Switchable Crystal Filters

I am going to redesign this board. The layout is sub-optimal. But for now, I am going to assemble it and see if I can make 2 filters worthy of use.

The one thing that has always frustrated me with many SSB/CW homebrew rigs is that you get one filter and one filter only. I have seen some use varactor diodes to solve this problem, by varying a voltage on the diodes you can vary the capacitance and this the width of the filter. I thought about going down that route myself for a while, but in the end, settled on having 2 filters as I have lots of relays already.

The plan is for an SSB filter that is about 2200hz wide and a CW filter about 500hz wide. I am also using the good crystals for these also. 4.91520Mhz xtals sorted to within 10hz of each other.

Each crystal was loaded up into my you beaute xtal tester and the frequency measured to within an inch of its life.

My function gen also has a frequency counter that counts down to 1hz resolution. So I measured my crystals and sorted them into piles within 10hz of each other. If i cannot make acceptable filters with such tightly grouped crystals, then there is no hope in hell i can ever make a good filter ;

Out of this pile of crystals I have 6 groups of 8 crystals all within 10hz of each other and a few more groups all within 20hz of each other. Buying these crystals was certainly well worth it. The pile in the bottom right corner are the outliers. It seems even good quality crystals have some in each batch that should be rejected.

Got the parts all sorted, just gotta build it now. Will update this post once i have built and tested each filter.

Part 2: Well i kind of expected that to not take forever, but anyway here is the update.

The build is pretty straightforward and all went to plan

Other than the width being a little wide in both cases and a bit to much ripple, I will say that I am happy with the shape of both filters. I can narrow both up easy enough, just need to change the cap values and the ripple, well it might be a bit of work, I will need to play with transformer values. I used 4:1 impedance transform, but 3:1 might actually be closer to the mark. I will have to play there and see what i can come up with, but a better match will remove some of that ripple.

For now though, I can keep on building the rest of the project as these will be acceptable to get things working. I can make them perfect later. Next job is IF Amps and mixers.


4 Band, Band Pass Filters

Double Tuned Bandpass filters

Being invasion day here in Australia with not much doing, between watching the cricket, sleeping and doing not a lot more I managed to build and test the bandpass filter board.

Overall, well its MEH. The filters are nothing to rave home about, they will work for now, but I do think i am going to redesign the board using a different topology and get much better attenuation in the stop band.

I ran all the filters though the bode plotter and remembered to screen grab 3 of them which i will now present here for laughter and ridicule.

40m bandpass filter is a 2 humped camel. The width between the peaks is fine, but i will have to play with the coupling cap value to see if i can take some of that hollow out.

20m bandpass filter looks ok, its just to narrow.

The 15m band filter is also to narrow and the high side attenuation is kind of poor at best.

The 80m filter was also MEH but i forgot to screen grab it, It was double humped, but only -3db down in the middle and the high side attenuation was also poor.
Anyway, these will allow me to get building and testing more parts of the radio and think about changing the topology and laying out a new board and having it made. I have 2 other board sitting there waiting to be sent to JLCpcb, the 3rd iteration of the audio board and the first of a final PA board. More on those to come.


Receiver Front End: Pt_2

http://vk4ffab.info/2018/12/22/receiver-rf-front-end/ To see where things have come from, here is a link to the original post:

So between the first iteration and the 2nd, I have changed from using Sprint Layout to using Diptrace. I have tried a few different programs from Kicad to the free version of Altium and I just could never get the hang of how to make components and footprints in either of them

Diptrace on the other hand I find to be much more intuative and straight forward to use and I can make components and footprints in it with easy, as will be seen later when i get the latest batched of board back from china and write a blog post. So many new parts all created by me in the one library so i can find all my parts easily.

Anyway, to the board at hand. Its still got a couple of minor mistakes on it, the cap footprints were in 2.54mm not 5mm which the caps I use mostly are, the 5v regulator and caps are in the way of the right angled SMA connector and I am not sure that the variable RF preamp is actually varying. Circuit wise it should work, but the spectrum analyzer was not showing any gain change.

