Radio Bebek 2nd If Amp and 2nd Mixer

Radio Bebek Receiver Almost Complete

So today I populated the 2nd IF mixer, the 2nd mixer and some of the switching circuitry and gave all those elements a little bit of a test.

To follow on from the test of the first IF amp, I put a 10mv signal into the first IF amp and tested the whole IF strip through the Xtal Filter and out the 2nd IF amp buffer and it all seems to be working quite well.

But there is one issue and I am not sure what the cause is. The 470R resistor from voltage divider on Q6 does not go straight to ground and trying to find its ultimate path to ground has been elusive.

To make matters worse there are q Q10 in the schematic and Q10 and 21 on the board, there are 2 diode on the board not on the schematic and I cannot for the life of me work out what the actual cause is here. Either one of the Q10’s is not being biased on or there is some part missing or either VR7 or VR2 are not providing the emitter ground path on the emitters of those Q10 transistors.

Anyway, I think what i will do is pull both the Q10’s out and related circuitry, part of which is for a signal meter and just hard wire the 470R voltage divider to ground. That will at least leave me to be able to complete the receiver and actually test it.

At one point today I had fed a 9mhz signal into the IF strip and was getting a 600Hz tone out the audio amp, so that in itself is promising as I was worried about the dodgy design 2nd mixer. Which looks like a ring mixer but does not have trifilar windings on the inputs and outputs, but some crazy bifilar winding on one side and a variable resistor and cap divider on the other. I am not sure about this mixer design and I have not done any google foo to see if i can find it, but it does work

Oh and for the diodes in the mixer i did not use the typical 1n4148 diodes that everyone uses, but rather I used a Schottky Diode 1N5711, it probably means i have way to much drive from the BFO as the switch on of these are much lower, but its working and for now, I call that a win, particularly so when i really do not understand the mixer topology being used.

Oh and as far as audio goes, of the 2 AF stages, I am only using the LM386 first stage because the TDR2003 is motor boating or oscillating or just acting like a tard and I am not sure why here either

I do hope the next post will be a video of it receiving signals. Failure is always an option here, but so far this build has been very challenging, but its been rewarding and even if it ends up being a pile of junk, i will have learned a lot.

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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.

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Radio Bebek Xtal Filter

Radio Bebek Xtal Filter

I thought before i added in the IF Amps i would build the xtal filter first and see how it looked. The values of 33pf seemed very small and would end up with quite a wide filter. Typical values for a 2K wide filter are usually around the 100pf mark, so i started there.

Obviously i was wrong, but i am thinking 50pf might be closer to the mark after looking at the bode plot. Its not going to break any performance records and the skirts are kind of garbage for a 5 xtal filter. But, I will see how it goes on receive once i have the rest of the radio built.

Radio Bebek Xtal Filter Bode Plot

As far as xtal filters go, this is one of the better looking ones I have built HEHE, so I guess the others have been really crap. It is kind of symmetrical looking and only 3db-ish of ripple. Might be ok. We will see.

2200hz IF Filter

Ok, so after playing with cap values i got the filter about the right width, but there is a lot of variance in the passband. 8db to be exact. Way to much. But as there is no way to impedance match this, there is not a lot i can do. It is what it is as they say and as I am only building this as a receiver, I can live with it.

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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.

4. WSPR

QRP Labs WSPR Kit

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.

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The Weather Station Returns

The Weather Station

Yes its true, the weather station returns for 2019. I finally pulled my finger out and pulled down the old one, removed its electronics, put it in a new box and moved from the western facing side of my house to the eastern side where it will not face direct sun that obviously skews the temperature readings
All that failed on the old setup, which would have been obvious if i went and took a look at it, was the USB connectors from the solar panels to the battery charger rusted and lost a stable ground connection. Obviously a bad thing when using lithium batteries that do not like to be drained and kept drained. And so, it killed the cells
So at the moment the weather station is running off a wall wart, but I am going to add battery backup for the odd time that the power goes down. Not that losing a little data here or there would really be a big issue anyway. But it gives me something to tinker with.

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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.

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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.

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