[PCB] Improve VHF harmonic suppression

[VanceVagell]

Improve the VHF filtering, I'd like the second harmonic to be even more supressed (currently it's just over -40dB, which is the minimum for part 97).

[VanceVagell]

I already have a v1.6 design that is meant to fix this, should have test boards soon to confirm.

[nhendin]

I've just ordered a v 1.6 PCB, and have access to decent spectrum analyzers at work. (Keysight portable as well as an Rohde & Schwarz FSV3 series).

What's your test setup? I assume it's just transmit into an appropriate attenuator (or coupler to a dummy load) and measure 2nd harmonic suppression?

I can attempt to duplicate the test that when I get my boards and parts set up (I just ordered a set of stuff from Amazon / Aliexpress / Signal Stuff) yesterday so it will be a little while.

[VanceVagell]

Thanks for the offer, more testing is always welcome. I test with a TinySA spectrum analyzer, transmitting through a 30dB attenuator (the TinySA usually adds a few more dB of attenuation as it figures out the graph, something like 37dB total usually). I'm expecting the v1.6 PCBs to be delivered today and will update here after my own testing.

If you have any PCB design experience, I'd really love any help improving it overall. I suspect there are many RF-related improvements that could be made (e.g. better via stitching, impedence matched traces, etc.). I'm fairly new to PCB design.

[nhendin]

What's the spec you are trying to hit for harmonic suppression? If it's relative (like -40dBc) then the total attenuation doesn't matter as long as the level is low enough to not cause non-linearities (e.g. more harmonics) in the spectrum analyzer itself. If it's an absolute spec, (e.g. less than -45 dBm) then calibration of the attenuation might matter.

--Neil.

On Wed, Oct 16, 2024 at 10:39 AM VanceVagell @.***> wrote:

Thanks for the offer, more testing is always welcome. I test with a TinySA spectrum analyzer, transmitting through a 30dB attenuator (the TinySA usually adds a few more dB of attenuation as it figures out the graph, something like 37dB total usually). I'm expecting the v1.6 PCBs to be delivered today and will update here after my own testing.

If you have any PCB design experience, I'd really love any help improving it overall. I suspect there are many RF-related improvements that could be made (e.g. better via stitching, impedence matched traces, etc.). I'm fairly new to PCB design.

— Reply to this email directly, view it on GitHub https://github.com/VanceVagell/kv4p-ht/issues/24#issuecomment-2417490824, or unsubscribe https://github.com/notifications/unsubscribe-auth/AAX7Q27ZDGQX2Y72I5OP6RLZ32QG3AVCNFSM6AAAAABQBPTMGKVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDIMJXGQ4TAOBSGQ . You are receiving this because you commented.Message ID: @.***>

[nhendin]

Also happy to help with PCB layout reviews. I'm a RF Engineer by profession and lately doing more engineering management.

On Wed, Oct 16, 2024 at 10:55 AM Neil Hendin @.***> wrote:

What's the spec you are trying to hit for harmonic suppression? If it's relative (like -40dBc) then the total attenuation doesn't matter as long as the level is low enough to not cause non-linearities (e.g. more harmonics) in the spectrum analyzer itself. If it's an absolute spec, (e.g. less than -45 dBm) then calibration of the attenuation might matter.

--Neil.

On Wed, Oct 16, 2024 at 10:39 AM VanceVagell @.***> wrote:

Thanks for the offer, more testing is always welcome. I test with a TinySA spectrum analyzer, transmitting through a 30dB attenuator (the TinySA usually adds a few more dB of attenuation as it figures out the graph, something like 37dB total usually). I'm expecting the v1.6 PCBs to be delivered today and will update here after my own testing.

If you have any PCB design experience, I'd really love any help improving it overall. I suspect there are many RF-related improvements that could be made (e.g. better via stitching, impedence matched traces, etc.). I'm fairly new to PCB design.

— Reply to this email directly, view it on GitHub https://github.com/VanceVagell/kv4p-ht/issues/24#issuecomment-2417490824, or unsubscribe https://github.com/notifications/unsubscribe-auth/AAX7Q27ZDGQX2Y72I5OP6RLZ32QG3AVCNFSM6AAAAABQBPTMGKVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDIMJXGQ4TAOBSGQ . You are receiving this because you commented.Message ID: @.***>

[nhendin]

AFAIK, it looks like 47 CFR, Chapter I, subchapter D, part 97, section 307(e) is the relevant document:

The mean power of any spurious emission from a station transmitter or external RF power amplifier transmitting on a frequency between 30-225 MHz must be at least 60 dB below the mean power of the fundamental. For a transmitter having a mean power of 25 W or less, the mean power of any spurious emission supplied to the antenna transmission line must not exceed 25 µW and must be at least 40 dB below the mean power of the fundamental emission, but need not be reduced below the power of 10 µW. A transmitter built before April 15, 1977, or first marketed before January 1, 1978, is exempt from this requirement.

Link

[VanceVagell]

@nhendin I just tested the v1.6 PCB design and unfortunately the 7th-order Chebyshev lowpass filter I designed is barely working at all, it's FAR worse than the v1.5 PCB design. The new filter is only around -11dB from the first harmonic which is atrocious.

Could you please take a look and see if you can help figure out what's wrong, and how to properly improve filtering?

I used this modeling tool: https://markimicrowave.com/technical-resources/tools/lc-filter-design-tool/

Attached is a screenshot of the values for the filter I designed, and I confirmed the BOM matches these.

So I'm not sure why there's such a major discrepency from the model and what I'm measuring. Could it be something about the actual component layout, e.g. the inductors too close together or something? Seems unlikely...

[nhendin]

I'd have to take a look at the layout. Do you have access to a network analyzer? We could measure the filter frequency response by itself -- I don't have my boards yet.

[nhendin]

Not having the board, the things I could think are possible, are coupling across the PCB that is bypassing the filter, parasitcs from the PCB changing the values, though the caps are large enough that I would not think that was a factor.

A VNA measurement of just the filter response would be really interesting.

[kiavash-at-home]

The frequency of the 7th order harmonic is high enough that the parasitic capacitance of the inductors may start to impact the response. It is possible to simulate the design including these parasitic values and compare with the measurements.