ssb61.html

I viewed a video where the operator was in a QSO, and he adjusted the bandwidth several times. My impression was the audio sounded "tinny" when adjusted to one of the extremes.

There is a schematic of the radio online.

Ted, KX4OM

ajparent1/kb1gmx
 

Tentec in their older radios had that filter approach using varicaps.  I was called
a Jones filter {patent holder} and the design is also seen in the Elecraft K2 and
there is on line (the 'net) a design with a constant center frequency.

Look here: http://www.kitsandparts.com/mpp/crystal_filters/CrFilterQEX20044.pdf

Not new and a bit tricky but works best with lower fequency crystals.

Allison

Brent Minchey
 

I'll throw my weapon of choice into the pile: the NEScaf  switched capacitive audio filter

From the description:
It is more flexible than previous filters for QRP rigs. It has two panel-mounted controls — a single center-detent potentiometer that sets the center frequency of the filter’s bandpass (default value settable between ~415Hz and ~1kHz), and a dual-ganged potentiometer that provides a continuously variable bandwidth control (from ~60Hz to ~1500Hz). This ability to smoothly vary the bandwidth is very useful.

QRP Club of New England used to offer this as a kit. I scratched together a Manhattan build and I'm tickled with it. Sound quality of the two cascaded Butterworth filters is fine and doesn't get weird until cranked down to 60Hz.  I find tuning tuning the center frequency over to the answering station a lot easier and more intuitive than messing with a receiver offset. 

Brent WT4U

jimamos@...
 

The kitsandparts.com multiPIG+ receiver used a variable 5 pole Cohn filter.    I built and measured one of these and found that as designed the passband was initially rather poor as you tuned for larger filter BW's. The transmitter portion of the radio itself was CW,  but the radio could be tuned anywhere in the bands if desired, so a SSB filter was also useful.     CW was good, but as you varied the BW, the mismatch caused the passband to go pretty wonky.  

The Cohn design has the problem that the BW of the filter changes, the impedance changes.   I added a resistive termination to the filter by adding a JFET source follower circuit and got a great SSB response.   I tried various termination values and found one that gave a good SSB response and a useful CW response (still a compromise I think though between the various BW Settings).   I posted some of these results on the yahoo groups for the multipigplus in January of 2005.   

Allison has pointed out the Jones filter, which is definitely an improvement in trying to keep the filter matched across all BW's.  

Jim N8CAH

kh6sky
 

Here is the implementation of the TenTec variable filter in my TenTec Rebel.  Looks easy and works great.

Jim KH6SKY