Broadcast - 29 June 2025
The main event at last week’s club room technical night was provided by Trevor VK7TB ably assisted by Colin VK7ZCF.
It was a working demonstration of band pass filter characteristics. It covered the use of a spectrum analyser when analysing the characteristics of a crystal band pass filter. A spectrum analyser has the ability to visually show the voltage or level of all frequencies between upper and lower limits defined by the user.
This particular spectrum analyser was a Rigol DS-A815 which also incorporated a sweep generator. Simplistically the sweep generator side of the spectrum analyser generated a constantly increasing sinewave frequency between two set limits at a very accurate voltage or amplitude. If the sweep generator is fed into the spectrum analyser input you should get a horizontal line showing that the sweeping frequency level is constant. If anything is placed between the sweep generator and input you then have a visual representation of the effect that it is exhibiting on the signal at various frequencies.
Trevor started of connecting a high quality German filter unit that was in the same league as used in Collins communications receivers. He was able to display the characteristics of the pass band filter, namely, centre frequency, bandwidth and the filter slope. Trevor also visually demonstrated the change to characteristics when adjusting the filters trim capacitors.
He then moved on to looking at testing a single crystal. Needless to say he had previously designed and constructed a test jig, HiHi. This allowed an individual crystal to be held for testing as well as matching its impedance to the 50 Ohms of the test equipment it was connected to. This allowed him to display the series resonance pass band of the crystal under test. Trevor came prepared, he had a bank of 100 crystals, purchased at the same time and identical within manufacturing repeatability and cost. These are the same crystals mentioned in a broadcast last year, the ones that were subjected to hundreds of tests by his Vector Analyser to test and measure then record all working equivalent parameters of the crystals. He was able to show the variations in characteristics by randomly picking crystals from his wide selection
As might be obvious now Trevor has a need for a crystal filter and is currently focusing his research towards this. For those technically minded Trevor is designing a ten pole crystal ladder filter with 100 dB out of band rejection and 0.1 dB band pass ripple. To achieve this he requires crystal matching to within 10 Hz with a “Q” greater than 160,000.
We then moved over to a computer. Trevor showed us an interactive programme that is a “Crystal Ladder Filter Calculator” called DISHAL by DJ6EV. Now that’s a pretty good starting point for this work in progress!
Thank you Trevor and Colin for a very informative night and demonstration of what is often only talked about.
As always pictures will be available on the NTARC Web site under “Blogs” for this broadcast. NTARC Blogs
Yes! NTARC club rooms will be opening up to watch the REAST “Chirpsounder Technology and Propagation Tool Interpretation” presentation this Wednesday the 2nd July. Doors will be open by 7:00 pm, with the live REAST YouTube channel on line by 7:30 pm. See you there.
UPCOMING EVENTS
On Air Test and Technical Net session - Every Wednesday, Test-Net and CW course on 3.580 MHz from 7 pm, then a Technical Net on 3.567 MHz from 7.30 pm till 8.30 pm. Your host for the evening is Nic VK7WW.
Special Club Room night session - Wednesday the 2th. July, live stream of REAST “Chirpsounder Technology and Propagation Tool Interpretation”. Doors open at 7:00 pm.
Mid-year Club get-together - Wednesday 9th July at Iron Horse Bar and Grill.
Club Room Technical night session - The next session will be Wednesday the 23rd July, from the usual time of 6.30 pm at the Club Room Archer Street, Rocherlea.
Coffee Morning - Held every Friday in the NTARC Club rooms. Time is from 10 am to noon.
Finally – If you have any items of news please email them to the Secretary at the following address news@ntarc.net all items to be received no later than 5 pm on the Friday prior to the Broadcast.
That’s all folks,
73, Stefan VK7ZSB, Secretary NTARC Inc.
