Tuesday, July 25, 2017

A New Line of Transceivers ~ DifX

Dissing the Dishal Filter

(AKA Ditching the Dishal Filter)

07/27/2017 ~ Updated with some additional data.
Several months ago I started a project that would use a filter designed using the Dishal method. I have finally concluded that it is time to say Basta. For those who claim they have a working filter I suggest you go back a fully evaluate what you really have.
A short review of my reasons for Dissing (Ditching) the Dishal. After resolving the issue that every time I downloaded the software my virus checker put the Dishal.exe file in quarantine (not a good sign) I started off with a 6 crystal filter. That did not end well! Then I shifted to a 4 crystal filter and the curve below is the result of the second effort. This is a real plot of my data and from what I see, it has the classic Dishal shape and far different than my plot of the failed six crystal filter. For reference purpose I have now added what the theoretical 4 Pole. Dishal Plot should result based on my data

Mind you I did all of the steps relative to measurements using the G3UUR oscillator and employed my SDR transceiver to measure the loaded frequency shift to 1 Hz. The crystals that were selected out of the large batch I purchased all were identical as to normal and loaded frequencies. I purchased precision close tolerance MLCC COG (NPO) caps so we had many factors in place when the filter was measured and built. Essentially I used selected 120 PF and 150 PF caps that read higher but where each value was identical --ie two higher reading 120 PF caps to get you close to the nominal 121.6 PF computed value. The same applied with my 150 PF caps.
N6QW's Plot of the 4 Pole Dishal Filter

4 Pole Filter Plot from Dishal

Based on this plot I installed the filter (11.5 MHz Cf [nominal]) in a working 40 Meter transceiver and should mention that I used a second external oscillator to accurately develop the BFO (USB/LSB) frequencies. Once these were determined then that data was used in the sketch. It was important that my four pole filter was installed in a known working transceiver so that the only variable was the Dishal Filter. That installation is shown in the photo below.
The next phase of testing was to tune across the band and just listen to what I heard. The first notable item was that received signals (LSB) seemed to have an unusual fidelity like in what the "Enhanced SSB Crowd" likes. There was no evidence of a pinched audio or that there was sideband cutting. It was like hi-fi. Next I shifted to USB and tuned away 3 kHz and boom I could copy the signal in USB. So that was most telling in that two things were happening: 1) The opposite sideband rejection was terrible and 2) that the filter was too wide. Now the hi-fi sound made sense.
Next I tried the rig on transmit (into a dummy load) and while in LSB tuned an outboard receiver to USB and there I was on USB (not quite as strong but definitely not 50 dB down). So throwing caution to the wind I connected my rig to an antenna (the worst I could be was to have a DSB signal).

In short order I found a very loud station in San Diego (200 miles away) and gave him a call. Boom he came back to me and reported "you sound absolutely terrific". I then asked if he could check my other sideband --and got a shocking answer (he was an extra class ham, newly minted) and he said I don't know how to do that. I thanked him and quickly said 73's. I already knew I had a DSB signal.
About 15 minutes worth of work I installed a filter from a Yaesu FT-101 (3.180 MHz Cf) and after repeating listening to my signal into a dummy load --there was no USB component. True it did not have the fidelity of the Dishal; but it was a real sideband filter.
Oh the 15 minutes -- 5 of it was to install the filter and load the code in the Pro-Mini --the other 10 minutes was to install the new paint job -- Juliano Blue V.2. You can just see the 3.180 MHz filter where the Dishal was installed.
The New Juliano Blue Rig!
I must challenge those who built a Dishal Filter -- have you looked at the Opposite Sideband Rejection both on receive and transmit? You might be surprised. Even my good friend Bill, N2CQR commented that he had less than sterling results using Dishal and he uses the AADE software or the Program in the EMRFD DVD.  While the purists want to say I built all of this rig, even the crystal filter, you may actually have a marginal filter in the rig if you use the Dishal method. If you are getting a lot of negative 40M SDR Police reports --you surely are getting a message about your filter.
By the way my local Home Depot was having a sale on spray paint and so look for some future rigs in  "Hot Pink" and "Cranberry" colors. I am reminded of Henry Ford who initially produced only black colored cars and when asked about other colors smartly said -- "you can have any color you want so long as it is black". Why do so many of the commercial rigs come only in black? Now I do know that the Hiberling PT-800A, (a $17K rig) did come in a sorta Juliano Blue --but not sure they are still being produced.
Final word -- the Dishal software has been exorcized from my computer. Based on my experience I would not recommend the use of the Dishal method. The results simply don't match what you hear once installed in a rig. I truly believe those who say they have built a successful Dishal Filter may need to revisit their data and re-evaluate the filter performance in a real rig.
Pete N6QW