Minima - Smaller Bandpass Filter

OK, ok, . . . I have NOT finished my Minima Audio Modules yet,  . . . What am I doing designing another RF circuit?

Well, I like RF circuits, and not so much for Audio Circuits. As I have stated before, the audio Capacitors are too big for my liking and building interests.

This new RF circuit is a Trial Bandpass Filter using very-VERY small crystals (2x2.5mm). I know, small SMT crystals may not perform well as a Bandpass Filter, but I could not pass up the chance to experiment with them.

My first Minima Bandpass Filter is 1x2 inches (2 sq inches).

My Original Minima Bandpass Filter Module

The new Filter will only be 0.8x1 inches (0.8 sq inch).

New Smaller Bandpass Filter
About 2.5 times smaller than the Original

The boards are on order from OshPark.com, three boards will only be $4.00.

I am not sure this will work well for the Minima Bandpass Filter, but it will be a fun experiment and, I really enjoy small projects.


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Si5351A Circuit Build - Success

After the Failure as described in previous post, I modifying my Si5351A VFO PCB layout for proper Crystal Pad Pattern, and finished the build.

This circuit will be used as a LAB Test Oscillator and for my experimental Farhan Transceiver VFO and BFO.

Si5351A Module Ready For Test

The 3.3Volt Regulator is the black chip next to the top pin of the power header, the black chip near the bottom header pin is the Si5351A, and below that is the very small 27MHz Reference Crystal. The three BN-47-2402 Binocular Cores provide ground loop isolation for each of the SMA RF Connectors.

Si5351A and Si570 Modules
Ready for Use

One unfortunate aspect of both of these modules is; the Si570 and Si5351A chips do not have options for changing their I2C access address. And therefore, only one of each can be connected to single I2C control circuit without a special I2C switch.

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Si5351A Circuit Build - Failure

As I have mentioned in previous posts, I want to build an Si5351A RF VFO, which is similar to the Minimal Si570 VFO.

The Si5351A has three RF Outputs that I would like to have available on my LAB Test Bench, and I will also try it as replacement for both the VFO and BFO for my experimental Farhan Minima Transceiver. The Si5351A is programmed via I2C, which is similar to the Si570.

Si5351A Three Port RF VFO

Today, I created the Si5351A PCB via the Toner Transfer Method, and I installed all of the necessary "vias", and 95% of the SMD parts. All that was left to install were the SMA RF Connectors, Power Header, and the Crystal.

I planned to install the Crystal next.

But then, . . . . Failure !

Small Crystal - Big PCB Pattern
(As Viewed Through the Microscope)

Dang, . . . I used the wrong Pad Pattern for the Crystal on the PCB, it is much too big for the Crystal that was purchased for this purpose. The Crystal is shown up-side-down in the photo (pads up) for pad comparison.

According to the Spec Sheet, the size of the CX3225SB Crystal only is 3.2 x 2.5 mm (0.125 x 0.098 inch).  The Crystal is only about as big as one of the Pads of the four Pad Pattern that I used.

I will correct the Crystal Pad Pattern in the layout, and try AGAIN!

This is one of the down-sides of not using "Ugly Style" construction, many time there is NOT an acceptable recovery.

I am getting better-and-faster at creating Double-Sided Homebrew Toner Transfer PCBs, and better-and-faster at connecting the two sides with very small vias. So, . . . I guess when necessary, I can now, Fail Faster :-)

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UPDATE: Apr 7, 2014 10:45
For future reference, the Crystal Pad Pattern that I fond that appears to fit is labelled "CFPX-5". It is interesting to note that "Pin 1" is not marked, "Pin 2" is the indexed pin (see the inside filleted corner).

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Minima - BFO Module

I have finished constructing the Farhan Minima Transceiver BFO Oscillator Module for my Transceiver Experiments.

I used the same Toner Transfer Method that I have used for all of my other modules. The PCB is double sided, with oscillator power connection and ground plane on the backside. The size of this module is 1.0x1.5 inches.

Mimima - 20MHz BFO

This module supplies 20MHz to the Transceiver's BFO Mixer (via SMA connector on right) with frequency off sets for Upper Sideband (USB) and Lower Sideband (LSB). The Relay selects one of the two Sidebands via one of the two adjustment Trimmer Caps used with the oscillator.

On the initial power-up, the measured output was about 800mV PP (will this be enough?).

My circuit is slightly modified version of Farhan's published circuit, I wanted to use the colpitts oscillator and adjustment strategy that I have used in many of my other projects. I have kept the circuit's interface the same to maintain compatibility with Farhan's Minima Transceiver circuit.

The upper left pin header is for power (12v). The lower left header selects the Sideband, the extra (upper) pin in this header provides a quick way to select the Sideband with just a simple jumper, which will be useful while testing this module as a stand alone circuit. The white block in the center left is the Relay with its control circuit far center left. The BFO colpitts oscillator and output transistor are center right. The crystal (of course) is lower right. A red power-applied LED is upper left.