Anyway, the design will not be changing now, its built, tested and it works. And i can stay like that :

The gerbers can be downloaded from HERE: http://vk4ffab.info/Files/rf-frontend.rar


4 Band Lowpass Filter Board

Got to do some building on this the other day and knock up the LPF board. Ran it though a battery of tests and other than Elsie screwing up the values of the filters, everything worked at designed. I gotta suck the inductors out and add some turns. Not a fun job but its gotta be done. Gerbers for the board can be downloaded, from HERE: http://vk4ffab.info/2018/12/16/4-band-lowpass-filter-board/

This picture will give a bit of an idea on the stackup and how these things well all fit together. Using the 10w ebay PA for now for testing until I find a transmitter design i want to use and lay a board out for it. But as an idea, it will be using a pair of RD16HHF1 conservatively biased for 10w or so.


Radio Bebek 1st IF Amp

Radio Bebek First IF Amp

I did a bit more on this today, went through and added in the first IF amp and tested it to ensure it was working. After sorting out some probe issues with my scope, it was all systems go.

Radio Bebek First IF Amp

After working out where the input to the first IF amp was and also buzzing it out back to the first mixer, I fed a 100mv signal into the amp and scoped the output on the other end of the xtal filter to ensure signal was going in and coming out. As you can see from the oscilloscope output above it works.

The blue trace is the 100mv signal from function generator and the yellow trace is the signal on the other side of the xtal filter. Now to build the 2nd IF amp and test it also.


2019 A Years Projects

Its new years ever and I have been thinking about the things I have achieved in 2018 and its been quite a lot, but at the same time, there has been a lot that I have not completed or even made a dent in and others that I have failed at.

So this list is the list of projects I plan to complete in 2019. Some of these I have had for sometime, some are not even in existence yet but are coming soon and some might be absolute dismal failures. Some of these can be competed in an afternoon and others are going to take some time, but here we go with the list.

1. Bebek Radio

First on the list is to finish the bebek, at least to receiver level. I do not think i am brave enough to tackle the transmit side of it. Its been an interesting and frustrating project so far, the sooner its over the better.

The Radio Bebek

2. Pixie Transceiver

The Pixie Tranceiver

Ok, so i have build one or two of these before, but I have this one in the draw still and I should build it and attempt to make a contact with it, or at least get spotted on the Reverse Beacon Network calling CQ with it.

3. 49’r Transceiver

The 49r Transceiver

There is a similar kit on ebay called the Frog Sounds that I built ages ago. The receiver in it actually worked quite ok, but i never ever transmitted with it, let alone made a single contact with it.

So what I plan to do with this kit is modify it to use a DDS VFO and make it frequency agile. To achieve that i will add a bandpass filter to the front end and use a AD9850 DDS to inject a signal where the Xtal oscillator currently is.



I was given this maybe 2 years ago from a friend, Simon VK6MSC. Its been sitting in my draw waiting and waiting to see the light of day and get built. Now that I can actually use digital modes, I am itching to get this happening and on the air every other moment I am not using my radio and antenna.

5. Super Rockmite

The Super Rockmite Kit

Another ebay kit i have has sitting around for a long time. I am not sure weather to build as standard to modify it to make it frequency agile. At this stage it might just be an afternoon build and see if I can make a contact with it. The built in keyer is a nice touch.

6. QCX

QRP Labs QCX Kit

Now we are getting into serious build territory. The QCX is more than a toy kit, it is a quality transceiver kit that I will take my time building and will enjoy using.

7. VK4FFAB 4 Band Receiver

PCB Board for the VK4FFAB Receiver Project

This has been an eternal project that has no end in site. I would like to have completed the board above built and connected together to see how it actually works as a receiver. Each of the boards has been built and modified in isolation and what is pictured above are 2nd and 3rd iterations of some boards. Along with just stuffing the boards there is a whole lot of software to write to control the whole thing. While its mostly done, there is still a lot of work to go.

8. QRP Labs QSX

QRP Labs 10w PA for QSX

Not a lot i can write about this, other than I will be dropping everything and building one once the kit is fully released. All i have at the moment is the 10w PA. This kit i am excited to get my hands on.

