My Low Pass Filters

With repairs of my boat almost complete, I can return to some Ham Radio projects.

The following are the files and artwork that I have used to create the Low Pass Filter, see my previous blog posts. As previously posted, the value of components were selected from junk box parts which are a close match as computed. The artwork is provide here as an ".svg" file suitable for use with Inkscape or other vector graphics programs. The intent is to allow space for as many identical parallel capacitors as necessary to provide the correct values. The coils are wound and measured as necessary. PCB SMA edge connectors are soldered on both sides of the PCB and are used to make connections easy.

30m Low Pass Filters

Inkscape SVG Low Pass Filter Artwork

My public URL of the SVG filer is: https://dl.dropboxusercontent.com/u/46313528/Published/OutputFilter_03.svg

Let me know if you build a Filter.

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PA47 Now Ready for On The Air Tests

Thus far I have NOT connected my PA47 Amplifier to an antenna, because I was not sure of it output harmonic content (see previous posts).

Last night at our monthly P&C group meeting, Wayne - NB6M presented a demonstration of his new DSA815 Rigol Spectrum Analyzer with one of his oscillators and the effects of a output filter devices. His filter completely eliminated (down into the noise floor) any harmonic content.

As a second example and demonstration, I provided the PA47 Amplifier and a LowPass filter. A 50 Ohm load and 40db tap was used to reduce the input to the Analyzer. Actually, both the 1 Watt and the 15 Watt versions of the PA47 were checked, the results were about the same.

The 1 Watt and 15 Watt Version of PA47

The output filter that I used, is a simple Chebyshev 5 Pole Filter (as previously posted).

30m LowPass Filter as Designed with LtSpice

The filter was originally designed via an Online Filter Design program, and then available multiple 330pF 1206 SMD caps were used to obtain the nearest values, 3 caps for each end, and 4 for the center. The cores were wound as necessary by measuring the results on an AADE LC meter.

30m LowPass Filter as Implemented

The Results

Wayne declared the PA47 fit for Amateur Radio Service. The second harmonic was down 51db and the third was down 67db from the fundamental.

PA47 Connected to
DSA815 Rigol Spectrum Analyzer

I will soon connect the 1 Watt version of the PA47 to an Antenna, and use if for QRSS Beacon Service on 10.140050MHz (+-50).

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30m Low Pass Filter Results

Here are the measured results of the 30m Low Pass Filter (see previous post).

Here is the Propeller Output without a Low Pass Filter. The tall center signal is the Spectrum Analyser's LO, the Propeller 10.140MHz signal is to the right of the LO, and the negative images are seen on the left side. The third and fifth harmonics can be easily seen, odd harmonics are typical of a square wave output.

Without LP Filter

And, here is the Output with a Low Pass Filter

With LP Filter

Here is the Propeller and Low Pass Filter test configuration.

Prop and Low Pass Filter

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More 30m Low Pass Filters

Today was again, Low Pass Filter Day.

I built three 30m Low Pass Filters to use with the Propeller as the Keyer and RF exciter. Previous filter builds included a DC blocking Cap to avoid DC loading the Prop, but a Low Pass Filter should not include elements that prevent Low Frequency signals to pass even if they are not wanted. DC blocking should come before the LP Filter on the Prop.

My previous built filter were a little flimsy, they were built on .031mil PCB material, components in line with the axes tended to crack and stop working. For these board, I used .064mil PCB material.

The circuit I am using was calculated via the CalculatorEdge online service (there may be other online calculators that would work as well, but this is the one that came up first in Google search).

The Calculator suggested the following, which became (=>) the available parts used.

• Two Inductors 594.4nH => 680nH as hand wound (I could have been more precise here, but, . . .  :-)
• Two End Caps  1086.5pF => 990pF (3 - 330pF)
• Middle Cap 1416.1pF => 1320pF (4 - 330pF)

30m Low Pass Filter

I updated the circuit on my CircuitLabs Workspace, and ran some plots with the slightly different component values that I have available. The second harmonic rejection is reported as down -45db.

Down -45db at 20MHz

The results:

30m Low Pass Filters

I am going to use SMA connectors for all of my projects now, thanks to Jeff - KO7M I had enough connectors to complete two filers. More SMA Connectors and Jumpers Cables are on order.

One of the above Filters will be sent to a friend.

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CircuitLab Software

I have been playing with the "CircuitLab" on-line circuit simulation software, which is very easy and fun to use.

Here is the circuit for the 30m LowPass filter that I am using with the Propeller Beacon, as documented in previous posts.

Click here to play with the Filter on-line. Note: Because it is on-line, that version my not appear exactly the same as shown here, I am editing it while still playing.

The Circuit

The Simulation Results

I like the software, but it indicates that I may need to work on the circuit. a little :-(

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