Characterizing a Bencher YA-1 low-pass (TVI) filter

Some time ago I bought a couple of low-pass / TVI filters from a ham-fest. It is mandatory to ensure that you are not producing out-of-band or otherwise harmful interference and so having such a filter assists in ensuring that you are in compliance with the regulations. I picked up a Bencher YA-1 52Ω 1.8-30MHz 1500W low-pass filter for about $20 and a B&W FL-10/1500 filter for around $10.

Low-pass / TVI filters

Bencher and B&W low-pass filters

DSA815-TG being prepared to test a Bencher YA-1 filter

DSA815-TG being prepared to test a Bencher YA-1 filter

Having somewhat recently purchased a spectrum analyzer with tracking generator, I was naturally curious to see how the two filters performed. I started by “normalizing” the test system to account for any impedance mismatch and transmission line losses inherent in the system prior to the addition of the DUT (device under test).

Normalizing the DSA815-TG for filter measurements

Normalizing the DSA815-TG for filter measurements


To normalize the test system, you must make a connection from the tracking generator output to the spectrum analyzer input using the cables which will be part of your test-bed. I used a BNC-BNC barrel connector in the photo but changed to a SO-259barrel for the actual measurements.

The advantages of the BNC connectors are as follows: they are rated for higher frequency than SO-259/PL-259 connectors and they are also “quick connect.” My rationale for using a SO-259 barrel was to allow the inclusion of the BNC-to-PL-259 adapters which are necessary for the test (since the filters are equipped with SO-259 connectors.)

Normalized trace for filter measurement

Normalized trace for filter measurement

The normalization procedure was done from 9kHz to 500MHz at a 1MHz RBW (receiver bandwidth) with no input attenuation and the tracking generator set to output at 0dB. The spectrograph shows a nice flat curve which is what you should expect as the spectrum analyzer is mathematically accounting for any impedance mismatches, etc.


Bencher YA-1 filter being "swept" by the DSA815-TG

Bencher YA-1 filter being “swept” by the DSA815-TG

Upon inserting the DUT into the system, it is immediately apparent as to what is going on.

The tracking generator’s signal sweeps from 9kHz to 500MHz and shows a very characteristic low-pass response curve. The device under test shows very little attenuation (approx. -0.6dB) and then a nice steep roll-off starting around 40MHz.

Bencher YA-1 9kHz-500MHz 1MHz RBW

Bencher YA-1 9kHz-500MHz 1MHz RBW

If we examine the spectrograph, we see that there are a few points of interest:

  • insertion loss
  • -3dB point (the bandwidth point)
  • -23dB point
  • any “spurs” or peaks

It should be noted that this spectrograph was taken with an RBW of 1Mhz which is not very precise.

Bencher YA-1 9kHz-500MHz 10kHz RBW

Bencher YA-1 9kHz-500MHz 10kHz RBW showing increased detail

As we see with an RBW of 10kHZ we can see some more detail, as well as lowering the noise floor of the test-system.

The effect of lowering the noise floor as well as increasing our relative sensitivity shows us some minor oscillations in the filter where there is a 10dB difference in filtering ability in the 100-500MHz range. This is completely normal.

By further decreasing the RBW to 1kHz, we can see even more detail as seen in the accompanying spectrograph below.

Bencher YA-1 9kHz-500MHz 1kHz RBW

Bencher YA-1 9kHz-500MHz 1kHz RBW

The system floor now exists around the -85dB point where we can clearly see the periodic increase and decrease in signal passing ability. We are still talking about attenuation of 75dB at worst. Recall from decibel math that 10dB means 10x and 20dB means 100x .. so -70dB means that the signal is 10000000x less powerful coming out of the filter in this range.

Notice also the peak at 425MHz where the attenuation is -53dB – this is still very good but it is interesting to note all the same. This indicates that the filter is less able to attenuate signals around 425MHz, however, it is still an effective filter all the same.

Bencher YA-1 9kHz-100MHz 1kHz RBW

Bencher YA-1 9kHz-100MHz 1kHz RBW

If we examine the filter a little more closely, limiting the frequency range to between 9kHz and 100MHz, we can see that the filter’s “roll-off” is nice and flat and that the insertion loss is approximately 0.6dB and that the -3dB point is a little more accurately shown as ~40MHz.


Bencher YA-1 35-45MHz 300Hz RBW

Bencher YA-1 35-45MHz 300Hz RBW

By further decreasing the window in which we are looking (ie. 35MHz to 45MHz at RBW 300Hz) we can quite accurately determine the parameters of interest.