Note: I use EB2-3NU 3Volt Relays in most of my projects, and therefore when they are used in 12Volt circuits a limiting resistor is needed.

So now, after doing a little head scratching, my questions are:

• What should the actual BFO Frequencies be for generating USB and LSB with a 20MHz IF?
• Or at least, What is the expected difference between the two Frequencies?
• Does this all depend on the width of the Crystal Filter?
• In normal Transceiver operations, does the VFO Frequency also have to change (a little) when selecting the opposite Sideband?

More research about Superheterodyne Receivers on my part is necessary (and I thought I knew this stuff :-).

Any feed back via comments would be appreciated.

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Minima Transceiver Module Testing

Jack - WA7KMR and I spent some time in his Lab, using his Test Equipment to characterize the first few modules of my homebrew implementation of Farhan's Minima Transceiver. We tested my; RF Mixer, Crystal Filter and IF Amplifier (see previous post).

Jack at the Controls

We first connected each module as a standalone device to ensure each worked as expected.  We were both most interested in the Crystal Filter performance measurements. My previous measurement attempts with the AIM-4170B were inconclusive, other than the shape of the bandpass.

Crystal Filter

With Jack's Lab Equipment we measured the bandpass and insertion loss of the Crystal Filter. The bandpass is 6KHz, and insertion loss is about 3db, with about 2db of ripple. With some tweaking, I think the ripple could be reduced.

We later checked the operation of the standalone RF Mixer, and then the IF Amplifier. The RF Mixer worked similar to one of Jack's known mixer. The amp provided about 30db of gain at 20MHz.

We connected all three modules together and I very pleased with the results.

RF Mixer, Crystal Filter, and IF Amplifier

Output of the IF Amp
Horizontal = 20MHz Center,  5KHz / Div
Vertical  = -30dbm at top line, -110db at bottom, 10db / Div
LO was 200mV RMS
The RF Signal was Detectable down to -90dbm

The Results: I am a very Happy-Camper.

More of my "Minima" module implementations will follow.

Thanks Jack, for the measurement help.

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A Homebrew Crystal Filter

I ordered and recently received a batch of 20MHz Crystals to build an experimental Crystal Filter similar to as shown in Farhan's Minima Receiver and my previous post.

Somewhere (link found) I read that for Homebrew Crystal Filters the HC-49US (short) crystals do not have as high of "Q" as the standard HC-49U (standard) crystals, but as with all of my projects "small is better". This experiment is being done to see what I can do with the short crystals.

To try to find matching crystals, I used an AIM-4170B Analyzer to sort the crystals into six,  5-digit groups (19.991 - 19.996MHz). The crystals sorted into a typical standard bell distribution. I think the measured frequency, which is lower than the expected 20MHz, is due to calibration, and/or because the crystals are not actually operating in an oscillator circuit.

For this build, I decided to use the 19.993MHz group, as it provided more crystals to further select from.

Initial Sorting of 20MHz Crystals

The Analyzer display provided the Resonate Frequency and other information.

AIM-4170B  20MHz Crystal Plot

I re-tested each crystal of the 19.993 group, sorting and recording the frequency to 7-digits. A green tape label was attached to each crystal to make sorting easier.

Sorted Crystals

I selected a set of eight crystals for my filter, several of which had the same values.

Selected Crystals

The selected crystals have the following frequency values:

• 2 - 19.99265
• 1 - 19.99266
• 2 - 19.99267
• 1 - 19.99269
• 2 - 19.99271

A double sided 1 x 2 inch PCB was created to mount the crystals and capacitors, the Toner Transfer Method was used to make the PCB.

20MHz Crystal Filter
Toner Transfer PCB

This is the results after Cutting, Drilling and Loading the board.

Completed 20MHz Crystal Filter

I think the AIM-4170B is NOT the best instrument to measure overall performance of a Crystal Filter, But, initial testing indicated there were some major "spikes" in the frequency response.

I initially loaded the PCB with two parallel 50pF capacitors at each of the five locations where Farhan's circuit called for a 100pF. The goal was to parallel two capacitors to lower series inductance and/or provide the option of replacing one-of-each with a variable capacitor (if needed).

Due to the observed "spikes", I replaced one-of-each set of two capacitors with a 4-47pF NP0 variable capacitors.

Tuning Caps Installed

With a little tweaking, the "spikes" were removed, and this is resulting SWR plot with a 50 ohm Load on the output. But still, the resulting curve is less flat than I expected,  . . .

SWR with 50 Ohm Load

The real test and performance evaluation will need a Spectrum Analyzer and/or a measurement done within a fully constructed receiver.

More Crystal Filter fun to follow :-)

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UPDATE: Feb 23, 2014 14:04
I just received an e-mail from Jack, we should be able to evaluate the Filter soon.

Hi Eldon,

Nice looking XTAL filter.

If your design is around 50 ohms in and out then my equipment should do the job.

