Monday, January 1, 2018

Looking for the perfect match..

After a delay for the holidays and my son coming into town from his USAF training at Sheppard AFB in Wichita Falls, Texas, I am finally resuming the NorCal 40A Anniversary project started by Chuck Adams, K7QO. This little juncture covers the crystal matching phase of the effort. And, typical of me, it's a start-to-prepare-begin-to-plan-to-organize-to-commence affair where I divide the two piles of crystals I got off of eBay into matched sets of six for the radios I am contemplating. (I will be assembling a 40-meter and 30-meter version; more about that later.)

In order to do this, I ordered the crystal Colpitt's oscillator checker boards from ALLPCB.COM per the project instructions to be found here. Here is the video on this sub-project:

Incidentally, as shown in the video, you order ten but the All PCB folks will reward you with any over-runs they have when manufacturing your order. In my case, I received 13 boards which I will use to do some experimenting with on Colpitt's oscillators. (I am not exactly satisfied with the results returned with the first two I built and want to determine how to optimize or augment the circuit. If nothing else, you can build up three or four of them with different crystal values and use them as sources of RF. But be advised that the Colpitt's oscillator -- at least this design -- has problems oscillating above 15 MHz.)

And the schematic for the little Colpitt's oscillator board can be found here. Chuck's thought was to use this connected to a Morse code frequency enunciator to determine a crystal's frequency. As each is measured, it is sorted -- matched -- with other crystals of similar frequencies. This is essential to determine the band pass frequency of the filter and the BFO of the radio we are building.

Chuck likes to match them to the nearest 10 Hz and does so as he shows below:

So, following this process, I did make a "grid" to help facilitate this matching process using Excel. It differs from Chucks but is useful in accomplishing the deed.

Note that the gross frequency for a batch was determined by measuring about 10 or 15 of them and generating the grid of one kilohertz spread by 10 Hz increments as Chuck did.

The sheet is laid over some 8x11 Styrofoam I have and as crystals are measured, they are "punched" into the appropriate slot. However, as I started measuring the crystals, I noted that the frequencies tended to drift (mostly upwards) and it was not convenient to determine the actual frequency. At first, I thought about experimenting with a higher value for R0 as Chuck mentions in the first video but, not wanting to go any further down the rabbit hole, I opted for the "just get 'er done" philosophy and opted for a "timed measurement" by letting the crystal "cook" for approximately 15 seconds to determine the frequency. Besides, I had just scored a couple of really, really neat stop watches manufactured in the USSR in the late eighties -- byproducts of the Soviet space program -- and thought it neat to put them to work.

Also, I felt a little more comfortable using a frequency counter instead of the CW enunciator. So, below is the setup halfway through my measuring/matching the 4.9152 MHz batch I got for the 40 meter version:

 The stop watch was just started and a crystal inserted and, after 15-20 seconds whizzed by, the crystal was "posted" to the Styrofoam in the correct slot. The stop watch just kept running so I could approximate the 15-20 seconds; better than reading the tiny second hand on my wristwatch and around 15 seconds was adequate for a crystal to stabilize.

This made fairly quick work of a pile of these little beauties but it was interesting to note that the 8.000 MHz crystals were more "spread out" than the 4.9152 MHz crystals.

4.9152 MHz Initial Batch

4.9152 MHz "Overflow" Batch

8.000 MHz Batch
While the 4.9152 MHz batch will not be hard to gather into six crystal groups, the 8.000 MHz batch will be a little problematic because of their distribution. But that's for another day.

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