We can see now that the insertion loss through the filter is 0.65dB and that -3dB point is at 39.8MHz and the -23dB point is 43.5MHz.

From the investigation of the Bencher YA-1 filter, it is clear that the unit performs well under the test conditions and should be an effective filter to prevent TVI / harmonic emissions.

Bencher YA-1 9kHz-1500MHz 10kHz RBW

Bencher YA-1 9kHz-1500MHz 10kHz RBW

For the sake of completeness, I’ve attached a spectrograph of the filter’s response curve over the range of 9kHz to 1.5GHz. Notice the filter is effectively unable to attenuate any signals around the 1.05GHz range.

The purpose of the filter is to knock out harmonics from the HF amateur band and so for example, at 20m (14MHz) we might expect to see harmonics being generated in the following series: 42, 70, 98, 126, 154, 182, 210, 238MHz as 3rd, 5th, 7th, 9th, 11th, 13th, 15th and 17th harmonics. In order for a transmission on 20m (14Mhz) to create interference on the 1.05Ghz band, a transmitting system would have to generate 75th harmonics! (rest assured the power content of 75th harmonics would be .. well .. purely academic in importance.

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5 Responses to Characterizing a Bencher YA-1 low-pass (TVI) filter

  1. Zaba OH1ZAA says:

    After having problems with the YA-1, today I drilled out the pop-rivets and opened it up. There is
    no need to tune a LPF for minor excursions of SWR when terminated in a 50 ohms load, but my
    curve had gone “wild”. Actually one cap had blown at one of the inner feed-through walls. All caps are made out of brass plates and (likely) teflon sheets. The screws at the top are not for tuning, but for tightening the brass plate tight against the teflon and the filter top wall. If you open them too much, you lose the nut and you have to remove the pop-rivets to get it fixed.
    For now I have to find out what the value of the feed-through caps is, and either replace the sheet or put in a high voltage capacitor, e.g. the button model, about 2 – 3 kV…. Cheers/73, Zaba OH1ZAA

    • James says:

      What about sourcing a new PTFE insulating sheet if that is what is damaged? Measuring the thickness of the sheet and the diameter of the brass plate, you could determine the (rough) capacitance of the brass plate / aluminium shell capacitor.

      C = ε(A/d) where:
      C = capacitance (pF)
      ε = permittivity (F/m) – dependent on dielectric; PTFE ~2.1 so ε = (8.85e-12 x 2.1) = 1.86e-11 F/m
      A = cross-sectional area (mm^2) of plates – surface area of brass plate
      d = separation of plates (mm) – thickness of PTFE sheet

      • Zaba says:

        Thanks James!

        It turned out, it was a combination of mica and PFTE sheets. Both were burned through in an ugly hidden manner. I guess I could do with a single PFTE sheet (and I have them). But I went the easy way in order to get an indication of cap value and the dependence, so I tried 3.5 kV 56 pF and 27 pF buttons. However, there seems to be more hidden damage, as I do not get change in the SWR curve when I change . I will keep you posted… Cheers/73, Zaba OH1ZAA

        • Zaba OH1ZAA says:

          After playing a few more hours, bending coils, changing caps I have the complete picture. I was interpreting the caps in the separation walls mainly as feed-throughs, but those are defining a major part of the low-pass characteristic. Ultimately I figured out that those caps are 220 pF, and now I have a 2 kV tubular version installed, grounded at an extra screw at the nearest SO-239 connector (hole drilled). With a 50 ohms termination the SWR stays below 1.3 : 1 up to 29 MHz, and the pass-band is symmetric. One day I may change it back to PFTE if I find the sheets, but I guess that is only after a new failure. The 5 kW peak specification may be a bit optimistic, but faulty lines and SWR can of course cause high voltages at lower power levels… Cheers/73, Zaba OH1ZAA OH2MZA OH3BCX OH4BCS OH5ZA OG6B NN0Y

  2. Andrew says:

    Hello there OM;

    Wonder, if you have any info related to TUNING of YA-1 filter.. This is probably easy done, but it would be handy to know procedure.
    On the top of a case we have 3 screw heads with slots for tuning. I got this filter from somebody, who decided to “tune” it…and I have to make this filter working agn..
    I got here CRO and little dymmy load.. so it probably will be enough to get it going.
    I can inject little power from my radio and observe how filter is doing within its working range..
    So.. any suggestions or help much appreciated..

    73, andrew

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