I expect to have the interconnect cable for my Spectrum Analyzer to Tracking Generator by next Tuesday.

Assuming both pieces of gear work as planned I will be testing about 4 RF filters I have on hand and your XTAL filter would be a lot of fun to test.

I can also provide a print out of the filter response curve. A camera shot of the CRT also works.

Jack

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Part Quest

Crystal Pin Sockets

Several days ago, I received my order for Crystal Pin Sockets (see previous post) and have not had time to use them yet. I have been busy working on some projects, and trying to get ready for Salmoncon.


The smaller Crystal Pin Sockets will be used in Propeller Projects, Homebrew PCB Projects, and the larger (longer) ones will be used in Manhattan and Ugly Style Projects. The 2-Pin Header and the single Machined Pin/Post are shown for scale. The crystal leads will be cut to length as necessary for the project.

The Stash

These are by far the smallest mechanical devices in my bag of project parts. And, they are probably the most expensive per weight, at $0.20 each they are several times the price of GOLD. But, . . I only need a few :-)


The first photos shows several styles and sizes that I have obtained. The smaller ones are the style that I had in mind for my initial Part Quest, they fit into 1mm plated holes, and do not protrude much beyond on the backside of the PCB. Others were some initial trial parts.

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My Small Trace 5mils 30m QRSS Transmitter

My small trace (5mils) 30m QRSS Transmitter board has been loaded with most of the parts (see previous post). Still needs a Crystal, Keyer, Battery, and Antenna connections.

So far, no screw-ups, and no shorts!

Now, awaiting the KID2 to arrive in the mail.

QRSS Tx Almost Finished

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Industry Standard Small Traces

Today I read that "Industry Standard Small PCB Traces" are normally as small as 6mils.

Because I like small things, I wanted to try to build a circuit with even smaller traces.  The circuit I choose is a new iteration on my Micro Power QRSS Transmitter - the same circuit that was used on the QRSS test that I ran last week.

I have re-designed the board for ALL 1206 SMD parts, using 5mil traces and 8mil clearance. I would have liked to use smaller parts, but 1206 size parts are the smallest that I have available. The traces do not need to be this small, I am just doing it to see if I can? - It is kind of a QRP thing for the Home Brew PCB Fabrication process!

The Toner Transfer and Etch process went well - The board DC Resistance checks OK, without loss of connections or shorts.

The size of the board is 1000mil x 850mil,
shown here at about 4X actual size.
The signal traces between pads are 5mils wide, 8mil clearance.
The GND Grid is 5mil Traces on 20mil Centres.
The Font is 3pt.

The major components of the circuit are; a 5 Volt Regulator, Crystal, 2N3904 Oscillator, and a KID2 QRSS ID'er.

Note: SMD 1206 parts are: 12mils long and 6mils wide.

The next step is to mount the parts - which could be a real challenge, . . . . for tomorrow.

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The Marker Family Photo

Dave Axe - W7AGJ came by my Shop today while I was working on board design for the replacement QRSS Receiver, previously I was working on the last Band Edge Marker Oscillator and had just gather them into a group on the bench.  Something was said about they looked like they were there for a Family Photo - So,  why not - here it is!

The pre First - Not shown in photo, was an "Ugly Style Leaded" oscillator soldered to the top of a 9volt battery clip, photo is available in another Album.

Here is the Family, from left to right:

• The First, was created to try to use SMD parts in Ugly Style.
• The Second was the previous circuit in SMD Flat Layout, not all parts are SMD, I had just started using PCB Layout Tools and found PCB-FAB-In-A-Box.
  
• The Third - using all SMD parts (except Crystal, Switch, and Antenna).
  
• The Forth - New Layout and added a previous forgotten part - a decoupling cap, which made for low output.
  
• The Fifth - New Layout, while trying to make it as small as possible.

Do I need five Markers? . . . . NO, they were just fun projects and I learned a Lot!

I do not currently have plans to expand the Family - Or, at least not until I can mount all of the components on the side of the Crystal, my skill level will need some improvement first.

This was a fun project - Now onto, Bigger Smaller and Better Things

Don't you just love this Radio/Electronic Stuff!

If NOT, you should not be reading this Blog

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Marker Project Resumes

The Micro-Tips for my soldering iron were received today. The unfinished Band Edge Marker Projects resumes, - and now is finished.

Marker Finished, and On Frequency

I know, they all look alike?  But, they are getting smaller, and they are still just fun to build. I have about $18.00 invested in this Marker (that is in small-lot-quantity part prices).

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30m Crystals Arrived

As promised, my previously ordered 30m Crystals arrived today - now I am ready for more 30m projects.

Planning New QRSS and WSPR Projects

I am planning several new 30m QRSS and WSPR projects, availability of the right Crystal is always a major  factor - so, I placed an order with "Expanded Spectrum Systems". They have a good price and ship the same day. For my projects, I need a few 10.140mHz crystals.