9. ZOI-40 Tranceiver

ZOI-40 Tranceiver

This is one of those projects that I may or may not get to. Its another board laid out by YD1JJJ from Indonesia. I have not taken a close look at things, but i kind of expect this to be a nightmare like the Bebek and something that I will have to buzz out and compare between board and schematic to complete.

10. Bitx 20

Bitx 20 PCB Boards

Where the BITX craze kind of went viral, the Bitx 20m will be something I tackle when i get time.

11. Other Notable Thoughts and Ideas

There are really only 3 other ideas floating around at the moment and that i would like to get to this year and they are all transmitters. A CW transmitter, a RTTY transmitter and a PSK transmitter. 3 individual or potentially combine into a single unit to pair with my receiver project. These will take precedence over some of the projects above and will make nice additions the the lineups.

To be honest, I have certainly listed out more stuff than I will probably get to, but these are the things I have in the pipeline and are my immediate list of goals. The hardest part with all this is not getting sidetracked and going off on a tangent.

Wish me luck.

Happy New Year to you.


Radio Bebek LP and BP Filters

Ok, this has been a fun section to build. I spent the better part of an hour scratching my head over this and wondering why it was not working. Well, this is how I approached this section of the build.

First thing i did was rummage around in my parts bins for canned inductors. I do not have a lot of these things, but i do have a few and those are of a few different types. From what I understood from the few bits in information i have been able to glean from the internet, they used 10.7Mhz IF cans. This then narrows it down to 3 or 4 actual IF Transformers.

Of the ones i have in my parts trays, only one looked like it had suitable values for a 7mhz filter, with windings that measured 120nH and 2.2uH. That was going to get me ballpark and allow me to simulate the required capacitor values.

LTSpice Simulation of Bebek Bandpass Filter

The fun begins when you go to buzz out the actual pcb, because the input series cap is not on the board, but its on the schematic, first issue that did my head in. The output series cap is on the board and on the schematic. Simulating without the input cap seemed to make no difference at all. So i just left it out rather than cutting traces and soldering it to the bottom side of the board. This is not the first time in this build that i have found such discrepancies either. In fact, it would be a miracle indeed if someone could stuff the board with parts and it just worked.

LTSpice Simulation of Bebek Lowpass Filter

Not feeling very confident at this point, I thought it would be best to simulate the lowpass filter just to make sure it was ballpark and surprise it was actually quite ok. Perfect in fact.

Radio Bebek filters built and ready for testing

Ok now this is where the fun began. First thing i do when building something is buzz it out to make sure there are no shorts. Well the low value windings on the canned inductors are only 100nh. They look like a short to ground at DC. So sticking my continuity meter on both the antenna pins caused it to go beep and I spent the next 30mins trying to track down the mystery short to ground, before i worked out what was happening

Testing the Bebek filters

So finally I wired up some power and injected a signal and started to trace it though the board all the way to the input port of the mixer footprint. Initially there was nothing, so i probed around and found RF at a mystery pad showing a 6.8uh inductor, which is in series with the 7pf coupling cap. After some head scratching, I think that this meant to be a capacitor so that you can series 2 caps together to get a lower value. I dropped a wire link in there and things finally worked.

Signal scoped at the RF Post of the First Mixer

0.02V signal injected at the antenna port, through the lowpass filter, through the bandpass filter into the RF Preamp and we have 50mV. I do not think the losses in the filters are all that high.

So that end this nightmare 🙂 Its all worked so far, just some IF amps, IF filter and 2nd mixer to go to have the receiver working. Still plenty that can go wrong from here, but its getting there.


Radio Bebek RF PreAmp

Radio Bebek RF Preamp
Powered Up and Testing
100mV in and 500mV out

So the RF preamp has about 5x gain. On 40m its probably not really needed unless there is a lot of loss in the filters. Which in this case might actually be the case as the RF path from the antenna goes through the LPF, into the Relay and then into the BPF. So a little gain ahead of the mixer might be a good thing.

One thing that I left out for now were the back to back clipping diodes. I am only building this as a receiver pretty much to begin with and I am not sure clipping diodes are really necessary. Next I will be working on the LPF and BPF. Need to do some LTSpice here to make sure my values are correct.